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May 23, 2024 42 mins

In this 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. (part 2 of 2)

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Speaker 1 (00:03):
Welcome to Stuff to Blow Your Mind production of iHeartRadio.

Speaker 2 (00:12):
Hey, welcome to Stuff to Blow Your Mind. My name
is Robert Lamb.

Speaker 3 (00:16):
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

(00:36):
some basics about the biology and taxonomy of parrotfish species.
We talked about their diet and feeding behavior. Parrotfish are
usually considered herbivores or something equivalent to herbivores. They survived
by grazing for algae, microorganisms, to trite us, and sometimes
invertebrate animals like coral polyps, along the surfaces of rocks

(00:58):
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 phyryngial mills.

(01:20):
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

(01:41):
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

(02:05):
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

(02:26):
and coral for food. Also true, other beliefs not quite
so accurate. For example, Rob, didn't you talk about this
idea that they cooperate altruistically to save one another from traps?

Speaker 2 (02:37):
Yes, and in this became an enduring and long lasting
symbol of friendship, which doesn't really hold up to how
they actually behave.

Speaker 3 (02:46):
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.

(03:08):
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 2 (03:29):
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 small businesses that are

(03:53):
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 to see various parrotfish

(04:13):
in action as well as other fish, I highly recommend
checking out Coral City Camera. 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.
It's pretty cool and if you watch long enough, you
will see some parrotfish.

Speaker 3 (04:33):
In action pursuing their new career as streamers.

Speaker 2 (04:36):
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 (04:51):
Oh?

Speaker 2 (04:51):
Do? I?

Speaker 3 (04:52):
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 parrotfish was one that I initially
got interested in just by looking at pictures, you know.
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

(05:14):
another parrotfish subtopic that I think one could come to
in exactly the same manner, and that is their mucous cocoons.
If you look for photos of parrotfish sleeping, you will
find images of these animals nestled down into cozy little
niches in the seafloor or within the reef structure, sometimes

(05:35):
kind of in a recess or a little heidi hole
in the reef, surrounded by what looks like some kind
of film. Sometimes it looks like the parrotfish is enclosed
within a gauzy, transparent orb covered like sometimes covered in glitter.
If you see these in motion and rob below the

(05:55):
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 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 fish is wrapped up inside
a huge funnel spider web that's just covered in sand.

(06:19):
Which funny thing about that the sand may of course
be the parrotfish's own excretion from earlier.

Speaker 2 (06:26):
Yeah, I'm looking at the video footage right now. It's
slimy but beautiful.

Speaker 3 (06:32):
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 mucous envelope, described by
an author named H. E. Wynn in a scientific article
in nineteen fifty five as a quote thin, transparent and

(06:55):
gelatinous mucoid substance which starts as a full 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 generating

(07:20):
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, as a side note,

(07:44):
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 provide a

(08:05):
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 kinds
of fish help with OSMO regulation. But these mucous coverings

(08:30):
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 such
as cuts and scrapes. It in some cases provides UV

(08:52):
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 2 (09:02):
So this is of course one of the potential issues.
And just handling fish such as then like catch and
release and so forth. The slime isn't just something that's
on the fish. It's an active barrier.

Speaker 3 (09:12):
Yeah, But that's all just the normal mucus coating common
to many many fish. What we're talking about here is
specifically this baggy mucus 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

(09:33):
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

(09:56):
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

(10:21):
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 those lines.

Speaker 2 (10:43):
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 (10:53):
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 and how and when
the 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 scas dubious, commonly known as the
regal parrotfish, and scas per pair. Oh wow, here's a word,

(11:17):
pers bisilattis 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 2 (11:35):
Yeah yeah, So Burn.

Speaker 3 (11:37):
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 founding around

(12:00):
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

(12:22):
keep shining the light on them, at least for twenty
four hours't 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 2 (12:36):
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 a highly advanced species shines
artificial light on me for twenty four hours.

Speaker 3 (12:47):
That's right. So the constant light means he 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 mucus 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

(13:08):
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 parrotfish 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

(13:29):
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

(13:52):
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

(14:17):
out of the buckle cavity, so sort of a breathing hole. Now,
I mentioned that in a lot of these photos, the
mucus cocoon 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

(14:39):
bubble appear thin and delicate. This is how A 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
wispy spider web or this very very thin kind of

(15:04):
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 2 (15:16):
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 mucous shielding. They would give
it the appearance of being super thin when in fact
it is probably thicker.

Speaker 3 (15:32):
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

(15:56):
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,

(16:17):
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

(16:39):
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 such

(17:04):
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 ticky. Yeah, And the authors point

(17:27):
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 larger fish.

(17:50):
But how do the fish protect themselves at night? The
idea behind this experiment was that maybe the mucous 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 sortied us. And the way

(18:11):
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,

(18:35):
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,
So huge difference.

Speaker 2 (18:53):
All right, Right, So Yeah, coming back to the mosquito
net comparison, it's 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 mosquitos and similar
insects that sort.

Speaker 3 (19:07):
Of thing, or maybe even something downstream from mosquitoes in
the analogy here, because the author is also investigated the
question of how energetically costly it is for the fish
to make these mucus orbs, and they calculated that it
takes about two point five percent of a fish's daily
energy budget to make the mucus bag. Now, when I

(19:31):
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 Grutter, 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 the size of a

(19:53):
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,

(20:13):
Grutterer mentions the possibility that the blood 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

(20:36):
the mosquitoes, but the malaria is much more concerning than
the mosquito. Malaria can be deadly. So 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

(20:59):
night without these tiny shrimp 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

(21:22):
what appeared to 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 mucous 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 bed.

Speaker 2 (21:43):
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 recoop that cost, and that might
mean consuming all that ectoplasm 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

(22:04):
various reptiles that will eat their own shed skin because
you know, why waste that, you know.

Speaker 3 (22:10):
YEA, 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
mucus bags help prevent against parasite attacks.

Speaker 2 (22:24):
Very fascinating, all right, and so for the rest of
the episode, we're going to turn to the world of
parrotfish reproduction and parrotfish sex. So, as we teach out

(22:45):
in the first episode, one of the other 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
environments conditions are right. It occurs as part of a
normal life cycle.

Speaker 3 (23:05):
And within a fairly predictable pattern, right.

Speaker 2 (23:07):
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

(23:29):
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 (23:43):
And this would feed back 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 2 (23:57):
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
some resources about the parrotfish. I also want to point

(24:20):
out though, 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 sequential hermaphroditism,

(24:47):
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

(25:10):
and in time, if that female lives long enough and
grows large enough, she transitions into a larger, terminal, reproductive male.

Speaker 3 (25:18):
Interesting.

Speaker 2 (25:20):
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.

(25:40):
I've seen it cited as less than twenty. I've also
seen it sited 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 parasites,

(26:02):
to eels trying to eat them, to human fishermen and
so forth.

Speaker 3 (26:06):
It's just hard to imagine like ten straight years of
biting and scraping on rocks with your teeth.

Speaker 2 (26:11):
Yeah, literally scraping by right now. Parrotfish display what is
referred to as indeterminate growth, which means that there's not
a full size 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 to

(26:31):
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 parrotfish just

(26:53):
to give us like a nice frame of reference, because
also the largest parrotfish is also pretty gnarly.

Speaker 3 (27:00):
Is this the bumphead we talked about last time?

Speaker 2 (27:02):
It is the bumphead, So we have a little more
on the bump edd 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 you know,

(27:24):
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 sexes.

Speaker 3 (27:43):
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 2 (27:55):
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.

Speaker 3 (28:16):
I see.

Speaker 2 (28:17):
The market won't allow it, you know. Now, bumphead 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 (28:33):
So I believe, based on what I've read, these are
the ones that are classic. We talked about the different
classifications of parrotfish feeding 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 2 (28:52):
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 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

(29:15):
and 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 (29:23):
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 2 (29:36):
Yeah, because it's one of the interesting things at about
reef environments, and this is something you're definitely instructed about
anytime you go out and snorkel or scuba dive. Certainly
I imagine around 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 or walk

(29:57):
on them or touch them for a number reasons, because
a lot of times they can be quite harmful. You
can 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 their own

(30:20):
dead parts. I see. 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 from a couple of sources. Here.

(30:43):
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 in Rasses and Parrotfishes, published in

(31:06):
the American Naturalist the same year.

Speaker 3 (31:09):
Okay, so how would this indeterminate growth factor affect how
sex is distributed and developed in a fish species.

Speaker 2 (31:17):
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 (31:32):
Meaning that like, a larger male has a better chance
of mating more.

Speaker 2 (31:37):
Right, and that and that that male large males and
this would be the terminal males 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

(32:00):
genes 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. You know, 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,

(32:23):
this is where 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 sex, and then you have the size to

(32:45):
prove effective. You're better at controlling territory resources, harems, etc.

Speaker 3 (32:51):
That's interesting, Okay, So it gives more individuals of the
species a chance to mate more often.

Speaker 2 (32:59):
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
Scaas typically exhibit the following reproductive characteristics. So we see this,

(33:23):
first of all, there is this proto Guynus female. First hermaphrotitism.
There's there are breeding territories, there are harems, and there
is external fertilization.

Speaker 3 (33:33):
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 activity you might be picturing that Instead, there's a
there's an external meeting of the game meats of these animals.

Speaker 2 (33:49):
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 in
the open. So Suz points out though that in the
past and and and really maybe not in the two
distant past, we've had these other hypotheses that there might

(34:09):
be a 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,

(34:31):
and the females don't just switch over at an earlier age,
but rather have more difficulty finding a mate.

Speaker 3 (34:38):
Oh, okay, so it might be kind of baked in
that they need to reach a certain size.

Speaker 2 (34:42):
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 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 don't dominate reproduction and and it makes more
sense from a reproductive standpoint start off as female and

(35:05):
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 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 in population density, that

(35:30):
sort of thing. In the same way that we see
examples and say the world of salamanders, where you know
there are 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 very if there are social or environmental
triggers that are involved in theory, we would be able

(35:54):
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
particular as perhaps being influenced by population density, growth and

(36:17):
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're just smaller
overall sizes in the terminal males, then this change may

(36:38):
trigger earlier onset of the sex change in the female
parrotfish in that population. So, like I say, it 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 is

(36:59):
actually caused 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 sequential hermaphroditism. So it's female,

(37:23):
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 this in some molluscs and crustaceans. Uh.

(37:46):
The size reproduction hypothesis is widely employed 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 parent fish.

Speaker 3 (38:00):
Hmm.

Speaker 2 (38:01):
That may just really have more to do with hermaphroditism
as a as an evolutionary trait in general.

Speaker 3 (38:09):
But the sequential version you're saying, it seems that there's
a 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 animals size and its
likelihood of successful reproduction, yes, exactly, or specifically the size
of males and successful reproduction. Yeah, and the fact that

(38:32):
the animals just keep growing.

Speaker 2 (38:34):
Yeah. Yeah, So it's fascinating. I 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 going
to take your word for it, and I'm gonna I'm
gonna look at it, and then I'll try to remember
to read about it later.

Speaker 3 (39:01):
Man, you would never guess that there is so much
interesting stuff about these fish just watching them scrape the rocks.

Speaker 2 (39:07):
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 (39:27):
In between recording these two episode parts, did you end
up googling more human parrotfish bites? I did. I don't
know why I did. I shouldn't have.

Speaker 2 (39:36):
Why would you do that?

Speaker 3 (39:38):
I don't know. I regret it. I wish I hadn't
done it. I just did.

Speaker 2 (39:42):
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. Yeah,
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 (40:03):
Yeah, I would not want that to be my bones. Yeah.
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 2 (40:20):
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

(40:43):
officer certainly possible. Who knows what that parrotfish was going
through that day?

Speaker 3 (40:48):
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 2 (40:57):
You, right, Yeah, I don't think they're they're coming.

Speaker 3 (41:00):
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 2 (41:14):
We have Cotajo Bodhi missed a bond with a fine
layer of algae coral dust. You will now drop you
into that of parrotfish.

Speaker 3 (41:26):
M No, not the bump che Okay does that do
it for parrotfish?

Speaker 2 (41:32):
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 parrotfish or various parrotfish
species that we didn't talk about right in, because we
would love to hear from you. Just a reminder that
Stuff to Blow your Mind here is primarily a science
and culture podcast, with core episodes on Tuesdays and Thursdays,

(41:55):
listener mail on Mondays, short form episode on Wednesdays, and
on Fridays. We set aside mostly concerns to just talk
about a weird film on Weird House Cinema.

Speaker 3 (42:03):
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 1 (42:25):
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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