May 24, 2025 • 43 mins
In this classic episode of Stuff to Blow Your Mind, Robert and Joe discuss the weird and wonderful parrotfish: changers of sex, poopers of sand and – if the myths and legends are true – great friends and a parent of fishes. (originally published 5/23/2024)
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Speaker 1 (00:06):
Hey, welcome to Stuff to Blow your Mind. It is
another Vault day here for you. So we have the
Parrotfish Part two. It originally published five three, twenty twenty four.
I hope you enjoy.
Speaker 2 (00:20):
Welcome to Stuff to Blow Your Mind production of iHeartRadio.
Speaker 1 (00:30):
Hey, welcome to Stuff to Blow your Mind. My name
is Robert Lamb.
Speaker 3 (00:33):
And I am Joe McCormick, and we're back with Part
two in our series on parrotfish, a group of related
fishes containing about ninety to one hundred species worldwide, notable
for their powerful beak like mouths made out of rows
of fused teeth. In the previous episode, we talked about
(00:53):
some basics about the biology and taxonomy of parrotfish species.
We talked about their diet and feeding behavior. Parrot Fish
are usually considered herbivores or something equivalent to herbivores. They
survive by grazing for algae, microorganisms, to trite us, and
sometimes invertebrate animals like coral polyps along the surfaces of
(01:15):
rocks and coral skeletons within coral reef environments. We talked
about the fact that some parrotfish species end up biting
or scraping off significant chunks of hard matter from the
rocks and coral that they scour for food, and then
grinding up these coral skeletons and minerals in their phryngial mills.
(01:37):
It's kind of like horror movie machinery in the back
of their throats. I think we compared it to like
a bone transmission gear and then defecating what's left in
the form of sand. And so as a result, parrotfish
are major figures in the erosion of coral reefs and
in the production of sand. So if you walk across
a white sandy beach in the tropics where there are
(01:59):
coral reefs nearby, there is a good chance that most
of the sand under your feet was at one point
parrotfish poop. This is true of reef islands built entirely
out of reef sediment, like the Maldives, but also true
of some white sandy beaches in places like Hawaii and
the Caribbean. We talked also about writings on parrotfish from
the ancient world, how the Romans prized certain parrotfishes as
(02:23):
food items, and why they thought they were in first
place among the culinary uses of fish. How they developed
a range of beliefs about these fish, some of which
were fairly biologically accurate, such as the belief that these
fish are herbivorous grazers they sort of are, and that
they produce audible sounds underwater as they scrape the rocks
(02:44):
and coral for food, also true. Other beliefs not quite
so accurate. For example, Rob, didn't you talk about this
idea that they like cooperate altruistically to save one another
from traps?
Speaker 1 (02:55):
Yes, and in this became an enduring and long lasting
symbol of free which doesn't really hold up to how
they actually behave.
Speaker 3 (03:04):
But we also talked last time about some interesting beliefs
about parrotfishes in Hawaiian mythology, where the parrotfish is sort
of a progenitor of other sea life and enters into
a pact with a Hawaiian mythical hero who can sort
of call upon his friendship with the parrotfish in order
to produce an abundance of fish catch for the people.
(03:26):
And then finally, we also talked about research on parrotfish teeth,
which are made out of some of the hardest, stiffest,
and most resilient biominerals known even to the extent that
they are being investigated as a model for high durability
synthetic materials in the lab. So that was part one,
and today we're back to talk some more about parrotfish.
Speaker 1 (03:46):
Yeah, and I just want to drive home that if
you haven't had the opportunity to observe parrotfish in the wild,
and you find yourself traveling to or in an area
where there are reef environments and there is some manner
of snorkeling going on, go check it out. Oftentimes, you
know there are various snorkeling companies and and you know
(04:08):
small businesses that are that are very approachable. You don't
have to have a lot of experience to try these out. Again,
these are often we're talking like shallow water environments, and
in these environments there's almost always some sort of parrotfish
to observe. And if that's not your cup of tea,
I want to add that for anyone who would like
(04:30):
to see various parrotfish in action as well as other fish,
I highly recommend checking out Coral City Camera. It's that's
just Coralcitycamera dot com, or you can look it up
and search it's an underwater camera streaming live from an
urban coral reef in Miami, Florida. Uh, it's it's pretty cool,
and if you watch long enough, you will see some parrotfish.
Speaker 3 (04:50):
In action, pursuing their new career as streamers.
Speaker 1 (04:54):
Yes. Yes, so we're going to get into parrotfish reproduction
here in a bit. But first up, we have this
other delightful area to look at something that I wasn't
familiar with at all concerning parrotfish. Joe, do you want
to get into their mucus?
Speaker 3 (05:08):
Oh? Do? I? I feel like I'm already into their mucus.
I've been in it all day. So I mentioned in
the previous episode that the topic of parrotfish was one
that I initially got interested in just by looking at pictures.
Speaker 2 (05:21):
You know.
Speaker 3 (05:22):
I was looking at like a photo of a parrotfish mouth,
and I started thinking, what is going on with this
awesome palisade of fused together teeth? And so there is
another parrotfish subtopic that I think one could come to
in exactly the same manner, and that is their mucus cocoons.
If you look for photos of parrotfish sleeping, you will
(05:45):
find images of these animals nestled down into cozy little
niches in the seafloor or within the reef structure, sometimes
kind of in a recess or a little heidi hole
in the reef, surrounded by what looks like some kind
of of film. Sometimes it looks like the parrotfish is
enclosed within a gauzy, transparent orb covered like sometimes covered
(06:09):
in glitter. If you see these in motion and rob
below the photos here, I did attach a link to
a video in the outline that you can look at
so you can see it moving. If you see these
in motion, they will they will appear to undulate in
the water, so they're kind of jelly like in movement
and substance. In other cases, this film looks like the
(06:30):
fish is wrapped up inside a huge funnel spider web
that's just covered in sand. Which funny thing about that
the sand may of course be the parrotfish's own excretion
from earlier.
Speaker 1 (06:43):
MM. Yeah, I'm looking at the video footage right now.
It's it's slimy but beautiful.
Speaker 3 (06:50):
So what is this gossamer bubble around a slumbering fish.
This is what's known as the parrotfish's mucous cocoon. Or
sometimes in the scientific literature, it's mucus envelope, described by
an author named H. E. Win in a scientific article
in nineteen fifty five as a quote thin, transparent and
(07:13):
gelatinous mucoid substance which starts as a fold at the
mouth and progresses backwards in folds to surround the body.
So parrotfish are daytime animals. They sleep during the night,
they wake and feed during the day, and some species
have been observed to spend roughly an hour before sleep
(07:37):
generating this jelly like sleeping bag out of mucus from
their mouths before actually getting to sleep, so it's like
a bedtime routine. As night is falling, they start spitting
out the mucus and it starts to envelop their body
from front to back. So the fish are making themselves
bedtime slime sacks. But why now? First, this is a
(08:00):
side note. I just wanted to say it is normal
for all kinds of fish species, not just parrotfish or
the other related fish. Like some rasses that make these
slime sacks. It is normal for all kinds of fish
to be covered in a thin layer of mucus on
the outside of their skin. This omnipresent slime barrier can
(08:22):
provide a number of benefits, one of which is OSMO regulation,
and that's maintaining the balance of internal water and solutes
such as electrolytes, so for example, and OSMO regulation function
within our bodies human bodies is maintaining the right level
of salt in our body fluids. Mucous coverings on all
(08:42):
kinds of fish help with OSMO regulation. But these mucous
coverings on the skin also cut down on friction. So
the slippery layer of mucus on the fish's skin makes
it easier for the fish to swim along. It's like
a lubricant for the interface with the surrounding water. It's
also just physical protection of the skin from contact trauma
(09:05):
such as cuts and scrapes. It in some cases provides
UV radiation protection. In some cases might protect the fish
from noxious chemicals or pollutants in the water, and provides
the fish protection against drying out.
Speaker 1 (09:20):
So this is of course one of the potential issues.
And just handling fish such as then like ketch and
release and so forth. The slime isn't just something that's
on the fish. It's an active barrier.
Speaker 3 (09:30):
Yeah, but that's all just the normal mucous coating common
to many many fish. What we're talking about here is
specifically this baggy mucous hyper sleep pod that forms around
some parrotfish and rasses through the night. So I started
looking at scientific papers about this to see what I
could find out. So first I was looking at a
(09:51):
marine zoology paper from the year nineteen seventy that investigated
a few species of parrotfish to see how and under
what conditions the fish would make these cocoons. So this
is by John E. Byrne, who was a professor of
zoology at the University of Hawaii. The paper is called
Mucus envelope formation in two species of Hawaiian parrotfishes, and
(10:13):
the paper begins by citing previous research by Win and
co authors on parrotfish from the coral reefs of Bermuda,
which were observed to make mucus envelopes at night. So
when and a co author named Bardak argued that the
purpose of the mucus cocoon of the slimesack was to
protect the parrotfish from predators while it was sleeping. And
(10:39):
this is a hypothesis that I've seen repeated in a
number of sources that maybe somehow the mucus covering will
help alert the fish more quickly if a predator gets
close to it, or may in some way help mask
the fish, maybe mask the fish is sent from predators,
or provide some kind of benefit along these lines.
Speaker 1 (11:01):
Okay, So either to some degree a cloaking system, a
cloaking device, if you will, or perhaps some sort of
like added security trip wire made out of mucus.
Speaker 3 (11:10):
There you go. Now we'll get to another explanation in
just a bit here, but we're not there quite yet.
First we're gonna look at like how and how and
when these things form. So the author of the study,
John Burn, begins by investigating envelope formation in a couple
of different species of parrotfish. There's scare Us dubious, commonly
known as the regal parrotfish, and scaus parsp pair. Oh wow,
(11:34):
here's a word, pers bisilatis. There you go, which is
commonly called the spectacled parrotfish. Both are found in the
reefs around Hawaii, and I think the spectacled parrotfish may
be one of the keyfish referred to as oohu and
some of the Hawaiian legends that we talked about in
the last episode.
Speaker 1 (11:53):
Yeah. Yeah, So burn.
Speaker 3 (11:55):
Did some experiments on these two species in his laboratory,
varying conditions of light and darkness within their aquaria, making
observations of behavior, and then examining the mucus producing organs.
So Previous field observations had found that as daylight intensities decrease.
As daylight goes down, fewer parrotfishes can be found swimming
(12:17):
around the reefs for the night time, these fish will
disperse their schools and go into recesses within the reef
to hide and sleep, and that's where they generate these cocoons.
Within the lab environment, Burne found that if you shine
a constant light on these fish for twenty four hours straight,
they will actually never make a mucus cocoon. You just
(12:39):
keep shining the light on them, at least for twenty
four hours. He didn't push the experiment to go that
much longer, because you know, it might just end up
harming them overall. But for twenty four hours straight, you
shine a light on them and it, you know, nothing happens.
Speaker 1 (12:54):
Yeah, corp, of course, I mean, there are a lot
of things I'm not going to do of some sort
of an intelligent being from highly advanced species shines artificial
light on me for twenty four hours.
Speaker 3 (13:05):
That's right, So the constant light means you never make
a pod. However, when darkness was introduced, you turned the
lights off. This triggered twenty two of the thirty parrot
fish tested to build mucous envelopes, and it was the
same frequency in the two different species. The fish took
different amounts of time to finish building their envelopes after
(13:25):
the light was turned off. The minimum was like thirty minutes,
maximum was two hundred and forty minutes. Average building time
was about seventy minutes. However, if you kept the fish
the parrot fish in the dark after it made its cocoon,
it did not stay in the cocoon forever. Eventually it
would emerge on its own. So what did they do
(13:46):
when they made these things? They would typically rest their
bodies on the floor of the tank and the seafloor
in the wild in an upright position, almost always with
one side of the body resting against a vertical surface
like a rock or a coral wall, or, in the
case of the lab experiments, the aquarium wall, and then
the cocoon begins. It begins formation at the front of
(14:10):
the fish around its mouth, and folds of mucus slowly
move back along the length of the body toward the tail.
Though interestingly, the mucus never completely closes over the body.
There is always at least a one to two centimeter
gap at the back end of the bubble, and Burn
believes this hole is to evacuate respiratory water that's forced
(14:34):
out of the buckle cavity, so sort of a breathing hole. Now,
I mentioned that in a lot of these photos, the
mucus coccoon seems to sparkle as if it is covered
in glitter. Burn writes that quote fine debris adhered to
the envelope's exterior and the outline was thus more clearly defined. However,
he says this coating of sediment and debris makes the
(14:57):
bubble appear thin and delicate. This is how Wind described
it in that article from the fifties. But Burn did
an experiment by injecting pigmented particles into the cocoon and
revealed that actually appearances can be deceiving here because the
cocoon often does look very thin. It's like a you know,
wispy spider web or this very very thin kind of
(15:21):
gossamer like material. But in fact he found when he
injected the pigmented particles in there, the mucus structure was
up to six centimeters thick in some places. So it's
not as wispy as it looks.
Speaker 1 (15:33):
Okay, This would just be the case of there being
like a thin layer of particles on top of this
otherwise translucent or semi translucent mucus shielding. They would give
it the appearance of being super thin when in fact
it is probably thicker.
Speaker 3 (15:49):
That's right. So dissection of the fish revealed that the
presence of gland tissue in the buckle cavity correlated with
whether or not the fish would make a cocoon. It
was found that fish that did not have this gland
tissue in the mouth cavity, they did not form the
mucus cocoons. So that's how they're made and win. But
(16:13):
what are they for? Well, again, the burn paper mentions
this hypothesis that the mucus sleeping bag somehow protects the
fish from large predators. A commonly mentioned predator in the
reef environment would be the more eel. You know, and
it can get down there in the recesses and attack.
But apparently there's some doubt about this because, for example,
(16:34):
when faced with reef dwelling predators such as more eels,
there is some evidence that sleeping fish within within a
cocoon are still vulnerable, like they still get eaten. But
actually I came across an interesting twenty eleven paper that
looked directly into the function of the mucus cocoon and
came to a different conclusion. So this was published in
(16:57):
the journal Biology Letters by Grutterer at All and the
title is this will give some of the findings away
fish mucus cocoons the mosquito nets of the sea. This
was in the year twenty eleven. So in this paper
the authors look at another hypothesis, which is the idea
that mucus envelopes actually protect the fish inside from parasites
(17:21):
such as ectoparasitic nathid isopods. These are these little bloodsucking
parasites that live throughout the ocean, often compared to terrestrial
mosquitoes and ticks. Raw I attached to a little photo
of these things for you to look at. They're kind
of shrimp like in appearance maybe look like a cross
between a shrimp and a tick. Yeah yeah, And the
(17:44):
authors point out that during the daytime, when parrotfish are
swimming around, they actually get some help. They get some
protection against blood drinking isopods from cleaner fish. You know,
this is a relationship where a smaller fish that wants
to eat these parasites will come along and help pick
them off to sort of groom the outside of the
(18:06):
larger fish. But how do the fish protect themselves at night.
The idea behind this experiment was that maybe the mucus
cocoon functions like a mosquito net to protect the sleeping
fish from these heimatophagous parasites. So they tested this hypothesis
on the coral reef parrotfish Chlorurus sordid us. And the
(18:29):
way they tested it was they got some of these fish,
they separated them into groups that would sleep with and
without the benefit of cocoons in the presence of these
isopod parasites. And actually, the way they did it was
they took a subset of cocoon fish and found a
way to sort of gently push them out of their
envelopes without waking them up. So what do they find Yes, indeed,
(18:53):
the fish without the mucus bag experienced way more attacks
by parasites. How much more well, about ninety five percent
of the fish without cocoons were attacked by isopods and
only about ten percent of the fish with cocoons were attacked.
Speaker 1 (19:08):
So huge difference, all right, Right, so yeah, coming back
to the mosquito net comparisons, Like, initially we looked at
it and we're like, this mosquito net must protect the
sleeper from bears, but in reality it protects them from
mosquitoes and similar insects that sort of thing.
Speaker 3 (19:25):
Or maybe even something downstream from mosquitoes in the analogy here,
because the authors also investigated the question of how energetically
costly it is for the fish to make these mucous orbs,
and they calculated that it takes about two point five
percent of a fish's daily energy budget to make the
(19:46):
mucus bag. Now, when I first saw that figure, I
kind of thought, oh, hey, that seems fairly cheap, only
two point five percent. But actually I was reading some
news reporting on this that quoted the lead author, Alexandra Grutterer,
and she framed it a different way. She said, quote
the amount of effort that goes into building these cocoons,
which requires fish to have developed very large glands about
(20:09):
the size of a quarter to produce the cocoons, is extraordinary.
Parasites must exert an enormous pressure on these fish in
order for the fish to have evolved such a specific
way of avoiding the parasites. So what could be so pressing?
Is it really just that you don't want to get
bitten by these isopods and have them drink some of
your blood? Well, Grutterer mentions the possibility that the blood
(20:34):
directly lost to the parasite might not be the only cost.
These isopods may also transmit a secondary endo parasite which
lives in the fish's blood, much like how mosquitoes transmit
malaria in humans. So the mosquitoes themselves are annoying and
you don't like the mosquitoes, but the malaria is much
(20:56):
more concerning than the mosquito. Malaria can be deadly in
a similar way. It's possible that it's worth it for
these fish to build these slimy bionets to protect themselves
from blood disease. Wow, so it seems like a good trade.
You spend a little energy to weave a slime tube
before bed. Every night, you sleep without these tiny shrimp
(21:19):
monsters drinking your blood, possibly giving you diseases of the blood.
And it all works out. And in fact, there was
one more observation from grutter speaking to the media that
kind of maybe there's a strategy to recoop some of
that nightly cost. So Grutterer says, quote, I have observed
on occasion a fish at dawn with what appeared to
(21:40):
be mucus stuffed in its mouth. And then she goes
on to say she has seen other fish, not parrotfish,
but related fish that also produce mucus cocoons pecking it
at its old cocoons in the morning. So like recooping
some of that nightly cost by eating the mucus that
you created before.
Speaker 1 (22:00):
I mean, that's just it's economically sound. It's like if
humans produce, say an ectoplasm defense shield at night, you
would want to to recoup that cost, and that might
mean consuming all that actoplasm again and getting all that
liquid back into your body. You know, it reminds us
of other examples we've looked at in biology, such as
(22:22):
various reptiles that will eat their own shed skin because
you know, why waste that, you know.
Speaker 3 (22:28):
Yeah, so may still provide some kind of benefit against
larger macroscopic predators too. Not certain about that, but it
does seem like there's a very good case that these
mucous bags help prevent against parasite attacks.
Speaker 1 (22:42):
Very fascinating, all right, And so for the rest of
the episode, we're going to turn to the world of
parrot fish reproduction and pair fish sex. So, as we
teach out in the first episode, one of the other
(23:04):
amazing aspects of parrotfish biology broadly is that they change
sex during the course of a normal lifetime. That's to say,
this is not something that occurs, you know, only when
certain environmental conditions are right. It occurs as part of
a normal life cycle.
Speaker 3 (23:22):
And within a fairly predictable pattern, right.
Speaker 1 (23:25):
That's right. Yeah, And they're going to be a couple
of exceptions. Again, as we've been distressing, there are a
number of different species of parrotfish, but still the vast
majority of them do follow this example that we're going
to be discussing. So they are proto gynos that means
(23:46):
female first, hermaphrodites that always turn into males if they
live long enough, so they're born female and then at
a certain point during their development they become male and
live out the rest of their life as a male.
Speaker 3 (24:00):
And this would feedback into something we talked about in
the first episode, which is sometimes difficulty in identifying parrotfish
species because they undergo these changes, and these changes come
with changes to their outer appearance.
Speaker 1 (24:14):
That's right along the way, multiple changes in colorization take place,
some of which have to do with just aging, some
of which have to do with changing their sex, and
others that have to do with diet and other factors.
This pointed out by the National Marine Sanctuary Foundation as
resources about the parrotfish. I also want to point out though,
(24:38):
that according to NAA fishery biologist Ronald J. Saals, gonecharism
has been reported for I think three species within the
parrotfish family, which is to say, there are at least
three species of parrotfish where we have the more typical
scenario of male female division as opposed to what we
predominantly see in parrotfish, which again is this this sequential
(25:03):
hermaphroditism in which the fish are born female and then
if they live long enough, become male, live out the
rest of their life as males. And so the basic
scenario is most parrotfish are born females, continue to grow
to reproduce externally as females, generally in the harem of
a larger protective male who also tends to a grazing territory,
(25:27):
and in time, if that female lives long enough and
grows large enough, she transitions into a larger terminal, reproductive male.
Speaker 3 (25:36):
Interesting.
Speaker 1 (25:37):
Now, in general, parrotfish experience what I've seen referred to
as moderate longevity. It's going to vary depending on the
particular specimen, and I think it even the general generalities
about how long they live is going to vary. I've
seen in general parrotfish life span sited seven to ten years.
(25:57):
I've seen it cited as less than twenty. I've also
seen it sighted as five to six. Again, we have
a number of different species we're talking about here, and
I'll throw out additional numbers for a specific species here
in a bit. We also have to remind ourselves that
these are creatures living in the ocean, and so there
are a whole number of factors from blood diseases to
(26:18):
parasites to eels trying to eat them. To human fishermen
and so forth.
Speaker 3 (26:23):
It's just hard to imagine like ten straight years of
biting and scraping on rocks with your teeth.
Speaker 1 (26:28):
Yeah, we're literally scraping by right now. Parrotfish display what
is referred to as indeterminate growth, which means that there's
not a full sized growth limit. They just keep growing
as long as they're alive. And so parrotfish just continue
to grow at a consistent rate. And this is important
(26:49):
to consider in making sense of their sex changes because
one of, if not the primary hypotheses for why they
do this, why they evolve to do this, does relate
to their size. Maximum size again, it's going to depend
on the species. I see ranges like one to four feet.
But let's go ahead and just talk about the biggest
(27:09):
parrotfish just to give us like a nice frame of reference,
because also the largest parrotfish is also pretty gnarly.
Speaker 3 (27:17):
Is this the bumphead we talked about last time?
Speaker 1 (27:20):
It is the bumphead, So we have a little more
on the bump head here. The bump head, according to
the NOAA, reaches size as a four point two feet
long and up to one hundred pounds, So one hundred
and thirty centimeters forty six kilograms, And not only are
they the largest parrotfish, but they're among the largest reef
fish period. Reef environments are generally shallow and tight, so
(27:41):
you know, they're not inviting places for larger fish. And
their namesake bump is used like a rams horns in
male to male competitions, though females also have smaller bumps, which,
of course, if the parrotfish lives long enough, is going
to grow in size once they have changed sect.
Speaker 3 (28:00):
Okay, so these are fish that just keep growing, even
though that's not necessarily the best for them in all ways,
like it might limit what coral surfaces they can access
and so forth.
Speaker 1 (28:12):
Well, these guys are just bigger anyway. This is just
But this would I guess, seem to be like the
maximum size that seems to fit into the evolutionary economy
of living around the reef. Okay, like I guess it
would be. It would be hard to argue that parrotfish
should get larger than this, because we have no living
parrotfish that get larger than this. I see, the market
(28:35):
won't allow it. You know, Now, bump head parrotfish can
live to be forty years old. I've read they don't
reach sexual maturity until five to eight years old, and
sadly their numbers are down except in protected reef environments.
Speaker 3 (28:51):
So I believe based on what I've read, these are
the ones that are classic. When we talked about the
different classifications of parrotfish feed behaviors based on like sort
of how hard they gouge the rock or the coral,
and these would be like the excavators, right, like they
are plowing into that stuff.
Speaker 1 (29:09):
Yeah, these guys take the big bites. I was reading
a little bit more about this on the NOAA website
and they said that, yeah, they take out those big
bites that also end up taking out a little bit
of live coral. But they stress that this is still
very healthy for the coral in all the ways we
already mentioned. I don't remember if we mentioned this. I
don't remember if we mentioned this or not. But there's
also the idea that they'll break down dead reef and
(29:33):
of course turn that into sand, dead bits and branches
that might otherwise break off in storms and damage other
parts of the reef.
Speaker 3 (29:41):
Oh yeah, I see, So it's better for it to
better for this chunk to get ground up in a
parrotfish's pharyngial mill and pooped out as sand rather than
knocked off in the storm and hit some other healthy
part of the reef.
Speaker 1 (29:54):
Yeah, because it's one of the interesting things about about
reef environments, and this is something you're def only instructed
about anytime you go out and snorkel or scuba dive.
Certainly I imagine around these these is that there is
like a hardness to them. Certainly they can also be
very like you certainly don't want to stand on them
(30:14):
or walk on them or touch them for a number
of reasons, because a lot of times they can be
quite harmful and scrape you out, they can cut you.
You don't want any of that. But on top of that,
they can be actually quite delicate, and they can be
easily broken. And so this would be another case of
where if the parrotfish are doing their thing, that limits
the amount of damage that they're going to sustain via
(30:36):
their own dead parts.
Speaker 3 (30:38):
I see.
Speaker 1 (30:39):
But anyway back to sex changes in parrotfish in general. So,
according to Jennifer Hodge, a postdoctoral researcher in the Department
of Evolution and Ecology in the UC Davis College of
Biological Sciences in twenty twenty. The indeterminate growth factor may
in fact be key. I was reading a couple of
(31:00):
from a couple of sources. Here. One is a UC
Davis article by Andy Fell covering her work titled male
size advantage drives evolution and sex change of sex change
in Refish. And then also there is a full paper
I was looking at, and this is by Hodge at
All titled Correlated Evolution of sex allocation and Mating system
(31:21):
in Rasses and Parrotfishes, published in The American Naturalist the
same year.
Speaker 3 (31:26):
Okay, so how would this indeterminate growth factor affect how
sex is distributed and developed in a fish species.
Speaker 1 (31:35):
It basically comes down to the fact the observation that
reproduction among parrotfish and also some of these other fish,
but for our interests here, the parrotfish is often dominated
by large males.
Speaker 3 (31:49):
Meaning that like, a larger male has a better chance
of mating more.
Speaker 1 (31:54):
Right, and that and that male large males and this
would be the terminal mail in parrotfish fishes. They are
the ones dominating like all of the mating. So if
you are not a large male, you are just not
going to be effective at reproduction. If you are a
small parrotfish male, your chances of passing on your genes
(32:18):
is rather slim. And remember the genetic mission is to
pass on your genes. Now, as a small parrotfish female, however,
it's less of an issue. The bigger males, they have
the advantage. They're going to form these harems. If you
are a small female, you can be part of that
harem and you are doing your reproductive part as a parrotfish.
And so that's according to this hypothesis, this is where
(32:41):
the evolution of sequential hermaphrotitism evolves as a strategy by
which all individuals have a better shot at participating in reproduction.
So start off small and female, you definitely get to reproduce.
And then if you live long enough and you grow
big enough, you shift to the male set, and then
(33:01):
you have the size to prove effective. You're better at
controlling territory resources, harems, etc.
Speaker 3 (33:08):
That's interesting, Okay, So it gives more individuals of the
species a chance to mate more often.
Speaker 1 (33:16):
Yes, yeah, that's the way I understand it. And I
was looking again at the writings of in Oa's fishery
biologist Ronald J. Salce and Salce points out that, yeah,
the largest parrotfish are always terminal males, and he points
out that the species, the various species in the genus
Skeras typically exhibit the following reproductive characteristics. So we see this.
(33:41):
First of all, there is this proto Guynus female first hermaphrotitism,
There are breeding territories, there are harems, and there is
external fertilization.
Speaker 3 (33:50):
Oh yeah, the external fertilization is a good point, because
I don't want to give the wrong idea when I
was mentioning mating that it's like, you know, the kind
of active you might be picturing that. Instead, there's a
there's an external meeting of the game meats of these animals.
Speaker 1 (34:06):
Right, And I think, I don't know humans, maybe we
have a problem imagining fish sex in general. But somehow
this makes it a little easier to sort of picture
how all this is happening. I think it's all out
in the open. So SAAL's points out though that in
the past, and really maybe not in the two distant past,
we've had these other hypotheses that there might be a
(34:27):
social trigger for the change in sex. But apparently, based
on what he wrote, this hasn't necessarily been observed, or
at least not in all cases or in a broad
array of cases, because we have scenarios where large terminal
males are removed from a population, such as by fishing,
(34:48):
and the females don't just switch over at an earlier age,
but rather have more difficulty finding a mate.
Speaker 3 (34:55):
Oh okay, so it might be kind of baked in
that they need to reach a certain size.
Speaker 1 (35:00):
Seems to be the case now, I don't, But again,
we're dealing with hypotheses here. I don't think that there
that anything is like one hundred percent proven out here.
There's still a lot of work that needs to be
done because a lot of it comes down to, Okay,
you can have this general idea that this practice evolved
because large males dominate reproduction and and it makes more
sense from a reproductive standpoint to start off as female
(35:23):
and then become male. But then what is the trigger
is it? Is it purely based on how big you grow?
Or are there environmental or social triggers? And Uh, ultimately
the size advantage explanation is just one hypothesis. Uh, there's
an you know, other hypotheses put more emphasis on possible
social or in mental environmental triggers such as changes and
(35:46):
in population density, that sort of thing, in the same
way that we see examples and say the world of salamanders,
where uh, you know something they're too many, or you know,
something goes on demographically in a collect in a certain group,
then you may have biological changes that result. But I
guess broadly if there are, if there are social or
(36:06):
environmental triggers that are involved in theory, we would be
able to observe them, you know, such as response to
overfishing of large males in response to changes in the
environment and so forth. Now there are individual species of
parrotfish where we might see some of those, like social triggers.
Perhaps I've seen discussion of the stop light parrotfish in
(36:29):
particular as perhaps being influenced by population density, growth and
mortality rates. So if terminal the idea here being that
it may be the case that terminal males in stop
light parrotfish populations, if they experience higher mortality rates so
more of them are dying, or if they are just
smaller overall sizes in the terminal males, then this change
(36:55):
may trigger earlier onset of the sex change in the
female parrotfish in that population. So, like I say, it's
still would line up with this idea that this evolved
because male parrotfish, large male parrotfish dominate reproduction. But it
would maybe be a slightly different case of like what
(37:16):
is actually causing it based on my understanding looking at
this documentation. But I like to say there's still I
think a lot of work going on here. Two thanks.
Keep in mind, though there is no evidence that any
species of parrotfish can undergo a sex change, reversal, or
a second sex change. Like it is, it is sequential,
(37:38):
sequential hermaphroditism. So it's female, then male. There are no
known cases where a male can then change back to
female do to you do to any kind of you know,
social pressure, environmental or what have you. It is female
and then male and again sequential hermaphroditism of one form
or another can be found in other fish. As I
mentioned the ras is. Apparently you see some version of
(38:00):
this in some molluscs and crustaceans. The size reproduction hypothesis
is widely e employed across the board, but I've also
seen I think the prevention of inbreeding being brought in
as another possible reason though I'm not sure if that
really pans out, particularly with the parrotfish. That may just
really have more to do with hermaphroditism as an evolutionary
(38:25):
trait in general.
Speaker 3 (38:26):
But the sequential version you're saying, it seems that there's
a similar evolutionary explanation given across these different classes of animals,
which is that it likely has to do with a
relationship between the animal's size and its likelihood of successful reproduction, yes, exactly,
or specifically the size of males and successful reproduction, and
(38:49):
the fact that the animals just keep growing.
Speaker 1 (38:51):
Yeah, So it's fascinating. Yeah, this is like a factoid
about parrotfish that I'd long heard, but I'd never really
looked into it. I guess one of the problems is
when you're in the water, it's it's really hard to
research stuff. You're just like, oh, I'm gonna take your
word for it, and I'm I'm gonna look at it
and then I'll try to remember to read about it later.
Speaker 3 (39:18):
Man, you would never guess that there is so much
interesting stuff about these fish just watching them scrape the rocks.
Speaker 1 (39:25):
Yeah, yeah, I mean just observing them, and I've observed
in plenty of times in the past. You know, it's
like you look at them and you're like, well, they're
a little bit goofy looking, they're beautifully colored, and then
you learn a little bit more about them, but there's still,
you know, greater depths of interest there. I guess that's
the that's the nature of most fish in the sea.
Never take them for granted.
Speaker 3 (39:44):
In between recording these two episode parts, did you end
up googling more human parrot fish bites?
Speaker 1 (39:51):
I did.
Speaker 3 (39:52):
I don't know why I did. I shouldn't have.
Speaker 1 (39:54):
Why would you do that?
Speaker 3 (39:55):
I don't know. I regret it. I wish I hadn't
done it. I just did.
Speaker 1 (40:00):
The only thing of that nature that I did run
across is when I was looking up pictures and looking
at articles about the big boys, the bumphead parrotfish is
there was an image of some coral with some big,
chunky bites taken out of it, and that was pretty impressive,
and it did cross my mind. It's like that I
would not want those bites taken out of my own flesh.
Speaker 3 (40:21):
Yeah, I would not want that to be my bones.
Speaker 1 (40:23):
Yeah.
Speaker 3 (40:24):
But to emphasize yet again, as we did last time, parrotfisher.
There's no indication that they're very aggressive or looking to
bite humans that like, these stories come from people who
were getting up in the parrotfish's business.
Speaker 1 (40:38):
Right, Yeah. I think I saw one account and this
is like, you know, this is I guess inherently unverified
of snorkelers or divers where someone was just sort of
casually bitten by a parrotfish. But in that thread, like
everyone was like, wow, that's weird. It's never happened to me.
So I don't know, you know, in the wild one
(41:00):
officer certainly possible. Who knows what that that parrotfish was
going through that day?
Speaker 3 (41:05):
Yeah. I guess any species of any fish could in
some case be aggressive, but it's not like generally thought like,
oh wow, you gotta be careful, like they're they're coming for.
Speaker 1 (41:15):
You, right, Yeah, I don't think they're they're coming for you.
Speaker 3 (41:17):
Yeah, I'm just because I'm imagining so like, you know,
the James Bond villain has a pool of piranhas that
he drops his henchman into when they make a mistake,
And I'm just thinking like, could they have gone with
a pool of parrotfish? How would that work out differently?
Speaker 1 (41:31):
We have cotajo bodhi, mister bond with a fine layer
of algae in coral dust, you will now drop you
into that of parrotfish.
Speaker 3 (41:43):
M No, not the bump edge. Okay, does that do
it for parrotfish?
Speaker 1 (41:49):
I think it does. You know, they may have more
mysteries that we didn't explore, but I think we hit
all the really interesting stuff here. But hey, if you
know of other dimensions to the parrot fish or various
parrotfish species that we didn't talk about, write in because
we would love to hear from you. Just a reminder
that's Stuff to Blow your Mind Here is primarily a
science and culture podcast, with core episodes on Tuesdays and Thursdays,
(42:12):
listener mail on Mondays, short form episode on Wednesdays, and
on Fridays. We set aside most serious concerns to just
talk about a weird film on Weird House Cinema.
Speaker 3 (42:20):
Huge thanks as always to our excellent audio producer JJ Posway.
If you would like to get in touch with us
with feedback on this episode or any other, to suggest
a topic for the future, or just to say hello,
you can email us at contact at stuff to Blow
your Mind dot com.
Speaker 2 (42:43):
Stuff to Blow your Mind is production of iHeartRadio. For
more podcasts from my heart Radio, visit the iHeartRadio app,
Apple podcasts, or wherever you listen to your favorite shows.
Speaker 3 (43:02):
West
Hey, welcome to Stuff to Blow your Mind. It is
another Vault day here for you. So we have the
Parrotfish Part two. It originally published five three, twenty twenty four.
I hope you enjoy.
Speaker 2 (00:20):
Welcome to Stuff to Blow Your Mind production of iHeartRadio.
Speaker 1 (00:30):
Hey, welcome to Stuff to Blow your Mind. My name
is Robert Lamb.
Speaker 3 (00:33):
And I am Joe McCormick, and we're back with Part
two in our series on parrotfish, a group of related
fishes containing about ninety to one hundred species worldwide, notable
for their powerful beak like mouths made out of rows
of fused teeth. In the previous episode, we talked about
(00:53):
some basics about the biology and taxonomy of parrotfish species.
We talked about their diet and feeding behavior. Parrot Fish
are usually considered herbivores or something equivalent to herbivores. They
survive by grazing for algae, microorganisms, to trite us, and
sometimes invertebrate animals like coral polyps along the surfaces of
(01:15):
rocks and coral skeletons within coral reef environments. We talked
about the fact that some parrotfish species end up biting
or scraping off significant chunks of hard matter from the
rocks and coral that they scour for food, and then
grinding up these coral skeletons and minerals in their phryngial mills.
(01:37):
It's kind of like horror movie machinery in the back
of their throats. I think we compared it to like
a bone transmission gear and then defecating what's left in
the form of sand. And so as a result, parrotfish
are major figures in the erosion of coral reefs and
in the production of sand. So if you walk across
a white sandy beach in the tropics where there are
(01:59):
coral reefs nearby, there is a good chance that most
of the sand under your feet was at one point
parrotfish poop. This is true of reef islands built entirely
out of reef sediment, like the Maldives, but also true
of some white sandy beaches in places like Hawaii and
the Caribbean. We talked also about writings on parrotfish from
the ancient world, how the Romans prized certain parrotfishes as
(02:23):
food items, and why they thought they were in first
place among the culinary uses of fish. How they developed
a range of beliefs about these fish, some of which
were fairly biologically accurate, such as the belief that these
fish are herbivorous grazers they sort of are, and that
they produce audible sounds underwater as they scrape the rocks
(02:44):
and coral for food, also true. Other beliefs not quite
so accurate. For example, Rob, didn't you talk about this
idea that they like cooperate altruistically to save one another
from traps?
Speaker 1 (02:55):
Yes, and in this became an enduring and long lasting
symbol of free which doesn't really hold up to how
they actually behave.
Speaker 3 (03:04):
But we also talked last time about some interesting beliefs
about parrotfishes in Hawaiian mythology, where the parrotfish is sort
of a progenitor of other sea life and enters into
a pact with a Hawaiian mythical hero who can sort
of call upon his friendship with the parrotfish in order
to produce an abundance of fish catch for the people.
(03:26):
And then finally, we also talked about research on parrotfish teeth,
which are made out of some of the hardest, stiffest,
and most resilient biominerals known even to the extent that
they are being investigated as a model for high durability
synthetic materials in the lab. So that was part one,
and today we're back to talk some more about parrotfish.
Speaker 1 (03:46):
Yeah, and I just want to drive home that if
you haven't had the opportunity to observe parrotfish in the wild,
and you find yourself traveling to or in an area
where there are reef environments and there is some manner
of snorkeling going on, go check it out. Oftentimes, you
know there are various snorkeling companies and and you know
(04:08):
small businesses that are that are very approachable. You don't
have to have a lot of experience to try these out. Again,
these are often we're talking like shallow water environments, and
in these environments there's almost always some sort of parrotfish
to observe. And if that's not your cup of tea,
I want to add that for anyone who would like
(04:30):
to see various parrotfish in action as well as other fish,
I highly recommend checking out Coral City Camera. It's that's
just Coralcitycamera dot com, or you can look it up
and search it's an underwater camera streaming live from an
urban coral reef in Miami, Florida. Uh, it's it's pretty cool,
and if you watch long enough, you will see some parrotfish.
Speaker 3 (04:50):
In action, pursuing their new career as streamers.
Speaker 1 (04:54):
Yes. Yes, so we're going to get into parrotfish reproduction
here in a bit. But first up, we have this
other delightful area to look at something that I wasn't
familiar with at all concerning parrotfish. Joe, do you want
to get into their mucus?
Speaker 3 (05:08):
Oh? Do? I? I feel like I'm already into their mucus.
I've been in it all day. So I mentioned in
the previous episode that the topic of parrotfish was one
that I initially got interested in just by looking at pictures.
Speaker 2 (05:21):
You know.
Speaker 3 (05:22):
I was looking at like a photo of a parrotfish mouth,
and I started thinking, what is going on with this
awesome palisade of fused together teeth? And so there is
another parrotfish subtopic that I think one could come to
in exactly the same manner, and that is their mucus cocoons.
If you look for photos of parrotfish sleeping, you will
(05:45):
find images of these animals nestled down into cozy little
niches in the seafloor or within the reef structure, sometimes
kind of in a recess or a little heidi hole
in the reef, surrounded by what looks like some kind
of of film. Sometimes it looks like the parrotfish is
enclosed within a gauzy, transparent orb covered like sometimes covered
(06:09):
in glitter. If you see these in motion and rob
below the photos here, I did attach a link to
a video in the outline that you can look at
so you can see it moving. If you see these
in motion, they will they will appear to undulate in
the water, so they're kind of jelly like in movement
and substance. In other cases, this film looks like the
(06:30):
fish is wrapped up inside a huge funnel spider web
that's just covered in sand. Which funny thing about that
the sand may of course be the parrotfish's own excretion
from earlier.
Speaker 1 (06:43):
MM. Yeah, I'm looking at the video footage right now.
It's it's slimy but beautiful.
Speaker 3 (06:50):
So what is this gossamer bubble around a slumbering fish.
This is what's known as the parrotfish's mucous cocoon. Or
sometimes in the scientific literature, it's mucus envelope, described by
an author named H. E. Win in a scientific article
in nineteen fifty five as a quote thin, transparent and
(07:13):
gelatinous mucoid substance which starts as a fold at the
mouth and progresses backwards in folds to surround the body.
So parrotfish are daytime animals. They sleep during the night,
they wake and feed during the day, and some species
have been observed to spend roughly an hour before sleep
(07:37):
generating this jelly like sleeping bag out of mucus from
their mouths before actually getting to sleep, so it's like
a bedtime routine. As night is falling, they start spitting
out the mucus and it starts to envelop their body
from front to back. So the fish are making themselves
bedtime slime sacks. But why now? First, this is a
(08:00):
side note. I just wanted to say it is normal
for all kinds of fish species, not just parrotfish or
the other related fish. Like some rasses that make these
slime sacks. It is normal for all kinds of fish
to be covered in a thin layer of mucus on
the outside of their skin. This omnipresent slime barrier can
(08:22):
provide a number of benefits, one of which is OSMO regulation,
and that's maintaining the balance of internal water and solutes
such as electrolytes, so for example, and OSMO regulation function
within our bodies human bodies is maintaining the right level
of salt in our body fluids. Mucous coverings on all
(08:42):
kinds of fish help with OSMO regulation. But these mucous
coverings on the skin also cut down on friction. So
the slippery layer of mucus on the fish's skin makes
it easier for the fish to swim along. It's like
a lubricant for the interface with the surrounding water. It's
also just physical protection of the skin from contact trauma
(09:05):
such as cuts and scrapes. It in some cases provides
UV radiation protection. In some cases might protect the fish
from noxious chemicals or pollutants in the water, and provides
the fish protection against drying out.
Speaker 1 (09:20):
So this is of course one of the potential issues.
And just handling fish such as then like ketch and
release and so forth. The slime isn't just something that's
on the fish. It's an active barrier.
Speaker 3 (09:30):
Yeah, but that's all just the normal mucous coating common
to many many fish. What we're talking about here is
specifically this baggy mucous hyper sleep pod that forms around
some parrotfish and rasses through the night. So I started
looking at scientific papers about this to see what I
could find out. So first I was looking at a
(09:51):
marine zoology paper from the year nineteen seventy that investigated
a few species of parrotfish to see how and under
what conditions the fish would make these cocoons. So this
is by John E. Byrne, who was a professor of
zoology at the University of Hawaii. The paper is called
Mucus envelope formation in two species of Hawaiian parrotfishes, and
(10:13):
the paper begins by citing previous research by Win and
co authors on parrotfish from the coral reefs of Bermuda,
which were observed to make mucus envelopes at night. So
when and a co author named Bardak argued that the
purpose of the mucus cocoon of the slimesack was to
protect the parrotfish from predators while it was sleeping. And
(10:39):
this is a hypothesis that I've seen repeated in a
number of sources that maybe somehow the mucus covering will
help alert the fish more quickly if a predator gets
close to it, or may in some way help mask
the fish, maybe mask the fish is sent from predators,
or provide some kind of benefit along these lines.
Speaker 1 (11:01):
Okay, So either to some degree a cloaking system, a
cloaking device, if you will, or perhaps some sort of
like added security trip wire made out of mucus.
Speaker 3 (11:10):
There you go. Now we'll get to another explanation in
just a bit here, but we're not there quite yet.
First we're gonna look at like how and how and
when these things form. So the author of the study,
John Burn, begins by investigating envelope formation in a couple
of different species of parrotfish. There's scare Us dubious, commonly
known as the regal parrotfish, and scaus parsp pair. Oh wow,
(11:34):
here's a word, pers bisilatis. There you go, which is
commonly called the spectacled parrotfish. Both are found in the
reefs around Hawaii, and I think the spectacled parrotfish may
be one of the keyfish referred to as oohu and
some of the Hawaiian legends that we talked about in
the last episode.
Speaker 1 (11:53):
Yeah. Yeah, So burn.
Speaker 3 (11:55):
Did some experiments on these two species in his laboratory,
varying conditions of light and darkness within their aquaria, making
observations of behavior, and then examining the mucus producing organs.
So Previous field observations had found that as daylight intensities decrease.
As daylight goes down, fewer parrotfishes can be found swimming
(12:17):
around the reefs for the night time, these fish will
disperse their schools and go into recesses within the reef
to hide and sleep, and that's where they generate these cocoons.
Within the lab environment, Burne found that if you shine
a constant light on these fish for twenty four hours straight,
they will actually never make a mucus cocoon. You just
(12:39):
keep shining the light on them, at least for twenty
four hours. He didn't push the experiment to go that
much longer, because you know, it might just end up
harming them overall. But for twenty four hours straight, you
shine a light on them and it, you know, nothing happens.
Speaker 1 (12:54):
Yeah, corp, of course, I mean, there are a lot
of things I'm not going to do of some sort
of an intelligent being from highly advanced species shines artificial
light on me for twenty four hours.
Speaker 3 (13:05):
That's right, So the constant light means you never make
a pod. However, when darkness was introduced, you turned the
lights off. This triggered twenty two of the thirty parrot
fish tested to build mucous envelopes, and it was the
same frequency in the two different species. The fish took
different amounts of time to finish building their envelopes after
(13:25):
the light was turned off. The minimum was like thirty minutes,
maximum was two hundred and forty minutes. Average building time
was about seventy minutes. However, if you kept the fish
the parrot fish in the dark after it made its cocoon,
it did not stay in the cocoon forever. Eventually it
would emerge on its own. So what did they do
(13:46):
when they made these things? They would typically rest their
bodies on the floor of the tank and the seafloor
in the wild in an upright position, almost always with
one side of the body resting against a vertical surface
like a rock or a coral wall, or, in the
case of the lab experiments, the aquarium wall, and then
the cocoon begins. It begins formation at the front of
(14:10):
the fish around its mouth, and folds of mucus slowly
move back along the length of the body toward the tail.
Though interestingly, the mucus never completely closes over the body.
There is always at least a one to two centimeter
gap at the back end of the bubble, and Burn
believes this hole is to evacuate respiratory water that's forced
(14:34):
out of the buckle cavity, so sort of a breathing hole. Now,
I mentioned that in a lot of these photos, the
mucus coccoon seems to sparkle as if it is covered
in glitter. Burn writes that quote fine debris adhered to
the envelope's exterior and the outline was thus more clearly defined. However,
he says this coating of sediment and debris makes the
(14:57):
bubble appear thin and delicate. This is how Wind described
it in that article from the fifties. But Burn did
an experiment by injecting pigmented particles into the cocoon and
revealed that actually appearances can be deceiving here because the
cocoon often does look very thin. It's like a you know,
wispy spider web or this very very thin kind of
(15:21):
gossamer like material. But in fact he found when he
injected the pigmented particles in there, the mucus structure was
up to six centimeters thick in some places. So it's
not as wispy as it looks.
Speaker 1 (15:33):
Okay, This would just be the case of there being
like a thin layer of particles on top of this
otherwise translucent or semi translucent mucus shielding. They would give
it the appearance of being super thin when in fact
it is probably thicker.
Speaker 3 (15:49):
That's right. So dissection of the fish revealed that the
presence of gland tissue in the buckle cavity correlated with
whether or not the fish would make a cocoon. It
was found that fish that did not have this gland
tissue in the mouth cavity, they did not form the
mucus cocoons. So that's how they're made and win. But
(16:13):
what are they for? Well, again, the burn paper mentions
this hypothesis that the mucus sleeping bag somehow protects the
fish from large predators. A commonly mentioned predator in the
reef environment would be the more eel. You know, and
it can get down there in the recesses and attack.
But apparently there's some doubt about this because, for example,
(16:34):
when faced with reef dwelling predators such as more eels,
there is some evidence that sleeping fish within within a
cocoon are still vulnerable, like they still get eaten. But
actually I came across an interesting twenty eleven paper that
looked directly into the function of the mucus cocoon and
came to a different conclusion. So this was published in
(16:57):
the journal Biology Letters by Grutterer at All and the
title is this will give some of the findings away
fish mucus cocoons the mosquito nets of the sea. This
was in the year twenty eleven. So in this paper
the authors look at another hypothesis, which is the idea
that mucus envelopes actually protect the fish inside from parasites
(17:21):
such as ectoparasitic nathid isopods. These are these little bloodsucking
parasites that live throughout the ocean, often compared to terrestrial
mosquitoes and ticks. Raw I attached to a little photo
of these things for you to look at. They're kind
of shrimp like in appearance maybe look like a cross
between a shrimp and a tick. Yeah yeah, And the
(17:44):
authors point out that during the daytime, when parrotfish are
swimming around, they actually get some help. They get some
protection against blood drinking isopods from cleaner fish. You know,
this is a relationship where a smaller fish that wants
to eat these parasites will come along and help pick
them off to sort of groom the outside of the
(18:06):
larger fish. But how do the fish protect themselves at night.
The idea behind this experiment was that maybe the mucus
cocoon functions like a mosquito net to protect the sleeping
fish from these heimatophagous parasites. So they tested this hypothesis
on the coral reef parrotfish Chlorurus sordid us. And the
(18:29):
way they tested it was they got some of these fish,
they separated them into groups that would sleep with and
without the benefit of cocoons in the presence of these
isopod parasites. And actually, the way they did it was
they took a subset of cocoon fish and found a
way to sort of gently push them out of their
envelopes without waking them up. So what do they find Yes, indeed,
(18:53):
the fish without the mucus bag experienced way more attacks
by parasites. How much more well, about ninety five percent
of the fish without cocoons were attacked by isopods and
only about ten percent of the fish with cocoons were attacked.
Speaker 1 (19:08):
So huge difference, all right, Right, so yeah, coming back
to the mosquito net comparisons, Like, initially we looked at
it and we're like, this mosquito net must protect the
sleeper from bears, but in reality it protects them from
mosquitoes and similar insects that sort of thing.
Speaker 3 (19:25):
Or maybe even something downstream from mosquitoes in the analogy here,
because the authors also investigated the question of how energetically
costly it is for the fish to make these mucous orbs,
and they calculated that it takes about two point five
percent of a fish's daily energy budget to make the
(19:46):
mucus bag. Now, when I first saw that figure, I
kind of thought, oh, hey, that seems fairly cheap, only
two point five percent. But actually I was reading some
news reporting on this that quoted the lead author, Alexandra Grutterer,
and she framed it a different way. She said, quote
the amount of effort that goes into building these cocoons,
which requires fish to have developed very large glands about
(20:09):
the size of a quarter to produce the cocoons, is extraordinary.
Parasites must exert an enormous pressure on these fish in
order for the fish to have evolved such a specific
way of avoiding the parasites. So what could be so pressing?
Is it really just that you don't want to get
bitten by these isopods and have them drink some of
your blood? Well, Grutterer mentions the possibility that the blood
(20:34):
directly lost to the parasite might not be the only cost.
These isopods may also transmit a secondary endo parasite which
lives in the fish's blood, much like how mosquitoes transmit
malaria in humans. So the mosquitoes themselves are annoying and
you don't like the mosquitoes, but the malaria is much
(20:56):
more concerning than the mosquito. Malaria can be deadly in
a similar way. It's possible that it's worth it for
these fish to build these slimy bionets to protect themselves
from blood disease. Wow, so it seems like a good trade.
You spend a little energy to weave a slime tube
before bed. Every night, you sleep without these tiny shrimp
(21:19):
monsters drinking your blood, possibly giving you diseases of the blood.
And it all works out. And in fact, there was
one more observation from grutter speaking to the media that
kind of maybe there's a strategy to recoop some of
that nightly cost. So Grutterer says, quote, I have observed
on occasion a fish at dawn with what appeared to
(21:40):
be mucus stuffed in its mouth. And then she goes
on to say she has seen other fish, not parrotfish,
but related fish that also produce mucus cocoons pecking it
at its old cocoons in the morning. So like recooping
some of that nightly cost by eating the mucus that
you created before.
Speaker 1 (22:00):
I mean, that's just it's economically sound. It's like if
humans produce, say an ectoplasm defense shield at night, you
would want to to recoup that cost, and that might
mean consuming all that actoplasm again and getting all that
liquid back into your body. You know, it reminds us
of other examples we've looked at in biology, such as
(22:22):
various reptiles that will eat their own shed skin because
you know, why waste that, you know.
Speaker 3 (22:28):
Yeah, so may still provide some kind of benefit against
larger macroscopic predators too. Not certain about that, but it
does seem like there's a very good case that these
mucous bags help prevent against parasite attacks.
Speaker 1 (22:42):
Very fascinating, all right, And so for the rest of
the episode, we're going to turn to the world of
parrot fish reproduction and pair fish sex. So, as we
teach out in the first episode, one of the other
(23:04):
amazing aspects of parrotfish biology broadly is that they change
sex during the course of a normal lifetime. That's to say,
this is not something that occurs, you know, only when
certain environmental conditions are right. It occurs as part of
a normal life cycle.
Speaker 3 (23:22):
And within a fairly predictable pattern, right.
Speaker 1 (23:25):
That's right. Yeah, And they're going to be a couple
of exceptions. Again, as we've been distressing, there are a
number of different species of parrotfish, but still the vast
majority of them do follow this example that we're going
to be discussing. So they are proto gynos that means
(23:46):
female first, hermaphrodites that always turn into males if they
live long enough, so they're born female and then at
a certain point during their development they become male and
live out the rest of their life as a male.
Speaker 3 (24:00):
And this would feedback into something we talked about in
the first episode, which is sometimes difficulty in identifying parrotfish
species because they undergo these changes, and these changes come
with changes to their outer appearance.
Speaker 1 (24:14):
That's right along the way, multiple changes in colorization take place,
some of which have to do with just aging, some
of which have to do with changing their sex, and
others that have to do with diet and other factors.
This pointed out by the National Marine Sanctuary Foundation as
resources about the parrotfish. I also want to point out though,
(24:38):
that according to NAA fishery biologist Ronald J. Saals, gonecharism
has been reported for I think three species within the
parrotfish family, which is to say, there are at least
three species of parrotfish where we have the more typical
scenario of male female division as opposed to what we
predominantly see in parrotfish, which again is this this sequential
(25:03):
hermaphroditism in which the fish are born female and then
if they live long enough, become male, live out the
rest of their life as males. And so the basic
scenario is most parrotfish are born females, continue to grow
to reproduce externally as females, generally in the harem of
a larger protective male who also tends to a grazing territory,
(25:27):
and in time, if that female lives long enough and
grows large enough, she transitions into a larger terminal, reproductive male.
Speaker 3 (25:36):
Interesting.
Speaker 1 (25:37):
Now, in general, parrotfish experience what I've seen referred to
as moderate longevity. It's going to vary depending on the
particular specimen, and I think it even the general generalities
about how long they live is going to vary. I've
seen in general parrotfish life span sited seven to ten years.
(25:57):
I've seen it cited as less than twenty. I've also
seen it sighted as five to six. Again, we have
a number of different species we're talking about here, and
I'll throw out additional numbers for a specific species here
in a bit. We also have to remind ourselves that
these are creatures living in the ocean, and so there
are a whole number of factors from blood diseases to
(26:18):
parasites to eels trying to eat them. To human fishermen
and so forth.
Speaker 3 (26:23):
It's just hard to imagine like ten straight years of
biting and scraping on rocks with your teeth.
Speaker 1 (26:28):
Yeah, we're literally scraping by right now. Parrotfish display what
is referred to as indeterminate growth, which means that there's
not a full sized growth limit. They just keep growing
as long as they're alive. And so parrotfish just continue
to grow at a consistent rate. And this is important
(26:49):
to consider in making sense of their sex changes because
one of, if not the primary hypotheses for why they
do this, why they evolve to do this, does relate
to their size. Maximum size again, it's going to depend
on the species. I see ranges like one to four feet.
But let's go ahead and just talk about the biggest
(27:09):
parrotfish just to give us like a nice frame of reference,
because also the largest parrotfish is also pretty gnarly.
Speaker 3 (27:17):
Is this the bumphead we talked about last time?
Speaker 1 (27:20):
It is the bumphead, So we have a little more
on the bump head here. The bump head, according to
the NOAA, reaches size as a four point two feet
long and up to one hundred pounds, So one hundred
and thirty centimeters forty six kilograms, And not only are
they the largest parrotfish, but they're among the largest reef
fish period. Reef environments are generally shallow and tight, so
(27:41):
you know, they're not inviting places for larger fish. And
their namesake bump is used like a rams horns in
male to male competitions, though females also have smaller bumps, which,
of course, if the parrotfish lives long enough, is going
to grow in size once they have changed sect.
Speaker 3 (28:00):
Okay, so these are fish that just keep growing, even
though that's not necessarily the best for them in all ways,
like it might limit what coral surfaces they can access
and so forth.
Speaker 1 (28:12):
Well, these guys are just bigger anyway. This is just
But this would I guess, seem to be like the
maximum size that seems to fit into the evolutionary economy
of living around the reef. Okay, like I guess it
would be. It would be hard to argue that parrotfish
should get larger than this, because we have no living
parrotfish that get larger than this. I see, the market
(28:35):
won't allow it. You know, Now, bump head parrotfish can
live to be forty years old. I've read they don't
reach sexual maturity until five to eight years old, and
sadly their numbers are down except in protected reef environments.
Speaker 3 (28:51):
So I believe based on what I've read, these are
the ones that are classic. When we talked about the
different classifications of parrotfish feed behaviors based on like sort
of how hard they gouge the rock or the coral,
and these would be like the excavators, right, like they
are plowing into that stuff.
Speaker 1 (29:09):
Yeah, these guys take the big bites. I was reading
a little bit more about this on the NOAA website
and they said that, yeah, they take out those big
bites that also end up taking out a little bit
of live coral. But they stress that this is still
very healthy for the coral in all the ways we
already mentioned. I don't remember if we mentioned this. I
don't remember if we mentioned this or not. But there's
also the idea that they'll break down dead reef and
(29:33):
of course turn that into sand, dead bits and branches
that might otherwise break off in storms and damage other
parts of the reef.
Speaker 3 (29:41):
Oh yeah, I see, So it's better for it to
better for this chunk to get ground up in a
parrotfish's pharyngial mill and pooped out as sand rather than
knocked off in the storm and hit some other healthy
part of the reef.
Speaker 1 (29:54):
Yeah, because it's one of the interesting things about about
reef environments, and this is something you're def only instructed
about anytime you go out and snorkel or scuba dive.
Certainly I imagine around these these is that there is
like a hardness to them. Certainly they can also be
very like you certainly don't want to stand on them
(30:14):
or walk on them or touch them for a number
of reasons, because a lot of times they can be
quite harmful and scrape you out, they can cut you.
You don't want any of that. But on top of that,
they can be actually quite delicate, and they can be
easily broken. And so this would be another case of
where if the parrotfish are doing their thing, that limits
the amount of damage that they're going to sustain via
(30:36):
their own dead parts.
Speaker 3 (30:38):
I see.
Speaker 1 (30:39):
But anyway back to sex changes in parrotfish in general. So,
according to Jennifer Hodge, a postdoctoral researcher in the Department
of Evolution and Ecology in the UC Davis College of
Biological Sciences in twenty twenty. The indeterminate growth factor may
in fact be key. I was reading a couple of
(31:00):
from a couple of sources. Here. One is a UC
Davis article by Andy Fell covering her work titled male
size advantage drives evolution and sex change of sex change
in Refish. And then also there is a full paper
I was looking at, and this is by Hodge at
All titled Correlated Evolution of sex allocation and Mating system
(31:21):
in Rasses and Parrotfishes, published in The American Naturalist the
same year.
Speaker 3 (31:26):
Okay, so how would this indeterminate growth factor affect how
sex is distributed and developed in a fish species.
Speaker 1 (31:35):
It basically comes down to the fact the observation that
reproduction among parrotfish and also some of these other fish,
but for our interests here, the parrotfish is often dominated
by large males.
Speaker 3 (31:49):
Meaning that like, a larger male has a better chance
of mating more.
Speaker 1 (31:54):
Right, and that and that male large males and this
would be the terminal mail in parrotfish fishes. They are
the ones dominating like all of the mating. So if
you are not a large male, you are just not
going to be effective at reproduction. If you are a
small parrotfish male, your chances of passing on your genes
(32:18):
is rather slim. And remember the genetic mission is to
pass on your genes. Now, as a small parrotfish female, however,
it's less of an issue. The bigger males, they have
the advantage. They're going to form these harems. If you
are a small female, you can be part of that
harem and you are doing your reproductive part as a parrotfish.
And so that's according to this hypothesis, this is where
(32:41):
the evolution of sequential hermaphrotitism evolves as a strategy by
which all individuals have a better shot at participating in reproduction.
So start off small and female, you definitely get to reproduce.
And then if you live long enough and you grow
big enough, you shift to the male set, and then
(33:01):
you have the size to prove effective. You're better at
controlling territory resources, harems, etc.
Speaker 3 (33:08):
That's interesting, Okay, So it gives more individuals of the
species a chance to mate more often.
Speaker 1 (33:16):
Yes, yeah, that's the way I understand it. And I
was looking again at the writings of in Oa's fishery
biologist Ronald J. Salce and Salce points out that, yeah,
the largest parrotfish are always terminal males, and he points
out that the species, the various species in the genus
Skeras typically exhibit the following reproductive characteristics. So we see this.
(33:41):
First of all, there is this proto Guynus female first hermaphrotitism,
There are breeding territories, there are harems, and there is
external fertilization.
Speaker 3 (33:50):
Oh yeah, the external fertilization is a good point, because
I don't want to give the wrong idea when I
was mentioning mating that it's like, you know, the kind
of active you might be picturing that. Instead, there's a
there's an external meeting of the game meats of these animals.
Speaker 1 (34:06):
Right, And I think, I don't know humans, maybe we
have a problem imagining fish sex in general. But somehow
this makes it a little easier to sort of picture
how all this is happening. I think it's all out
in the open. So SAAL's points out though that in
the past, and really maybe not in the two distant past,
we've had these other hypotheses that there might be a
(34:27):
social trigger for the change in sex. But apparently, based
on what he wrote, this hasn't necessarily been observed, or
at least not in all cases or in a broad
array of cases, because we have scenarios where large terminal
males are removed from a population, such as by fishing,
(34:48):
and the females don't just switch over at an earlier age,
but rather have more difficulty finding a mate.
Speaker 3 (34:55):
Oh okay, so it might be kind of baked in
that they need to reach a certain size.
Speaker 1 (35:00):
Seems to be the case now, I don't, But again,
we're dealing with hypotheses here. I don't think that there
that anything is like one hundred percent proven out here.
There's still a lot of work that needs to be
done because a lot of it comes down to, Okay,
you can have this general idea that this practice evolved
because large males dominate reproduction and and it makes more
sense from a reproductive standpoint to start off as female
(35:23):
and then become male. But then what is the trigger
is it? Is it purely based on how big you grow?
Or are there environmental or social triggers? And Uh, ultimately
the size advantage explanation is just one hypothesis. Uh, there's
an you know, other hypotheses put more emphasis on possible
social or in mental environmental triggers such as changes and
(35:46):
in population density, that sort of thing, in the same
way that we see examples and say the world of salamanders,
where uh, you know something they're too many, or you know,
something goes on demographically in a collect in a certain group,
then you may have biological changes that result. But I
guess broadly if there are, if there are social or
(36:06):
environmental triggers that are involved in theory, we would be
able to observe them, you know, such as response to
overfishing of large males in response to changes in the
environment and so forth. Now there are individual species of
parrotfish where we might see some of those, like social triggers.
Perhaps I've seen discussion of the stop light parrotfish in
(36:29):
particular as perhaps being influenced by population density, growth and
mortality rates. So if terminal the idea here being that
it may be the case that terminal males in stop
light parrotfish populations, if they experience higher mortality rates so
more of them are dying, or if they are just
smaller overall sizes in the terminal males, then this change
(36:55):
may trigger earlier onset of the sex change in the
female parrotfish in that population. So, like I say, it's
still would line up with this idea that this evolved
because male parrotfish, large male parrotfish dominate reproduction. But it
would maybe be a slightly different case of like what
(37:16):
is actually causing it based on my understanding looking at
this documentation. But I like to say there's still I
think a lot of work going on here. Two thanks.
Keep in mind, though there is no evidence that any
species of parrotfish can undergo a sex change, reversal, or
a second sex change. Like it is, it is sequential,
(37:38):
sequential hermaphroditism. So it's female, then male. There are no
known cases where a male can then change back to
female do to you do to any kind of you know,
social pressure, environmental or what have you. It is female
and then male and again sequential hermaphroditism of one form
or another can be found in other fish. As I
mentioned the ras is. Apparently you see some version of
(38:00):
this in some molluscs and crustaceans. The size reproduction hypothesis
is widely e employed across the board, but I've also
seen I think the prevention of inbreeding being brought in
as another possible reason though I'm not sure if that
really pans out, particularly with the parrotfish. That may just
really have more to do with hermaphroditism as an evolutionary
(38:25):
trait in general.
Speaker 3 (38:26):
But the sequential version you're saying, it seems that there's
a similar evolutionary explanation given across these different classes of animals,
which is that it likely has to do with a
relationship between the animal's size and its likelihood of successful reproduction, yes, exactly,
or specifically the size of males and successful reproduction, and
(38:49):
the fact that the animals just keep growing.
Speaker 1 (38:51):
Yeah, So it's fascinating. Yeah, this is like a factoid
about parrotfish that I'd long heard, but I'd never really
looked into it. I guess one of the problems is
when you're in the water, it's it's really hard to
research stuff. You're just like, oh, I'm gonna take your
word for it, and I'm I'm gonna look at it
and then I'll try to remember to read about it later.
Speaker 3 (39:18):
Man, you would never guess that there is so much
interesting stuff about these fish just watching them scrape the rocks.
Speaker 1 (39:25):
Yeah, yeah, I mean just observing them, and I've observed
in plenty of times in the past. You know, it's
like you look at them and you're like, well, they're
a little bit goofy looking, they're beautifully colored, and then
you learn a little bit more about them, but there's still,
you know, greater depths of interest there. I guess that's
the that's the nature of most fish in the sea.
Never take them for granted.
Speaker 3 (39:44):
In between recording these two episode parts, did you end
up googling more human parrot fish bites?
Speaker 1 (39:51):
I did.
Speaker 3 (39:52):
I don't know why I did. I shouldn't have.
Speaker 1 (39:54):
Why would you do that?
Speaker 3 (39:55):
I don't know. I regret it. I wish I hadn't
done it. I just did.
Speaker 1 (40:00):
The only thing of that nature that I did run
across is when I was looking up pictures and looking
at articles about the big boys, the bumphead parrotfish is
there was an image of some coral with some big,
chunky bites taken out of it, and that was pretty impressive,
and it did cross my mind. It's like that I
would not want those bites taken out of my own flesh.
Speaker 3 (40:21):
Yeah, I would not want that to be my bones.
Speaker 1 (40:23):
Yeah.
Speaker 3 (40:24):
But to emphasize yet again, as we did last time, parrotfisher.
There's no indication that they're very aggressive or looking to
bite humans that like, these stories come from people who
were getting up in the parrotfish's business.
Speaker 1 (40:38):
Right, Yeah. I think I saw one account and this
is like, you know, this is I guess inherently unverified
of snorkelers or divers where someone was just sort of
casually bitten by a parrotfish. But in that thread, like
everyone was like, wow, that's weird. It's never happened to me.
So I don't know, you know, in the wild one
(41:00):
officer certainly possible. Who knows what that that parrotfish was
going through that day?
Speaker 3 (41:05):
Yeah. I guess any species of any fish could in
some case be aggressive, but it's not like generally thought like,
oh wow, you gotta be careful, like they're they're coming for.
Speaker 1 (41:15):
You, right, Yeah, I don't think they're they're coming for you.
Speaker 3 (41:17):
Yeah, I'm just because I'm imagining so like, you know,
the James Bond villain has a pool of piranhas that
he drops his henchman into when they make a mistake,
And I'm just thinking like, could they have gone with
a pool of parrotfish? How would that work out differently?
Speaker 1 (41:31):
We have cotajo bodhi, mister bond with a fine layer
of algae in coral dust, you will now drop you
into that of parrotfish.
Speaker 3 (41:43):
M No, not the bump edge. Okay, does that do
it for parrotfish?
Speaker 1 (41:49):
I think it does. You know, they may have more
mysteries that we didn't explore, but I think we hit
all the really interesting stuff here. But hey, if you
know of other dimensions to the parrot fish or various
parrotfish species that we didn't talk about, write in because
we would love to hear from you. Just a reminder
that's Stuff to Blow your Mind Here is primarily a
science and culture podcast, with core episodes on Tuesdays and Thursdays,
(42:12):
listener mail on Mondays, short form episode on Wednesdays, and
on Fridays. We set aside most serious concerns to just
talk about a weird film on Weird House Cinema.
Speaker 3 (42:20):
Huge thanks as always to our excellent audio producer JJ Posway.
If you would like to get in touch with us
with feedback on this episode or any other, to suggest
a topic for the future, or just to say hello,
you can email us at contact at stuff to Blow
your Mind dot com.
Speaker 2 (42:43):
Stuff to Blow your Mind is production of iHeartRadio. For
more podcasts from my heart Radio, visit the iHeartRadio app,
Apple podcasts, or wherever you listen to your favorite shows.
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