Stuff To Blow Your MindIn this episode of Stuff to Blow Your Mind, Robert and Joe discuss biological asymmetry in the natural world – including various examples of organisms in which side A doesn’t quite match up with side B. (originally published 06/30/2022)
Hey you welcome to Stuff to Blow your Mind. My name is Robert.
Lamb and I'm Joe McCormick, and it's Saturday. We are bringing you an episode from the vault. This one originally published on June thirtieth, twenty twenty two, and it's part two of our series The Lesser of Two Crab Claws, about asymmetry in the natural world.
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'm Joe McCormick, and we're back with part two of our series on asymmetry in life. Now. In the last episode, we talked about the concept of bilateral symmetry, where basically all of the higher animals have body plans where the left and the right sides are more or less a copy of one another. In other words, along one of the three dimensions of space, our bodies are approximately mirrored, at least on the outside. Now, in most organisms, there are minor variations on this type of symmetry, but occasionally there are species with isolated but radical deviations, where like one feature on the outside of an otherwise mirror flipped half of the body is drastically different from what you find on the other side. Examples that came up last time were the tusk of the nar wall, where in most cases it's actually the left maxillary canine tooth so weird it's the left fang basically of this whale stabbing through the upper lip and it becomes a single tusk. We also talked about the blowholes and skulls of toothed whales, such as the sperm whale, where in many cases these have developed left right mismatches that seem to have evolved to support the capacity for echolocation. We also talked about the cock eyed squid, which has two extremely different eyes for looking into extremely different worlds, one for the water above, which is filtering sunlight, and one for the water below, which may contain flashes of bioluminescence. And so today we wanted to pick up the series by talking about some more fascinating examples of lopsided animal evolution. Animals with halves that mostly match but in one capacity or another do not, and why that would be. Now, there are many great examples of asymmetrical evolution incrustations, and we may actually save some of these for the next part in the series. I know we're going to go to at least three parts here, but for today's episode, I wanted to start by getting out the lemon and the drawn butter, because this is an asymmetry that you don't have to be a specialist a marine biologist to notice for yourself. If you've ever eaten, or even just seen a cooked lobster, you probably have noticed a weird mismatch between the lobster's two claws. Rob, I assume you've you've seen this for yourself.
Yes, yes, they're not not as recently as you have. Because I believe this this was the inspiration for this episode. Right, you recently ate a lobster?
Oh, I don't. I don't think I even told you that, But yeah, this probably had something to do with it. I can't confirm the inner workings of my subconscious mind. But not too long ago, I was in the I was in New England where where lobster is king. I'm not gonna do the accent, but lobster is king. And I did. I did, in fact eat a lobster, and yeah, and I noticed stark differences between the claws, even not just looking at them, but in my fingers, you know, one claw was sort of a pleasure to crack open and get the meat out of, and the other one. When I handled the inside of the pincers, they were much sharper, and the spines within them were much smaller and kind of were irritating and unpleasant to the fingers.
Fascinating.
So what's going on with this claw mismatch? Oh and by the way, we should be clear that we're talking specifically about the American lobster or Homarus americanas this is the lobster you find along the northern edge of the eastern coast of North America, so all up through like the north half of the eastern United States and up into Canada.
This is like the red lobster lobster, the lobster from your grocery store that has rubber bands on its claws, not like the Caribbean lobster.
Yeah, not the rock lobster, though I hear those can be good eating too. I've never had man. But anyway, so the American lobster. So you look at these two claws, and what you'll notice is that usually one claw is longer and flatter, with a longer I don't know what the technical term for this is the danger zone, the space between the two pincers and the insides of the pincers on this flatter, longer claw seem to be sharper, more like a kind of spiky pair of scissors. And then the other claw is shorter in length but bulkier, thick with muscle, and the inside edges of its pincers have a sort of rounder, larger grain texture, almost pebbled, rather than with tiny spines. These claws are commonly referred to as the cutter and the crusher, respectively. I think the cutter is sometimes called the pincher also, But yeah, they are what they sound like, the cutter and the crusher. So what's going on? Why the two different claws on the same lobster? How does a lobster end up with two very different claws and what are they for? Well, to answer this question, I was reading what I thought was a really interesting older article in American Scientist magazine. So this is from nineteen eighty nine by an author named C. K. Govind who was a professor of zoology at the University of Toronto, and it's called asymmetry in Lobster claws. Seems like a lot of Govin's research focused on crustaceans, and so Govin begins by pointing out a number of different examples of asymmetry and animals. He talks about lateral dominance or handedness in humans and even mentions I thought this was interesting. In some songbirds, such as canaries, you have bilateral asymmetry in their singing apparatus. Song production seems to be centered on structures in the left half of the syrinx, And so when you see asymmetries like this, you can ask all kinds of questions about them. But one thing is that you might just assume them to be permanent, fixed features of anatomy, hard coded by genes and express through early development. But it's interesting that there are some cases where asymmetry in an animal's body seems to be reversible. Just for one example, In some cases of lateral dominance, damage to the dominant side of the brain or body can cause the non dominant side to assume some functionality previous localized to the side that has now been incapacitated, and this can lead us to wonder how do these asymmetries develop in the first place. So Govind argues that by examining the lobster, and this is the American lobster Homarus americanas we can see an example of a symmetry emerging not purely as a result of genetic coating, but actually as a result of how the lobster interacts with its environment during a crucial early period. And this is what brings us back to the crusher claw and the cutter claw. So I want to read from Govin's introduction here quote. As any self respecting gourmet knows, the paired claws of the American lobster have decidedly different morphologies. One claw, called the crusher or major claw, is short, stout, and heavy, with molar like teeth on its biting surface. I think that's a good comparison, molar like teeth. It's the pebbles are like your back teeth. It's hard to imagine them snipping something off. Instead, it's like they would sort of grab hold of it and be able to smash it real good.
Yeah. When I'm looking at a picture of this, I can't help but imagine the lobster putting on a puppet show with just its pincher and its crusher and each of them have you know, different characters like like hey on the crutcher, Hey on the pincher, and they interact.
You know, definitely the cutter claw has the higher voice.
Yeah.
Yeah, Let's say if they're street fighter characters, crusher claw is zangief and cutter claw is is what maybe maybe embison?
Yeah yeah maybe so longer does.
That cheap spinning move? Yeah? Okay, So anyway, so that's what that is. That that's the molar teeth on the biting surface. But then, to continue the quote, the other called the cutter or minor claw is long and slender with incisor like teeth or your incisors are your front teeth, the ones that you use to bite off things, you know, not to mash them up, but to separate them from what they're originally stuck to and pull them into your mouth. They're for cutting. So what Govind writes is quote what the gourmet might may not know, and what lobstermen know painfully well, is that the cutter claw can give a quick, nasty pinch. Indeed, it's dactyl, meaning the part of the claw that moves, the closing part can close against the opposing polyx within twenty milliseconds, which is several times faster than any human reflex. In contrast, the crusher claw closes very slowly, but with enough force to crack open the shells of oysters, muscles, and other bivalves. And it's true, molluscs such as muscles are a big source of food prey for the American lobster. So it crawls along the ocean floor in its adult phase, and what does it eat. Well, it might eat some worms of various types and stuff, but it's really going to be looking for mollusks such as muscles. It wants to crack those shells open and get that meat inside. Also, while we're mentioning anatomy, this is unrelated, but I just have to say American lobsters pe out of their faces. You kind of can't bring lobsters up without mentioning that they face pee, and they face peece specifically at each other, whether it's a mate or rival. So lobster sees another lobster, they're probably going to be peeing out of their faces at them. Though, is best I can tell. The face peeing is symmetrical.
Okay, well that's good to know, But coming.
Back to the claws. So the difference in the speed of pinching between the two claws is evidence of an underlying difference, and not just the shape of the claw, but its muscular composition. These claws have different types of muscle in them. About ninety percent of the space of a lobster's claw is taken up by the closer muscle. This is the muscle responsible for bringing the pincers together. Only a relatively tiny muscle is devoted to opening the claw, which is why a lobster might be able to pinch with massive force, but a simple rubber band can render its claw harmless by holding it closed. It has way more strength for closing than it does for opening.
I think that's a great point.
Yeah, I didn't double check this, but I just remembered hearing a fact that may or may not be true about alligator and crocodilian jaws like that when I was a kid. That you know, So they can close their jaws with massive force, but you can actually quite easily hold their jaws together and they can't open them back up.
Oh, so this would be the secret of the feet of the crocodile or alligator wrestler. Yes, Okay, Now coming back to the lobster, does that mean that the closer muscle is the delicious part? This is like that prize sliver of meat from the claw.
Well, I don't know about relative flavors. The closer would be the big one, and the opener is obviously you know, it's like ten percent. It's like one ninth the size of the closer muscle. So I don't know exactly what you're getting. When you have a cooked lobster and you pull it out of there and eat it, you probably some combination of the two.
Well, I guess in my experience, like the bigger the meat you pull out of a crustacean, like the greater the sense of victory. Yeah, And likewise, the more that one is picking through the crustacean with and removing tiny slivers to consume, the delicious they may be. But the more I feel like I'm some sort of like a creature stooped on a primordial shore scavenging pieces from a dead animal.
Getting the pieces from the tiny legs and the tiny parts makes you feel more like it's the road, you know, like looking for seeds or something to eat.
But pulling out that big piece of claw meat. You feel like a king.
That's right, that's luxury. Okay, So you got these different muscles. You got the closer muscle, the opener muscle. What makes the difference in the speed of pinching between the crusher claw and the cutter claw is the type of muscle fiber that the closing muscle is composed of. The cutter claw is made of about ninety percent fast muscle fiber, which is exactly what it sounds like. It's designed to move quickly along with what Govind calls a quote small ventral band of slow muscle, whereas the crusher claw is almost entirely or not almost, I think is entirely one hundred percent slow muscle fiber. So it closes more slowly, but can close with incredible force. And as a result, cutter snaps fast and sharp. Crusher closes slowly, but but it's massive. So you could compare this to handedness in humans, but Govind notes that while the majority of humans are right handed, the distribution of claws on adult lobsters seems equally probable both ways. It's not like the crusher claw is always on the left side. It's it's a coin flip.
Which, which would mean that there's that natural selection is not pushing it one way or the other.
Right, it's not, I mean it's push. It's clearly pushing the lobsters to have two different types of claws for the asymmetry to exist. But it doesn't seem to matter which side is which, at least not in a way that's universal across lobsters. It is it is decided by each individual lobster in development. So a great question then is, Okay, if the bilateral asymmetry is individual to each lobster and it's a random coin flip at least from a statistical point of view, what causes the change? How does the individual lobster's body when it's growing pick which side becomes which. Well, we can look at lobster larval development to see this. So when they're tiny little things swimming around before they become the big lobster, as we recognize, during the early larval stages, the claws of the lobster are undifferentiated. They're exactly the same. Both claws have what Govind calls quote a central band of fast fibers sandwiched dorsally and ventrally by slow fibers. Then during the later juvenile stages this would be like the fourth and fifth molting stages, there begins to be some variability in the amount of slow and fast fibers in each claw. But then the changes really start to become apparent during the sixth stage of molting. Quote, when the putative crush sure claw becomes slightly shorter and stouter with a central molar like tooth, while the putative cutter claw remains long and slender with a central incisor like tooth. A corresponding asymmetry in the composition of the closer muscle also develops. The muscle of the cutter claw gradually acquires fast fibers by transforming the slow fibers of most of its cross sectional face. The exception is a ventral band. The muscle of the crusher claw gradually transforms all of its fast fibers to slow fibers. In succeeding juvenile development, the paired claw's further diverge toward well defined cutter and crusher claws. So the divergence happens sometime in the childhood of a lobster, sometime around its fourth and fifth molting stages and really starts to appear during the sixth molting. But then Govin mentioned something I thought was really intriguing, an experiment going all the way back to nineteen o eight, way back to a researcher named Victor Immel, who found that if you remove one of the lobster's claw during the fourth or fifth stage, so you just pull that claw off, the claw that is left behind, still attached to the lobster, always becomes a crusher claw, and meanwhile the animal regenerates a new claw where the old one was torn off. A lot of crustaceans can do that. It grows a new claw, and the new claw always becomes the cutter claw. But this only happens if you do it early. So if you pull off a lobster's claw after the larval stage, when it's already approaching adulthood, when the asymmetry is already beginning to show up, the original arrangement stays intact. The claw you pulled off will regenerate as whichever type it already was. So this seems to show that claw laterality is determined sometime during the molting stages of like four to five, and it probably won't change after that, so what causes asymmetry to become fixed to during this stage in a young law's life? And here begins a long, twisting and to my mind fascinating journey of experiments trying to pin down how this happens. Most of these experiments Govind himself was in some way directly involved in, and for the sake of brevity, I'm going to gloss over some details in this section, but you can look up the article for yourself if you want the more zoomed in version with all the details and citations. I'll try to give a more sky level view. So, first of all, Govin and colleagues notice some things we already know leading into these experiments. One is that the triggers for developing different claws must be randomly distributed under normal conditions to explain the random distribution of claws in the wild, but not random once a claw is lost. And this naturally suggested something about use the way the claw is used. When one claw is pulled off and has to grow back and new, it isn't getting used, so the remaining cl law is getting used, and maybe it's something about getting used more that makes a claw into a crusher. This would align with the fact that the juvenile stage in which the claws become asymmetrical also coincides with a change in the lobster's lifestyle. So when the claws start to become asymmetrical is around the time when lobsters transition from swimming amongst the plankton to living on the ocean floor and crawling around on the substrate and burrowing in the substrate substrate meaning the stuff that lines the ocean floor. Now, some research had been done which found that if you take a bunch of lobsters and you raise them in smooth plastic trays environments with no stuff to mess with on the bottom of the water, lobsters do not, in fact develop crusher claws at all. In smooth environments. They just get symmetrically paired cutter claws, two cutters exactly the same. But if you put a lobster that's already reached the stage where its claws split into different types into a smooth environment, it keeps its crusher claw, So again it gets fixed sometime early on.
All right, So it seems like it's the environment that is key here, right.
It seems to be something about the texture of the environment at the right stage that causes crusher claws to emerge. So here's a new experiment. You raise lobsters in a smooth plastic environment versus one scattered with pieces of broken oyster shells? Does a lobster grow up differently with different distributions of claws on smooth plastic versus oyster chips? And the results were oyster shells give you normal asymmetrical lobsters with a crusher and a cutter. The smooth, no substrate gives you a pair of identical cutters. This was fascinating to me. Okay, so it's like what it's crawling around on determines how its claws develop. So they wanted to refine this answer further. Why is this it? Is this result something about oyster chips in particular, or could it be any substrate. So they tried the experiment again, but instead of oyster shells, they used different stuff. They used gravel, they used mud with debris, and they even used tanks with plastic shirt buttons and all of these produced normal lobsters with one crusher, one cutter normally randomly distributed crusher claws, and in a control they had a flat smooth substrate that had been painted to look like oyster chips, but it did not but did not have any actual stuff to crawl or burrow around in, and this did not facilitate differentiation. So on the one that was flat and smooth but painted, it still produced identical symmetrical cutters. Another experiment to refine this what about lobsters in smooth plastic trays, but putting them together instead of by themselves. This plays on the fact that lobsters are not very friendly to each other. They are aggressive and tend to fight each other, and typically when they were put in together, they would duel a bit and one of the lobsters would often get one or both claws removed in battle. Yeah, I know, this is kind of going into bug fights territory, but the lobster. So what they found was the lobster with both claws left would split like in the wild with an asymmetrical distribution with one crusher claw. So if there is no substrate, if you don't have any mud or oyster chips to root around and fighting will also do to split your claws into different types.
Okay, that this makes sense. It's about having It would seem to have something to do with the sorts of things you're encountering with your claws, be it chunks of oysters or the hard body of another lobster combatant.
Right, it seems to have something to do with doing something with the claws that produces one crusher of the two. Oh and as a control I thought this is also interesting. They're like, well, we want to make sure it's not just the appearance of another lobster that causes a crusher to develop. So they tried one with a smooth container but a mirror, so that if the lobster could see its reflection, would this make a differentiate But nope, heerio, you just got two cutters.
It's got to be the tactile experience.
Okay, yeah, so so far this is all lining up with the hypothesis that it's something about the claw getting used more that caused them to split and one to become a crusher. So they tried a new experiment with the hypothesis that if you put a lobster in a normal environment with a substrate, but you prevent only one of a lobster's claws from opening and closing, that's going to get less use. That's going to turn into the cutter claw and the other one will become a crusher. Okay, So they tried this with various methods, such as holding one claw shut with a rubber band or with a dab of glue, and they found, to their surprise, this did not produce the result they expected. They thought, if a claw can't open and close, that the other one's going to become the crusher. But no, Instead, with these lobsters, you still got random lateralization. In some the right became a crusher and some of the left became a crusher. Wow.
So it because that was going to be my guess that it depends on how that particular claw is being used, But we see this random distribution occurring even when that one claw is, say, rubber banded shut right.
So here they were like, well, maybe it has something to do with the claw being used, but not with it being able to open and close, and maybe it's something else. So from here they proceeded to a number of different anatomical experiments and to try to quickly summarize their findings. First of all, they found if you cut a tendon, preventing only one of a lobster's claws from opening or closing. This does stop it from becoming a crusher, possibly by preventing fast muscle fibers from transforming into slow muscle fibers. And they reason from this maybe it could be a result of severing reflexive nerve pathways in the process. So they tried to see what happened if you sever a nerve running from the claw to the sort of central nervous system control for the claw. A lobster's nervous system is not exactly like ours. They don't just have one central brain leading out to everything. They've got ganglia, you know, the centralized nodes sort of within the nervous system. So you would want to be severing the reflexive fiber running from the claw to the ganglia. Unfortunately, when they did this by severing that nerve somewhere in there, it almost always resulted from the lobsters after they came out of anesthesia, they would perform what's known as autotomy on themselves, So the lobster's nerve is cut, and then the lobster says, okay, don't need this claw anymore, and they would sever their own arm and grow a new one. This is a standard reaction. Actually of crustaceans, when a limb is trapped or damaged, they just cut it off and grow a new one, which is a fantastic thing to be able to do. And apparently severing the nerve within the claw seemed to trigger something in the lobster that suggested it needed to do that. That causes the behavior in the lobster. It says, something's wrong with this lamb. I'm removing it and I'll just get a new one.
Fun tie in for tomorrow's Weird House Cinema episode.
Oh brilliant. I didn't even make that connection. Yeah, So instead the researchers tried to incapacitate the part of the central nervous system leading to the clause, so incapacitating it more centrally to the body. This did work as expected. It did prevent the claw in question from becoming a crusher. So it has it must have something to do with nerve inputs from the claw that causes the asymmetry to develop. And they tested this with some target exercise regimes. Actually, and here I thought this was great. So I just wanted to read from the article. So Govind writes quote in an inspired moment we thought of enhancing activity by exercising one of the paired claws in a substrate free environment. The lobster was held and its claw law gently stroked with a small paint brush so that the bristles were gripped several times during a sixty second session. This regimen was repeated three times daily at five hour intervals through the entire fourth and fifth stages, the molting stages, that is, for about a month. A control group of lobsters was reared under identical conditions, including being handled but not exercised. While the lobsters in the control group developed paired cutter claws, the experimental lobsters developed a crusher on the exercised left side. The fact that the only perceived difference between the two groups of lobsters was the amount of exercise strengthened our belief that some minimal level of reflex activity in the claw is probably needed to differentiate a crusher claw. However, then they said, okay, what happens if we do the exact same thing, but we exercise both claws with the paint brush, will this give us two crusher claws, which is, by the way, something that we basically never find in nature. I think they cite one example of a lobster that had two external morphologies looking like crusher claws, but the muscles inside did not match. So that pretty much never happens in nature, and they found no. In fact, their experiment could not produce two crusher claws either. In fact, it was not only not able to produce two crusher claws, it gave them just the opposite. Whereas tickling only one claw with a paint brush and making the claw close around the paint brush by reflex that made that the crusher boss claw. Tickling both claws but the paint brush equally turned the lobster into a symmetrical beast with two identical cutter claws only. Huh.
So, if there are any chefs out there who believe that the crusher claw is superior, and they're looking for ways to create the pure crusher lobster, it thus far seemsossible to pull off.
It does not seem possible, though, though I will say this is an older article, I've not looked into subsequent attempts to create two crusher claw lobsters, but I doubt it. I don't think you can do that. That's just not part of the lobster's destiny, not part of its genetic destiny. So a lot of this seems to add up to show that it's not just stimulation or use of a claw that causes it to become a crusher. But that's something that seems to be important, is that it is differential use, which is somehow weighed or compared internally by the lobster's nervous system or ganglia, and the side that gets more use becomes a crusher. So it's not you can't exercise both sides and make them two crushers. If you do them equally, you get no crushers. You've got to get one side getting more stimulation or reflex exercise than the other one, which suggests that there is I don't know that there is some kind of internal comparison module going on in the nervous system. And so Govin concludes, quote in nature, as in the laboratory, initial use or contact of one claw with a substrate sets in motion an increasingly greater activity on that side. The greater neural input of that side determines in the central nervous system its fate as a crusher, and at the same time inhibits the opposite side from ever becoming a crusher, and Govin ends up using an analogy of a teeter totter. I thought this was funny, actual illustrations of crab claws on a playground teeter totter. So he shows, okay, you can have a balance where one side is a crusher and one's a cutter, and so cutter is up in the air, crusher is down. You can have balanced with both sides straight. You know, they're both hanging up in the air and they're both cutter claws. But if you tried to have two crusher claws, something doesn't work there. It's like the pole in the middle of the teeter totter will break, it won't support it. So whatever the exact calculus of experience in the nervous system leading to claw development, these experiments seem to make it clear that claw asymmetry is a prime example of what Govind calls quote experience modulating inherent programs, and this seems to be the underlying principle behind much of how any organisms body is formed and how it behaves, which kind of cuts through a lot of naive all or nothing nature nurture reasoning like much of what an animal is is necessarily a product of both it's genetically innate materials and programs that are the starting sort of the building blocks, and then life itself, the experience of living in an environment and the experiences that the organism has those determine how those innate materials and programs are expressed, leading to vastly different outcomes, even completely opposite outcomes, flipping the sides on which the crusher claw exists. So in the case of a lobster, the metaphor is kind of profound.
You know.
It starts with potential for a crusher on the right, a crusher on the left, or no crusher at all, but probably not two crushers, and then eventually the fixed form of its adult body depends on some early experience, whether and how it digs around in oyster shell chips or even shirt buttons, some early experience, some experience of moving one claw more than the other, or getting having some kind of sensory input causing reflexes. Maybe something that is information fed into the central ganglia through the nerves and the claws determines Okay, this claw is getting more use than the other one. That's the one that's going to be the crusher for the rest of my life.
Fascinating. Now. I had to look this up real quick to see if there was any information about the taste debate between these two claws. I did find an article. This is a two thousand and eight Associated Press article titled the Great Lobster Debate, Claws Versus Tails. As the title implies, this is mostly about taste differences between the claws of a lobster and the tails of a lobster, and it points out that that of the two claws, the crusher claw generally is tougher than the pincer claw. It doesn't get into the taste differences between the two, but it does point out that the tail, on the other hand, is meteor and more flavorful, in part due to the fact that or in large part due to the fact that the tail is used more the tail muscle is used more than the claw muscles. And this is cited to Brian Beal, a lobster expert and professor at the University of Maine.
How many how many lobsters did he have to eat for that experiment?
But it does make me think that maybe, certainly, if you're talking about claw vers's claw. It's probably going to be a matter of personal preference, but it could be wrong on that. Perhaps lobster are aficionados out there have some input on this. Maybe there's one claw they find themselves going to before the other. Maybe there are even certain dishes where oh, well, you only want to use this claw for this dish, and then you want to save your pincher or save your crusher for some other dish. I don't know.
Well, wait, which one did you say was the one that was usually tougher? Was it the crusher?
This article says the crusher claw, the larger of the two used to crush things generally, is tougher than the pincer claw.
To make sense apart, Yeah, so that the crusher claw is the slow muscle fiber and the pincher is the fast muscle fiber. And oh, I hesitate to say this because they may but they may not be comparable at all. I'm not sure how the analogy goes across different you know, fi love of animal life, but I mean, if you think about a chicken, like the breast meat is generally the fast muscle fiber and the dark meat is generally the slow muscle fiber, and that translates to different types of taste and texture within the meat. Like generally chefs would cook dark meat to a higher temperature because it has more sort of that needs to render out of it to render it tender.
Yeah, this article indicates that generally speaking, your lobster tail, that's what's going to get deep fried or whatever, while the claw meat is going to be more tender, and that's what's going to go into your lobster rolls and your lobster club sandwiches.
I'm sure there are people who want to fight about whether you're supposed to deep fry a lobster or not. We're not here to settle that debate. But anyway, I found this little research journey fascinating trying to pin down how and why this happens in a lobster, and it also just seems like somehow rich for metaphor.
Yeah, yeah, this is fascinating. I don't know that I'd really given much thought to this, in part because there are certainly clawed crustaceans out there that are more pronounced in their asymmetry, and I think we're going to get into those more in the third episode in this series.
Yes, there are some crustaceans, specifically some crowd that take claw asymmetry to a ridiculous extreme. That we'll save those for next time.
All right, So sticking to the aquatic world here, I thought i'd go in another direction that I imagine a lot of people were thinking about as we were talking about asymmetry, and particularly in the last episode, you know we were talking about the blowhole of the toothed whale migrating up to the top of the head. Well, we have to talk about the flatfish. There are some eight hundred species of flatfish in global waters, and probably one of the most famous groups here is the flounder. And if you haven't seen a flounder, do look up a picture. The pictures are. A picture of a flounder is always amusing or unsettling. Basically, what has occurred here is that the flatfish's eye left or right, depending on the variety of fish, has migrated to one side of its body to facilitate a sideways life in which it camouflages itself against the ocean floor, like basically living on the floor, kind of like array or something would live on the floor. But it acquires its flatness by being sideways. Both eyes are on the side facing up.
This looks hilarious, and I think it's different, unless I'm forgetting one. I think this is different than any of the other asymmetry examples we've talked about before, because I think all of the other ones have been cases where there is something that's originally symmetrical on both sides of the body, and then they develop in different ways, like one you know, maybe one hole opens and the other one closes, or one claw grows bigger and with a different shapes than the other, different muscle fibers or something like that. They just develop in different ways or two different extents. This is a case of a symmetry where something that was originally symmetrical has one of the two elements migrate to the opposite side, so they're actually switching sides instead of just like one growing bigger than the other or something.
Yeah.
Yeah, it's it's super weird looking. And one of the telling things about these fish is that you can also look at larval flounders, for example, and see eyes on both sides of the larva's head. So it's only as they develop and mature that the eyes move to the other side. Also, like with some of the whale examples we were looking at, you can look back in the fossil record and make out a halfway point in the evolution. Particularly, you can look at the fifty million year old fossil of Anthistium, which has an eye that has migrated to the top of the head, but no farther.
Oh that's interesting. So it would have like it's evolved enough that it has one eye pointing up and another one sort of pointing perpendicular to that right right, Okay.
It has not reached its final form. I guess you would say. Was reading an article from two thousand and eight in the journal Nature by A. Matt Friedman titled the evolutionary origin of flatfish asymmetry, and he points out that Eocene fossil evidence here with Amphistium and another species, heteronic ties, both demonstrate the intermediate form.
I mean, I guess isaat right angles is better than one eye just looking down in the dirt.
Yeah, yeah, but it's it's again. This one makes me think back to the cock eyed squid. It makes me think of just some of the strange challenges of aquatic life in general that lead to these adjustments in a creature's form. It's so fascinating.
Yeah.
Yeah, And of course, well one might ask, well, you know, why don't we see more examples of things like this in the surface world. And I think the answer to that is we do. You know, we talked maybe not in so much in terms of asymmetrical solutions, but in terms of like eyes moving with the evolution of a particular species. I mean, we talked about, we've talked about on the show, the difference between the position of eyes on a herbivore versus the eyes on a predator. You know, do you need your eyes in a position where you can basically see all around you as much as possible at a given time, or do you need those things hyper focused on the thing that's in front of you? You know, these are positions that are reached via evolution.
Yeah, and actually, to take that even further though, this has less to do with the placement of the eyes, but more about the shapes of the actual eyeballs themselves. You know, we talked not too long ago about research even finding differences in common eye shapes based on whether a predator is like is an active predator or an ambush predator you know that tends to specialize for different types of vision, Like do you need to have really good vision for estimating the distance needed for one pouncing jump, or do you need the kind of vision needed for chasing over a period of time.
Yeah, yeah, exactly. So that's a fun example that I just had to bring up. But I have another one here, and this is another famous example of asymmetry, and this time we are dealing with surface species and surface creatures. And it's also an interesting example because it's an example of a symmetry leading to more asymmetry. What happens when an asymmetrical creature is your preferred prey, Perhaps you become more asymmetrical in order to take advantage of it.
Oh that's interesting. So you'd imagine, like I'm just making this up, but if you're fighting some kind of giant lobster and one of its claws is bigger and more dangerous than the other, if over evolutionary time, your species develops i don't know, tougher skin or defenses on the side that matches the more dangerous lobster.
Claw, Yeah, assuming there's some consistency in which side of the giant lobster of the money claws on.
That's true. I guess if it was the American lobster would be random. So you're just out of luck there.
Yeah, but in this case we're dealing talking about the food. First, we're dealing with snails, and snails are obviously asymmetrical, possessing either clockwise or counterclockwise spiraling shells, and as as a side note, slugs are also asymmetrical. Slugs, of course, are evolutionarily speaking, they are snails that no longer need to carry their homes with them, have put that sort of lifestyle behind them, but they really retain the asymmetry.
Wow, I did not know this. So if I understand right, you're saying that the slugs evolved from ancestral snails, they evolved from creatures that did have shells, and they evolved to lose them to internalize them.
Yeah. We cover this on an old episode of Stuff to Blow your Mind many years back, and so some of the details are a bit foggy, but yeah, this is the basic story of slugs and snails. And by the way, if you look at a slug, you can still you can visually mark the asymmetry if you look for a particular it looks like a little circular feature, a little hole or orifice on their body that is the Numa stone. And yeah, it's on one side as opposed to the other.
That's really cool because that's another case where it's like, I don't know, just imagining evolution operating in a direction opposite to what you would have just naively assumed. It's like knowing that whales evolved from mammals that used to be land walkers, you know, the four quadrupedal land mammals spent more and more time and water and eventually became fully aquatic. So here these would be not that slugs evolved to gain shells, but that snails evolved in some cases to lose their shells.
Yeah. I mean, like we've covered time and time again, nature is flexible when it comes to evolution. The card is always subject to change. The card will change, and it's the species that evolved in sells into a corner sometimes that find it the hardest to survive long term.
Okay, So in modern snails and slugs, you've got this asymmetry, You've got some part of their biology having a kind of clockwise or counterclockwise component, and this would of course, be of relevance to any kind of creature that interacts regularly, especially with the snail that's got a hard external part, that's got this clockwise or counterclockwise.
Shell right, and that brings us to a species of snake known as Awasaki's snail eater also known as Awasaki's slug snake, and these are found in the Yayama Islands of Japan, and they specialize in eating snails, and they have specialized jaw structures that enable them to prey on clockwise, spiraling or dexterraal snails. However, as a result, they have a harder time preying on counterclockwise or sinestral snails. Soasically, the way this works out is the snake's mandibles have evolved for extracting snail bodies from their shells, and this evolved independently, apparently in at least three subfamilies, according to Hoso and Hori, writing in The Herpetological Review in two thousand and six, the snake inserts its mandibles into the snail's aperture and moves each mandible forward and back to extract the body. And these two individuals Hoso and Horri, they've written several papers on the snake. If you look up Ilasaki Snail Leader, you'll all often or always find these researchers involved, including a two thousand and seven paper titled right handed Snakes Convergent evolution of asymmetry for functional specialization.
This is one where you should look up a picture if you can. The one I'm looking at now, Rob that you supplied has the snake biting onto the snail shell, and it's got its upper jaw position on the outside of the shell and its lower jaw. I guess that's the mandible reaching in underneath the tube of the shell. So the lower jaw is what's getting inside.
Yeah, yeah, And just looking at the pigure it can be kind of hard to work out exactly what's going on. So I want to read a quote here from this two thousand and seven paper. They right. We found that snakes in the subfamily Peritinae, except for non snail eating specialists, have more teeth on the right mandible than the left. In feeding experiments, a snail eating specialist pere Iwasaki, completed extracting a dextraal soft body faster with fewer mandible retractions than a sinestral body. The snakes failed in holding and dropped sinnestral snails more often owing to behavioral asymmetry when striking.
Wow, so it's been preying on these snails so long that it's specialized for having one side of its mouth ready to get at a certain side of the tube of the snail's shell. And if it attacks a snail of the opposite handedness in its spiral, it's going to be at a real disadvantage.
Exactly. Yeah. So again a fascinating example of asymmetry leading to more asymmetry in the food chain.
I'm so scared that at some point here I've said snail when I'm at snake and vice versa. I apology, I apology, I apology if I did that.
So anyway, Yeah, definitely look up images of the snakes, especially if you find an image of it actually feeding on a snailshell. All right, Well, we're going to go ahead and cap this one off right here, but we will be back. I think it's going to be the episode after next in which we return with part three in this series, but there will be more creatures of note. There will be more fascinating evolution and adaptation, and the crabs will finally arrive on the scene, don't they always Yes, there's no stopping them in the meantime, if you would like to check out other episodes of Stuff to Blow Your Mind, our core episodes publish on Tuesdays and Thursdays. You can also enjoy, if you like, our listener Mail episodes on Mondays, our short form Artifact or Monster Fact episodes on Wednesdays and on Fridays. We set aside most serious concerns and just talk about a weird film.
Huge thanks as always to our excellent audio producer Seth Nicholas Johnson. 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.
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