From the Vault: The Parrotfish, Part 2
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Hey, welcome to Stuff to Blow Your Mind. It is another Vault Day here for you. So we have the Parrotfish Part 2. It originally published 5-23-2024. I hope you enjoy. Welcome to Stuff to Blow Your Mind, a production of iHeartRadio.

Hey, welcome to Stuff to Blow Your Mind. My name is Robert Lamb. And I am Joe McCormick, and we're back with part two in our series on parrotfish, a group of related fishes containing about 90 to 100 species worldwide, notable for their powerful beak-like mouths made out of rows of fused teeth.

In the previous episode, we talked about 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 survive by grazing for algae, microorganisms, detritus, and sometimes invertebrate animals like coral polyps along the surfaces of 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 pharyngeal mills. This 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 coral reefs nearby, there is a good chance that most of the sand under your feet was at one point parrotfish poo.

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 food items and why they thought they were in sort of 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 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?

Yes, and this became an enduring and long-lasting symbol of friendship.

Which doesn't really hold up to how they actually behave. 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.

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. 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, there are various snorkeling places.

companies and small businesses 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

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 in action, as well as other fish, I highly recommend checking out Coral City Camera. That's just coralcitycamera.com, or you can look it up and search CoralCity.

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 in action. Pursuing their new career as streamers. 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? 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. 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 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 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 hidey hole in the reef, surrounded by what looks like some kind of film that

Sometimes it looks like the parrotfish is enclosed within a gauzy, transparent orb, sometimes covered in glitter. If you see these in motion, 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 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, which funny thing about that, the sand may, of course, be the parrotfish's own excretion from earlier. Yeah, I'm looking at the video footage right now. It's it's slimy, but beautiful.

So what is this gossamer bubble around a slumbering fish? This is what's known as the parrotfish's mucus cocoon, or sometimes in the scientific literature, its mucus envelope.

Described by an author named H.E. Wynn in a scientific article in 1955 as a, quote, thin, transparent and 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 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 animals.

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, I just wanted to say it is normal for all kinds of fish species, not just parrotfish or the other related fish like some wrasses.

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 number of benefits, one of which is osmoregulation, and that's maintaining the balance of

internal water and solutes such as electrolytes. So for example, an osmoregulation function within our bodies, human bodies, is maintaining the right level of salt in our body fluids. Mucus coverings on all kinds of fish help with osmoregulation.

But these mucus 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, 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.

So this is, of course, one of the potential issues in just handling fish, such as in catch and release and so forth. The slime isn't just something that's on the fish. It's an active barrier.

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 hypersleep pod that forms around some parrotfish and wrasses 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 marine zoology paper from the year 1970 that investigated a few species of parrotfish and

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 the paper begins by citing previous research by Winn and co-authors on parrotfish from the coral reefs of Bermuda.

which were observed to make mucus envelopes at night. So Wynn and a co-author named Bardak argued that the purpose of the mucus cocoon, of the slime sack, was to protect the parrotfish from predators while it was sleeping.

And this is a hypothesis that I've seen repeated in a number of sources that maybe somehow the 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's scent from predators or provide some kind of benefit along those lines.

Okay, so either to some degree a cloaking system, a cloaking device, if you will, or perhaps some sort of like added security tripwire made out of mucus.

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 going to look at how and when these things form. So the author of the study, John Byrne, begins by investigating envelope formation in a couple of different species of parrotfish. There's Scarus dubius, commonly known as the regal parrotfish, and Scarus perspicensis.

Bicillatus, 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 key fish referred to as Uhu in some of the Hawaiian legends that we talked about in the last episode. Yeah, yeah.

So Byrne 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 around the reefs. For the nighttime, 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.

Uh, within the lab environment, Byrne found that if you shine a constant light on these fish for 24 hours straight, they will actually never make a mucus cocoon. You just keep shining the light on them, at least for 24 hours. He didn'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 24 hours straight, you shine a light on them and it, you know, nothing happens. Yeah, of course. I mean, there are a lot of things I'm not going to do if, uh,

some sort of an intelligent being from a highly advanced species shines artificial light on me for 24 hours. That's right. So the constant light means you never make a pod. However, when darkness was introduced, you turn the lights off. This trigger triggered 22 of the 30 parrot fish tested to build mucus envelopes. And it was the same frequency in the two different species. Yeah.

The fish took different amounts of time to finish building their envelopes after the light was turned off. The minimum was like 30 minutes. Maximum was 240 minutes. Average building time was about 70 minutes. However, if you kept 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 when they made these things? They would typically rest their bodies on the floor of the tank, 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 formation at the front of 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 Byrne believes this hole is to evacuate respiratory water that's forced out of the buccal 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. Byrne 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 bubble appear thin and delicate. This is how Wynn described it in that article from the 50s. But Byrne did an experiment by injecting pigmented particles into the cocoon and

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 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. Okay, this would just be the case of there being like a thin layer of particles in

on top of this otherwise translucent or semi-translucent mucus shielding that would give it the appearance of being super thin when in fact it is probably thicker.

That's right. So, dissection of the fish revealed that the presence of gland tissue in the buccal 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. Hmm.

So that's how they're made and when. But 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 mentioned predator in the reef environment would be the moray 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, when faced with reef dwelling predators such as moray eels, there is some evidence that sleeping fish within within a cocoon are still vulnerable, like they still get eaten. Hmm.

But actually, I came across an interesting 2011 paper that looked directly into the function of the mucus cocoon and came to a different conclusion. So this was published in the journal Biology Letters by Grutter et al. 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 2011.

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 as ectoparasitic nathid isopods. These are these little blood sucking parasites that live throughout the ocean, often compared to terrestrial mosquitoes and ticks, which

Rob, I attached 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 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, and, and help pick them off to sort of groom the outside of the, the 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 hematophagous parasites.

So they tested this hypothesis on the coral reef parrotfish Chlorurus sordidus. And the 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 cocooned fish and found a way to sort of gently push them out of their envelopes without waking them up. Yeah.

So what do they find? Yes, indeed. The fish without the mucus bag experienced way more attacks by parasites. How much more? Well, about 95% of the fish without cocoons were attacked by isopods and only about 10% of the fish with cocoons were attacked. So huge difference. All right. All 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 mosquitoes and similar insects, that sort of thing. 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 mucus orbs.

And they calculated that it takes about 2.5% of a fish's daily energy budget to make the mucus bag. Now, when I first saw that figure, I kind of thought, oh, hey, that seems fairly cheap, only 2.5%.

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,

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, Grutter mentions the possibility that the blood directly lost to the parasite might not be the only cost. These isopods may also transmit a secondary endoparasite, which lives in the fish's blood, much like how mosquitoes do.

transmit malaria in humans. So the mosquitoes themselves are annoying and you don't like 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 bio nets 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 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 recoup some of that nightly cost. So Grutter says, quote, I have observed on occasion a fish at dawn with 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 mucus cocoons, etc.

pecking it at its old cocoons in the morning. So like recouping some of that nightly cost by eating the mucus that you created before bed. 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 recoup that cost. And that might mean consuming all that ectoplasm again and getting all that mucus

liquid back into your body. You know, it reminds us of other examples we've looked at in the

in biology such as various reptiles that will eat their own shed skin because you know why waste that you know yeah so may still provide some kind of benefit against larger macroscopic predators too uh not not certain about that but it does seem like there's a very good case that these mucus bags help prevent against parasite attacks very fascinating colgate total may make your favorite toothpaste but

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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 teased out in the first episode, one of the other amazing, 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.

And within a fairly predictable pattern, right? That's right. Yeah. And they're going to be a couple of exceptions. Again, as we've been stressing, 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 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. 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. 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 was pointed out by the National Marine Sanctuary Foundation as resources about the parrotfish.

I also want to point out, though, that according to NOAA fishery biologist Ronald J. Sahls, gonachorism 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 sequential 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. And in time, if that female lives long enough and grows large enough, she transitions into a larger terminal reproductive male.

Interesting. Now, in general, parrotfish experience what I've seen referred to as moderate longevity.

And it's going to vary depending on the particular specimen. And I think even the general generalities about how long they live is going to vary. I've seen in general parrotfish life span cited seven to 10 years. I've seen it cited as less than 20. I've also seen it cited 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 these are creatures living in the ocean. And so there are a whole number of factors from blood diseases to parasites to eels trying to eat them to human fishermen and so forth. It's just hard to imagine like 10 straight years of biting and scraping on rocks with your teeth. 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-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 consider in making sense of their sex changes because one of, if not the primary hypotheses for why they do this, why they evolved to do this, does relate to their size. Hmm.

Hmm. 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 to give us like a nice frame of reference, because also the largest parrotfish is also pretty gnarly.

Is this the bump head we talked about last time? It is the bump head. So we have a little more on the bump head here. The bump head, according to the NOAA, reaches sizes of 4.2 feet long and up to 100 pounds, so 130 centimeters and 46 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 they're not inviting places for larger fish. And their namesake bump is used like a ram's 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.

OK, 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. 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.

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 won't allow, you know. Now, bumphead parrotfish can live to be 40 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. So I believe, based on what I've read, these are the ones that are classified. We talked about the different classifications of parrotfish.

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. 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, um,

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 or not, but there's also the idea that they'll break down dead reef 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.

Oh, yeah, I see. So it's better for this chunk to get ground up in a parrotfish's pharyngeal mill and pooped out as sand rather than knocked off in a storm and hit some other healthy part of the reef. Yeah, because it's one of the interesting things 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, uh, they can also be very like, you certainly don't want to stand on them or walk on them or touch them for, for a number of reasons, because, uh, a lot of times they can be quite harmful. They can scrape you up. 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, um,

damage they're going to sustain via their own 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 2020, the indeterminate growth factor may in fact be key. I was reading a couple of

from a couple of sources here. One is a UC Davis article by Andy Fell covering her work titled Male Size Advantage Drives Evolution of Sex Change in Reef Fish. And then also there is a full paper I was looking at, and this is by Hodge et al., titled Correlated Evolution of Sex Allocation and Mating System in Rasses and Parrot Fishes, published in the American Naturalist Society.

the same year. Okay, so how would this indeterminate growth factor affect how sex is distributed and developed in a fish species? It basically comes down to the fact, the observation, that reproduction among parrotfish, and also some of these other fish, but for our interest here, the parrotfish, is often dominated by large males. Meaning that, like, a larger male has a better chance of mating more?

Right. And that 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 genes is rather slim. And remember, the genetic mission is to pass on your genes. Now, as a small parrotfish female, however, it's

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, this is where the evolution of sequential hermaphroditism 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 prove effective. You're better at controlling territory, resources, harems, etc. That's interesting. OK, so it gives more individuals of the species a chance to mate more often. Yeah.

Yes. Yeah, that's the way I understand it. And I was looking again at the writings of NOAA'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 Scarus typically exhibit parrots.

the following reproductive characteristics so we see this first of all there is this proto-gynous female first hermaphroditism there's there are breeding territories there are harems and there is external fertilization oh yeah the external fertilization is a good point because i i don't want to give the wrong idea when i was mentioning mating that uh that it's like uh you know the kind of activity you might be picturing that instead there's a there's

an external meeting of the gametes of these animals. 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, uh, Saul's points out though, that in the past and, and, and really maybe not in the too distant past, we, we've had these other hypotheses that, um, there might be a social trigger for the change in sex. Um, but apparently based on what he wrote, this hasn't necessarily been observed or at least not, um, in all cases or in a, you know, a broad array of cases. Um,

Because we have scenarios where large terminal males are removed from a population such as by fishing and the females don't just switch over at an earlier age, but rather have more difficulty finding a mate. OK, so it might be kind of baked in that they need to reach a certain size.

Seems to be the case. But again, we're dealing with hypotheses here. I don't think that anything is like 100% 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 it makes more sense from a reproductive standpoint to start off as female 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 ultimately, the size advantage explanation is just one hypothesis. There's other hypotheses put more emphasis on possible social or environmental triggers, such as changes in population density, that sort of thing. In the same way that we see examples in, say, the world of salamanders, where there are too many or something goes on demographically in a certain group, then you may have biological changes that result.

But I guess broadly, if there are social or environmental triggers that are involved, in theory, we would be able to observe them, 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 changes.

some of those like social triggers, perhaps. I've seen discussion of the stoplight parrotfish in 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 stoplight parrotfish populations, if they experience higher mortality rates, so more of them are dying, or if they're just smaller overall sizes, then

in the terminal males, then this change may trigger earlier onset of the sex change in the female parrotfish in that population. Hmm.

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 actually causing it. Based on my understanding of looking at this documentation. But like I say, there's still, I think, a lot of work going on here.

Two things to 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 than male sequentially.

There are no known cases where a male can then change back to female due to any kind of 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 fishes. I mentioned the wrasses. Apparently, you see some version of this in some mollusks and crustaceans.

The size reproduction hypothesis is widely 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 trait in general. 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 animal's size and its likelihood of successful reproduction. Yes, exactly. Or specifically the size of males and successful reproduction. Yeah. And the fact that the animals just keep growing. Yeah. Yeah. So it's fascinating. 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 really hard to research stuff. You're just like, oh, I'm going to take your word for it and I'm going to look at it and I'll try to remember to read about it later. Colgate Total may make your favorite toothpaste, but it's also a science innovator committed to oral health. For instance, the Colgate Total Active Prevention System with

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Man, you would never guess that there is so much interesting stuff about these fish just just watching them scrape the rocks. Yeah, yeah. I mean, just observing them and I've observed them plenty of times in the past. You know, it's like you look at them and you're like, well, they're they're a little bit goofy looking. They're beautifully colored. Yeah.

And then you learn a little bit more about them, but there's still, you know, greater depths of interest there. And I guess that's the, that's the nature of, of most fish in the sea. Never take them for granted. In between recording these two episode parts, did you end up Googling more, uh, human parrotfish bites? I did. I don't know why I did. I shouldn't have. Why would you do that? I don't know. I regret it. I wish I hadn't done it. I just did. Yeah.

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. Yeah. I would not want that to be my bones. Yeah. But to emphasize yet again, as we did last time, parrotfish are...

There's no indication that they're aggressive or looking to bite humans that like these stories come from people who were getting up in the parrotfishes business. Right. Yeah. I think I saw one account and this is like, you know, this is, I guess, inherently unverified.

of, um, snorkelers or divers where someone was just sort of casually bitten by a parrot fish. But, uh, in that thread, like everyone was like, wow, that's weird. That's never happened to me. So I don't know, you know, in,

But in the wild, one-offs are certainly possible. Who knows what that parrotfish was going through that day. 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 got to be careful. Like they're coming for you. Right. Yeah. I don't think they're coming for you. Yeah.

Because I'm imagining, so like, you know, the James Bond villain has a pool of piranhas that he drops his henchmen 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? We have coated your body, Mr. Bond, with a fine layer of algae and coral dust. We'll now drop you into a vat of parrotfish. No, not the bump edge.

Okay, does that do it for parrotfish? 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...

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. Listener mail on Mondays, a 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.

Huge thanks, as always, to our excellent audio producer, J.J. 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 stufftoblowyourmind.com. Stuff to Blow Your Mind is a production of iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio app. Apple Podcasts are wherever you listen to your favorite shows.

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