From the Vault: Horseshoe Crabs

no memory of this one at all. Really. I mean, I vaguely remember researching some of the stuff about the blood and the usefulness of horseshoe blood, but I really don't remember this episode. Otherwise, I don't know why. I remember. This episode led me to discover a really cool book though, that was about horseshoe crabs and uh and an evolution. So yeah, yeah, it was a good one. Alright, let's dive in Welcome to Stuff to Blow Your Mind, a production of I Heart Radios has to work. Hey, you,

Welcome to Stuff to Blow your Mind. My name is Robert Lamb and I'm Joe McCormick, and today we're gonna be talking about an underappreciated evolutionary marvel, the horseshoe crab. Robert, this this episode was your idea, and I'm so glad you you thought of this. Yeah, and I'll bring up a little later in the episode. What what reminded me that the horseshoe crab should be a topic of discussion. But but really, this is a creature I feel like

that I've been admiring my whole life. It's frequently brought up.

It's frequently pointed out to me. I remember as a kid being if not shown an actual horseshoe crab, I mean maybe I was shown like the remnants of one that had washed up, or picture of one, and it was explained to me that, like, this is a unique organism that you don't find many things that I really like it on this earth, on the Earth today, and if you went back far enough in time, you would find them in in ages of of strange biological diversity

that what otherwise seem alien. But the horseshoe crab has remained largely constant. It is an olive colored lump from the Jurassic period and beyond. So one of the great things about it is it's sort of perfect fodder for our show, I think, because it's something that if you don't go deep on it, it might be uh, you might think of it as a kind of like lowly and uninteresting, just sort of lump in the mud with with some scuttling claws, and you know that there's not

really much to it. There's a lot to it. This creature is marvelous. And to start us off, I want to inspire some wonder by by reading a passage with a few abridgements from a really excellent book I've been reading this week by the British paleontologist Richard Forty, called Horseshoe Crabs and Velvet Worms, The Story of Animals and

Plants that Time Has Left Behind. It was published into the two thousand twelve Now velvet Worms by the way we spoke about them recently, it was potentially buried if you skip our movie episodes, and you shouldn't. Uh. We did an episode on the Tangler, that old Vincent Price horror movie, and the title character in or the title monster in that film, the tingler very closely resembles a velvet worm, so we discussed its unique biology so to be mentioned in the same sentence as the velvet worm.

That should I let you know that the horseshoe crab is no joke totally so. Richard Forty, the author of this book, is a former president of the Geological Society of London. He spent much of his career as a staff paleontologist at the British Natural History Museum, where his research included a special focus on our old friends, the trialo bites uh. And he's also done a lot of public natural history communication, appearing on BBC documentaries and stuff

like that. So in the opening chapter of this book, Forty is describing a massive gathering of horseshoe crabs that he witnessed one night on a beach in Delaware. So I'll begin reading here deep in the night, along the shores of Delaware Bay. The horseshoe crabs are stirring. The tide is now high, and there is no moon. Darkness rules, But even in the feeble starlight, the overwhelming flatness of the countryside can be made out, except along the room of the bay, where old sand dunes have built up

a levee heave with gentle movements. First, I noticed some very odd sounds. There is a general hollow clattering, a tapping and grinding sound somewhat like that made by knocking coconut shells together, once used on the radio to imitate horses hoofs, but altogether less rhythmic and with a kind of underlying push. Then, as my eyes get used to the darkness low shelly mounds, the size of inverted colanders can be seen slowly pushing and jostling all along the

shore and perhaps six meters up into the sands. They're bumping and clambering together. Is the source of those tap tapping percussive sounds. The flash of an infrared torch reveals

more details. The head shield of the horseshoe crab is domed upwards and carries a few weak spines at its back end to hinge marks a jointed boundary with a second large plate spiny at the edge, which can flap downwards, and beyond that again projects a stout triangular spike as long as the head, which can waggle up and down. Here at Kit's stomach. More crabs are gathered on the

mud flat seaward of the sands, waiting their turn. Strange green, black, slowly animated lumps further offshore again in the shallow seawater Tail Spikes project briefly above the gentle waves like raised radio antenna and are gone, showing we're still more horseshoe crabs vie with one another to get their place on the sand. So if that doesn't attempt you to buy the book, I will say that the whole thing I think is great like that Forty is uh he's a

great scientist but also a really great writer. In this book is just a fabulous read. Yeah. I like the details he gives to describing it. Like, one thing that I think stands out for me is the horseshoe crab always looked like an element from a suit of armor. You know, it has a looks it looks like a little tank um thing along the shoreline. Well, yeah, exactly, And there's a good reason it looks like that. I mean,

that is quite literally what it is. This is a creature that is mostly a suit of armor, especially if you're looking down from above right, it is quite literally biological armor. Now, Forty goes on to explain the marvels of the scene. He says, there are thousands of these creatures gathered on the beach and coming onto the beach

from the waves. Uh. He at one point finds one horseshoe crab upturned on its back in the sand, desperately bending its tail spike up and down in an attempt to flip itself back over, which is a strategy that I believe would probably work in the water, but not so much on the land. Um And despite his status as a scientific observer, Forty admits that he's unable to resist the urge to right the animal, and he does, grasping it by its head shield, and he flips it.

But once upright, of course, it doesn't say any thanks. It just kind of trundles away and gets back to business. But what is this business? It is mating. This is a giant convention of hor shoe crabs essentially for the purpose of an invertebrate orgy. And that's not forty. Forty himself uses the word orgy. I think that is the correct term for this. So he notes that the largest animals on the beach are digging down in the sand, so while their dorsal shields hide most of what's going on,

their jointed limbs underneath our industriously removing sand. And then some of the larger crabs end up digging themselves so far down that they're almost completely buried, and these larger creatures, the ones doing the digging, or the females, they will ultimately be burying their freshly laid eggs in the sand here on the beach. Meanwhile, smaller crabs are fighting to

climb on top of the buried females. These smaller ones are the males, and the reason they're fighting for positioning is that they're competing for a chance to fertilize the female's eggs with their sperm cells, which are called milt and Uh Forty realizes that much of the tapping he's talking about in that passage, I read that clacking noise that he heard in the dark comes from what he

calls tussles for dominance. Male horseshoe crabs knocking one another about by the exo skeletons as they fight for a chance to be the first in line to reproduce, and this can get really violent. Forty notes that finally, uh, many of them don't survive this night of mass invertebrate sex on the beach. In the morning, the shore is

just littered with assorted chunks of horseshoe crab carcasses. It's an amazing scene, and I wish I could be present to see this something I would go to Delaware for that. That would get me into Delaware. Yeah, they should put it on the license plate, right. Yeah, this does remind me of of a story that I've heard before. My wife's grandmother had an amusing tale of horseshoe crabs. I believe this is a tale from the Great Outer Banks

of the United States. Uh, though she also lived in Australia for a time when she was younger, so it might be a tale from Australia, but I'm pretty sure it's Outer Banks anyway. The story goes that she happened upon a bunch of horseshoe crabs on the beach and thought they were in danger. So um, my wife's grandmother then devoted an hour or so to collecting them and hurling them back into the sea, only to learn later that they had come ashore to make so she thought

that they were like beached whales essentially. Yeah, you know, they're usually not there here, they are in mass maybe something is wrong. They need help, you know, thrown back in before the birds get them, that sort of thing. So she meant, well, but it turns out she was interfering in their natural process. I bet those were some frustrated invertebrates. Uh. Now, these mass matings on the beach

bear a and actually pretty great ecological significance. Forty describes the eggs they lay as tiny and green, and he writes that they're they're laid together in these golf ball sized clumps of about four thousand to six thousand eggs apiece. Uh. He says up to fifteen or so males will have the opportunity to fertilize the eggs of a single female, and during a reproductive cycle, a single female horseshoe crab

might lay about eighty two a thousand eggs total. And yet Forty notes that on average, it's estimated that only about thirty three out of every million eggs survive into adulthood. So again this comes back to kind of in the invertebrate numbers game, much like we talked about in our Christmas Island Crabs episode. Uh. You know, there's a lot of larva and only a tiny fraction of them actually

ever become adult crabs. Uh. But this massive eggs and juveniles that don't survive, they're ecologically very important because like the Christmas Island crabs. They are an important food source for lots of animals living in and passing through the region. Longerhead turtles prey on the crabs even into adulthood. That is kind of weird imagining eating a horseshoe crab, because if you look at one, it just really does not

look like it would have much good meat. It looks like an animal made entirely out of shell and bone. So I was looking around on this and in some parts of the world that they are sometimes harvested for food, but only the eggs or row are edible, according to Malaysia Best dot net so, which is a blog post about this and some photos. You'll find them on the menu and some restaurants either grilled and flipped over for row consumption, or someplaces you can get the row already

scooped out and served to you. And I read that the eggs basically have a rubbery texture and a salty taste, so nothing really all that exotic in terms of you know, eating uh, you know, the the eggs of marine creatures like this, But again, the end of the air favorites among birds and other creatures, and the horseshoe crab is a keystone species for this reason, Like it's we're gonna spend a lot of time on this show talking about, you know, how ancient they are in their ancient origins.

It's easy to maybe fall into this notion that they are an outcast, that they're not really important, they're just a throwback. But no, they have a very important role in a number of species. Again, like migratory seabirds depend upon them. Yeah, you're exactly right about that. And Forty talks about this at length that you know, the birds will peck around in the sand to find the hidden caches, buried eggs. Um. So this scene that Forty describes so

vividly is not just an amazing spectacle of nature. We sort of alluded to this earlier, but it's also a window into the deep history of planet Earth and a way to think about the wonders of evolution across geologic time. So maybe we should take a closer look at the horseshoe crab and its anatomy and its body before we

kind of zoom out to the evolutionary picture. Yes, absolutely, and uh, and we do encourage you look up some footage or some images of the creature if you have a chance to while while we're discussing it here um, because that'll be helpful. Though. I think a lot of you, if not most of you, have seen a horseshoe crab before and have at least a basic idea in your

mind what it looks like. I guess maybe let's start from looking from above, right the way you would normally see one if you're looking down at it on the beach. So from above, the horseshoe crab is this closed dome

of armor that has three obvious body segments. The first segment is the headshield technically known as the pro soma, which is a solid, rounded, vaguely horsehoof shaped plate of protective kitanous material, which forty says is similar to the material you'd find making up the wings of a beetle. And then it's got on top of that headshield two obvious compound eyes poking up on on either side, and

those compound eyes are used for locating mates. One thing I was reading about them that was very interesting is that they they have drastically different levels of light sensitivity between the night and day cycle. So during the daytime, the receptors in those compound eyes are tuned way down, I guess, to make them, you know, less likely to

get blinded by the light coming in from above. But in the nighttime they get tuned way up, and that's I think primarily used for seeing a mate somewhere on the beach and navigating toward it in the dark, because again, this a lot of the spawning happens at night. I think the eyes are are kind of really key to the the human experience of the horseshoe crab, because you know, we like things with eyes that we helps us sort of connect with them and sort of even think of

a personality for them. The horseshoe crabs eyes, certainly at first glance anyway, they seem to have a certain seriousness to them, or even kind of a determination or a A seven a sinister quality, and so they're just all business. Yeah, they look they look very serious. They don't have like goofy eyes. You know, we've discussed animals on the show before that from the human perspective may even have googly eyes, but like Glenaria, these however, they look very serious and

so we kind of consider them seriously. I think sometimes well it turns out there even more serious than you think, because they've got those two big compound eyes that are that have that alternating sensitivity good for locating mates. But the horseshoe crab actually has something like ten eyes total. Uh. These are you know, less easily identifiable as eyes just by looking at them, but they've got basically ten photosensitive spots or organ that in some way help the creature

detect light and detect movement. Okay, so that's the big first part of it, the domed part of the head shield. Then you've got if you're going from front to back, you've got the next segment which connects behind the head shield, and this is the abdomen, also known as the opus the soma. Uh. This is a more flattened plate attached to the head shield by a Hinge as forty wrote, and uh, and it's got these backward facing spines along its lateral edges. And then finally, the horseshoe crab terminates

in this long, straight, rigid tail known as a telson. Uh. And if you've ever seen one of these animals moving or being handled, especially if you've seen you know, like a demonstration where somebody holds a horseshoe crab upright for you to see it's underside. You know, the tail can

kind of whip up and down dramatically. Now, other Arthur pods have Tellson's as well, such as the shrimp, And I think the shrimp is a great example because a lot of you have probably at least some you probably have some experience with shrimp tails, the consuming shrimp tails, so in shrimp, lobster's, krill, and crayfish. The the telson is important for what is known as the the karadid escape reaction, in which a downtail tail flip allows a

startled individual to dart backwards through the water. Which, again, that kind of comes back to you said earlier about how the thrashing of the tail would be more useful to to right oneself or to escape a stressful situation

in the water as opposed to on land. Yes, and that's a really interesting thing to look at too, because when you look at a display like the tail wagging up and down, this, you know, the the rigid pointed stick there basically, especially when the animal is handled, you might guess it's a defensive weapon, right, you'd think, like, okay, stinger of a scorpion, and a lot of people do assume that about the horseshoe crab that has got a stinger on its telson, and that guess would be half

right and half wrong. The correct part is that there probably is an evolutionary relationship at play in the similarity with the scorpions tail. Will come back to that in a bit, But the wrong part would be to assume that it is a stinger weapon. It is not, as we were talking about. Primarily, it's used to help the horseshoe crab navigate aquatic environment, so it can help the

animal steer its body while it's swimming. But it can also help the animal, like we said, right itself once it becomes flipped on its back, and you can imagine in its natural habitat this could happen quite a bit because this is a creature that's going to be dealing

with the physical tumult of the title zone. You know, you can imagine it might get rolled upside down in the surf, where might get rolled over while it's clambering around on something in the mud flats, and this popping up and down motion of the tail can help flip it back over like one of those spring flipping wind

up toys. Now I just mentioned swimming. One other very strange aspect of the horseshoe crab is that when it's time to swim, the horseshoe crab usually swims upside down, doing a kind of invertebrate backstroke with its head yield angled down towards the bottom and it's jointed legs paddling

towards the sky. If you can find video of this this, it will probably recommend you go look at video of several things in this episode, because a lot of a lot of this animal's movements and behaviors are are fascinating to see. But yeah, if you can find video of its swimming with its legs inverted up towards the sky, it looks very strange, very cool, and it well, yeah, it's basically just a backstroke. Well, as long as the animals flipped over here, let's discuss what's going on underneath

the armor. I think that's a great idea. So yeah, well you're flipping it over looking at its belly, and on this side it is bug city. You've got five pairs of jointed walking legs that look kind of like crab legs or spider legs, and these are known as pedipalps. The first four pairs of legs all end in a a pincer claw shaped tip, so imagine kind of a regular crab with claws on all of its walking feet.

But then the final pair of legs ends in what looks a kind of strange flower shape, which is apparently used mostly for digging, and then towards the front of the head. In in front of the walking legs, there's another smaller pair of appendages that are known as the chillissary, and we'll come back to their significance in a moment, but they're primarily used for guiding food toward the mouth. So here's maybe one of my This might be my

favorite partment. Where's the mouth? You might expect, in line with other crabs and invertebrates, that the mouth is in the front facing part of the head, but nope, in horseshoe crabs, the mouth is in the middle of the

underside between all of the animals jointed spidery legs. So as the legs move, any food caught underneath them is sort of shuffled inward toward a crevice running between the leg pairs, aided by these gripping spines that run along the inside of the appendages, so as the legs scuttle, the food is also partially chewed up by those legs and prepared for digestion. There's this grinding scraping action of

the exoskeleton parts of the legs and the joints. So, in perhaps illicit anthropomorphic terms, the horseshoe crab has crotch, mouth, and leg teeth. Yes, But on the other hand, I feel like if you really break down how any animal, including not especially humans, eat, it's it's all pretty gross. Soh No, I'm sure they think the way we eat is great, especially when we're eating their eggs. But but I do. I do agree that the footage is very interesting and one should check it out. Amazing. Yeah, look

at that there are videos of this online as well. Yeah. It also reminds me that a a fictional creature that the horseshoe crab is very typically compared to these days is, of course, the Zeno morph alien face hugger. Um Like even that that Malaysian blog post about eating them eating their their eggs at restaurant uh invoked the face hug of comparison. It's kind of inevitable at this point, though,

I'm not sure the face hugger eat. Um No, I'm well, okay, I mean that's a whole another description to start talking about the face hugger and how it matches up with uh with with actual biology. I'll have to save that for another episode. But but the face hugger does have the you know what, is kind of like a mouth that had it definitely has a tubular orifice that emerges from the underside and pretty much the same place one

would find the mouth of the horseshoe crab. But just in general, the horseshoe crab and the and the face hugger have kind of similar body layouts, even though they're

they're rather you know, textually different. I can agree with that, though, I guess maybe another similarity if the face hugger doesn't actually eat, and it's just like a you know, no no digestive system, reproductive organism, well I would I think you could even class say that the face hugger in alien and use a mobile sexual organ Yes, But but to bring it back to the horseshoe crabs, forty rights that mature adults can sometimes go for months at a

time without eating, So these things are tough. Uh. And then also to continue our exploration of the underside. Behind the legs and the mouth crack on the animals underside, you will see a series of overlapping leaf like flaps, and these are the animals gills, which allow it to breathe by absorbing dissolved oxygen from the water. And the horseshoe crab can survive out of water for a time if it can keep its gills wet. These organs are

a specific type of underwater breathing apparatus. Unlike many other animals gills, these are these are known as book gills. Now you might have heard us talk before about other arthropods with book lungs, animals such as the spider. And with that teasing detail, maybe we should take a break and then come back. All right, we'll be right back. Thank alright, we're back. So we were talking about the horseshoe crab. Ab, we're talking, and we talked. We mentioned

a little bit about spider. So let's let's get down to it. Let's get down to that that basic factoid that that I imagine most of you have heard plenty of times, and that is that the horseshoe crab is not a crab, despite the fact that we will refer to it often on as a crab, and you'll find plenty of, uh, plenty of studies where scientists will off handedly just refer to them as crabs. Everybody keeps calling

them crabs, but they're not crabs. True. Decapod crabs and horseshoe crabs both belong, of course, to the philum Arthropoda. They are both are arthropods, meaning they both have hard exoskeletons, They've got segmented bodies, and they've got multiple pairs of jointed legs, and the sharing of jointed legs is where the word arthropod comes from. Arthur pod means jointed leg or jointed foot. But after this, the evolutionary histories of

crabs and horseshoe crabs really diverge. Crabs, along with shrimp, lobsters, wood lice, and and many other creatures, belong to the sub phil um of Crustacea. We would call them crustations, and Forty points out that crustaceans have antennae or feelers on the head used for sensing the environment by touch and by smell, and Horseshoe crabs don't have these, so

what do they have? Instead? They have chillissary. Horseshoe crabs are not crustaceans their chill serata in evolutionary history, they are more closely related to arachnids like spiders, ticks, and scorpions, which if you look at the mouth parts of these creatures, like spiders and scorpions, you will find these similar little mouth parts that that guide food into the orifice. The

chillis sary. Uh. They're also Horseshoe crabs are also more closely related to an extinct branch of chill Serata known as the euryptorids, also known as sea scorpions. Now again in misleading names. Sea scorpions is somewhat misleading here because euryptorids were not actually scorpions and they didn't all live in the sea. But they are truly awesome. This is one of the great lines of extinct creatures we we talked about in the past episode. Didn't I think they've

come up before? Yeah, So, the Europtorids were briefly a diverse order of predatory animals, including the genus of the largest arthropod ever known to exist on Earth. Jclopterus, which based on some fossil remains found in Germany, is estimated to have grown up to about two point five meters or over eight feet in length. So that's definitely big enough to start its own harmon. Yeah, just try to

imagine it. So you got an eight foot arthropod, a sort of scorpion lobster like creature bigger than your whole body, with a plated exoskeleton and claws, scuttling around at the water's edge, or just hanging out in the shallows and ambush mode h I have before. I sometimes like to imagine these types of creatures surviving into modern geologic periods

and living alongside humans. And I wondered if the ancient Egyptians would have had a euryptorid headed god in place of the crocodile deity, so back that would be quite a quite a god to behold. You know, I mentioned that it is something like this would deserve its own horror movie. But now I'm remembering the creature in Deep Star six, the underwater horror movie from the director of the original Friday the Thirteen. Oh. I believe the monster in that is a uriptorid. Uh. It is a straight

up sea scorpion. I remember Miguel Faire exploding in the movie, but I do not remember what the creature looked like. Yeah he did, I guess he did explode. Um. He There's a scene where they're like, don't get in the escape pod, you'll go through explosive decompression, and he's like, no, I'm scared, and then he blows up. I just remember, I'm the most recent time I watched it, I was on an airplane on medication, and I remember just thinking it was a wonderful film. We'll see how that would

hold up over time. But yeah, well, I mean all those nine underwater horror movies are worth a watch, but certainly I think it was a case where they're, Okay, we need it underwater monster. Let's look at some real underwater monsters from the past. And they found one and they said, heck, let's not try and recreate the wheel here,

let's do that guy. And so that's what they did. Okay, So these would be the ancient closer relatives to the horseshoe crabs, the the you know, the arachnids, the scorpions, even the uptorids. There are a few extant species of horseshoe crab like animals, including a few species found in Asia, but the most common by far is the Atlantic horseshoe crab, which has the scientific name of Limulus polyphemus. So it's got the same name as the the cyclopid monster in

the Odyssey uh and and Limulus polyphemus. The Atlantic horseshoe crab can be found primarily along the East coast of North America, roughly from Mexico to Maine. So your wife's grandmother's story, I think probably more likely happened in in the outer banks on these tis. I'm like American percent sure it was. It was outer Banks, but yeah. So another thing that's interesting about the horseshoe crabs is something

that's more common to arthur pods generally. But like other arthropods, the development of their bodies as they mature happens through a fascinating process called molting. So since these animals have a hard, rigid exoskeleton, you might wonder how would they ever grow? How do they get bigger? Right? You know, if you've got you've got your bones on the outside,

and molting is the answer. Periodically during its life cycle, the horseshoe crab will bust out of its own exoskeleton and emerge as a softer critter from within, only to quickly have its soft, new larger outline harden again pretty rapidly in defense against the perils of the sea. Now, a normal horseshoe crab takes about ten years of growing and molting before it reaches sexual maturity, which seems like

a very long time for an animal of this kind. Uh, you know, imagine it has to grow for ten years before it's ready to mate in that orgy on the beach. Yeah, I mean, certainly when we compared to something like say a cat or a dog or you know, rat, something like that, where the it's pretty short turnaround. But this is this is ten years. But especially many other invertebrates,

we should think reach sexual maturity very fast. But after reaching sexual maturity, it never molts again, and instead it heads to the beach for mating to leave fertilized eggs in the wet sand and start the life cycle over again. And so this this life cycle has worked pretty well for the horseshoe crab, and it's worked that way for quite a long time. Yes, the earliest fossil evidence for

horseshoe crabs is incredibly ancient. The oldest fossil remnants resembling these modern animals, the modern horseshoe crabs, go all the way back to the Ordovician period. This is so long ago, it's unbelievable. Yeah, we're talking about four hundred and fifty million years ago. Um. I mean just consider that the for instance, the goblin shark is also something that is sometimes referred to as a living fossil, and we'll get

back to that terminology in a second. But the goblin shark own late dates back a hundred twenty five million years is an extremely long time. Yeah, um, still incredible. Crocodilia goes back some nine million years. Hagfish are virtually the same as they were three hundred million years ago. Lamp rays go back roughly three hundred sixty million years not Alloy's have evolved very little since roughly five hundred

million years ago. Although that's an example of a creature to where they were, there were certainly more varied two hundred million years ago. Yeah. About this idea of of a living fossil, Richard forty actually he kind of warns about this phrase. He calls it quote a paradox and an oxymoron rolled into one. Well, yeah, because I mean, on one level, it is not a fossil. I think that much is probably obvious to most people. It is

not a fossil. A fossil cannot be alive. A fossil is inherently the mineralized remains of something that once lived. Though he also sort of uses the word cautiously, his point is mainly that we shouldn't be lulled into the miss stake, an assumption that a species can exist for millions of years with no changes, for example, without significant genetic change. Genetic changes are always accumulating, They just build up over time, even if the overall form of the

animal stays very similar. Uh. And also the ecological surroundings of these organisms change over time. He talks about how even if the the the horseshoe crabs of today look extremely similar to the horseshoe crabs of the Jurassic Period, the animals all around them would have been completely different, and thus they there they were probably you know, they

had a different ecological niche. They were dealing with different relationships and different energy dynamics in the in the environment, but eating different things, titially being preyed upon by different things. But it is truly remarkable to see a type of animal that has survived. I think the horseshoe crabs have survived five different mass extinction events, definitely four, I think five, and they still exist today in in a body plan that is pretty close to the same animal you would

have found four fifty million years ago. And so as alternatives to the phrase living fossil, some paleontologists have proposed calling these types of organisms stabil a morphs. You know that that morph meaning like body basically and stabilo meaning stable, meaning that at some point long ago there was a body plan that was reached and it just has not

needed to undergo change much since then. Yeah, this is interesting and and definitely touches on something we've discussed in the past in terms of of body forms that work.

And sometimes you see those basic body form that works to the degree that it is acquired by by rather distantly related relatives, such as such as the basic dolphin or the basic dolphin form, and it's you can compare that to to various other fish forms and then also to the reptilian creatures that preceded them um uh Ichleosaurus, for example. I was just looking at a paper a while ago. I don't remember where it was, but there was something about how nature repeatedly has tried to build

crabs like crustacean crabs, Yeah, the actual crabs. That it's like a form that just kind of nature keeps coming back to from different evolutionary pathways and ending up in the same place. Yeah, it's like the most logical engineering solution to a given environment and a given set of challenges.

But of course, with the thesaurus, of course went extinct for first of some some essential reasons that I think we've touched on the past show on the show in the past, But with the horseshoe crab, it's rather different. It's like they acquired this form and that form has

remained relatively stable for the duration. Yeah. I mean those are two So you can talk in one sense about forms that are stable and you see that they're they're advantageous because of convergent evolution, different evolutionary pathways sort of arrived in the same place. But we're talking about animal forms that were achieved at some point in the past and then they just kept working over time, and they

didn't undergo significant changes in their lineages. Uh, you know, little changes here and there, but not major changes in the body form. And they never went extinct. Yeah, I mean, really, to invoke some more alien terminology, this is a perfect organism. It it perfectly survives in the environment for which it has evolved, and it has remained stable ever since. Then again,

surviving mass extinction events for four and fifty million years. Crazy. Yeah, there's another alternative name for what to call these types of creatures that was actually proposed by Darwin's bulldog, Thomas Henry Huxley, who just called them persistent types. I think that's pretty straightforward. Yeah, but it makes you wonder how on Earth is something like this possible? Like what makes animals like the horseshoe crab special? How does this type

of creature persist for so long with out huge morphological changes? Uh? And you know, surviving all these extinction events, never you know,

becoming all that different of an animal fundamentally. Yeah, because I can't help but think of think of it in comparison to say the business environment, you know, like like what are the persistent forms in the business world, Like what what brands or franchises or product types just survive for the duration through multiple like economic extinction events, and also surviving all the things changing around them, you know, predators and prey changing the way they behave and the

way they eat, you know, things in the in the natural world that are ever trying to find that new niche will allow them to to to themselves survive since the early Devonian nature has pivoted to video exactly. Um, so it's you know when we say that you know they're true survivors, that it's you know action. I mean, really you you're tempted to go that far and say, like something here is just really working that they have not been um surplanted by some of their creature along

the way. So many terrific seeming organisms have certainly proven to have a very tenuous role in the environment, but the horseshoe crab remains. Yeah, I mean, we got no room to talk, puny, Homo sapiens. What have we been around, you know, less than a few million years. Yeah, and and we're continuing to work hard and making sure that doesn't go too long. Um okay, So actually, so we don't know the answer for sure, why the horseshoe crab in particular and animals and other organisms like it has

persisted so long in its basic body type. But Forty has some general thoughts about the question of of what causes this sort of thing. He's got some arguments, and one that I thought was really interesting was that he talks about how survival is not just about the endurance of the animal, It's also about the endurance of habitat. So often the ability of an animal type to survive through the eons largely unmodified is a feature of the

animal's habitat more than the animal itself. Some habitats are just better equipped to sustain their adapted inhabitants through ecological catastrophe than others. And so what would be an example of this, Well, Forty mentions inter tidal zones of the ocean and goes into quote shallow subtitle habitats on muddy sandy shorelines. Now, why would this be a favored habitat

for survival through mass extinctions? Well, a big killer for ocean dwelling organisms during environment environmental catastrophe appears to be antoxy seas, where due to several cascading factors, you know and there's a big environmental catastrophe. Oxygen is often removed from a lot of the ocean water, and the animals in the water can no longer breathe, and they die even in these conditions. Fort just that these shallow, muddy ocean edge habitats could still be pretty well oxygenated quote.

After all, the wind still ruffled the waves on shore, and so many of the organisms adapted to this environment can make it do with very little food to begin with, and they tend to bury themselves in sediment at low tide and sort of feed on particles of food that wash in with the surf. And their access to the surface would keep them in oxygen, and their access to the tide would keep them supplied with particles of biomaterial

for food. Uh to quote forty again, I am tempted to return to the military metaphor maybe this habitat was like a tunnel that simply went under the front line. The luck came in if you happen to belong to that special battalion with access to the tunnel. And for this reason he points out mud flats as a special sort of extinction event survivor zone. Of course, we know that you would often find horseshoe crabs scuttling around in

mud flats and tidal areas. Um. Now, now, why don't ancient animal forms in zones like this get out competed and displaced by new arrivals. Uh to read from forty quote. Populations in many habitats are critically limited by the quantity of food available. However, in places such as mud flats, food may not be the limiting factor for filter feeding. Animals are rich food store is carried into the area with every high tide, or is brought from nearby land

during storms. The crucial things to find living space. The problem is not the food in the trough, but making a place at the stall. So if it can establish itself in its borrow lingula, a creature he's talking about is able to compete for food on equal terms with

a later arrival, geologically speaking, like a shrimp. This habitat does seem like a good place to be for an organism with conservative tendencies, And so he's sort of talking again about many of the creatures that survived these mass extinctions and go on for long periods of geologic time having kind of behaviorally or ecologically conservative behaviors. Uh. To quote him again, in one way, it is survival of the fittest, but of the fittest habitat with the right

design specifications to offer long term security. Stick in the MUD's last longest. Now, he also points out a few other things, a few other cases of evolutionary survival of an organism type over geological time, like if there is an animal that's a specialist and a specialist niche, and then their particular niche, by luck, persists over time. He gives the example of like lampreys and hagfish that survive

as parasites on bony fish. Um. And then uh, and he calls these areas that tend to accumulate long term evolutionary survivors together time havens. Yeah. I like that uh And And finally he points out also an interesting fact that longevity seems to be a common feature in long surviving animals, but it alone, of course won't preserve you remember again that that it takes horseshoe crabs ten years to reach sexual maturity, which is an unusually long period

of time for an arthropod. Alright, on that note, we're going to take a quick break. But when we come back, we will venture into the blood. Alright, we're back. All right, let's talk about blood. So one of the most astonishing things about the horseshoe crab is they're amazing blue blood.

It's literally blue. Now why would this be Humans and other mammals have red blood because of the presence of hemoglobin, which is an iron based compound that carries oxygen away from the lungs to the rest of the body and then transports carbon dioxide back the other way. Horseshoe crabs do not have hemoglobin. Instead, they have a protein called hemocyanin, which is based on copper instead of iron, and the

copper content makes the horseshoe crabs blood blue. I can't help but think of the I corp of Talos, the bronze automaton from Greek myth. But more specifically, I guess you have blue blood that shows up in science fiction like the alien opera singer in the Fifth Element we find out as blue blood. Oh yeah, like everyone else. I mainly remember multi pass um. So this hemocyanin based

blood has has some really amazing properties. So Richard Forty writes of how on this beach in Delaware when he's walking around looking at all these animals, he comes across many horseshoe crabs crawling around with signs of old wounds that look really like they should have been fatal, like a large hole punched in the middle of the head, or just part of the thorax missing, or you know, broken off tail, whatever, and the survival of such wounds may in part be due to the amazing clotting power

of horseshoe crab blood. Another anatomy fact that we didn't get into earlier was about the circulation of the horseshoe crab. Like many other arthropods, the horseshoe crab has an open circulatory system, and this is very different from our mammalian system, known of course conversely as the closed system, where blood is entirely contained within vessels. Right if you cut a person open the veins, arteries, capillaries, you have to to

rupture these containers for the blood to spill out. The horseshoe crab is is something closer to kind of like a box of free range blood. It has a heart type pump that circulates blood oxygenated blood from the gills, but then the blood sort of sluices around and bathes the creatures organs without being contained entirely within vessels. So how do horseshoe crabs survive the carnage of these mass mating battles, even having like chunks ripped out of the

thorax or holes punched in the head. Forty Rights quote such endurance as possible because the blood of Limulus polyphemus has exceptional clotting powers. The animal does not bleed to death because it's blood coagulates and walls off damaged areas. So this blood is unique, but it has also proven

quite useful to humans. Uh specifically, it's been become very important to the biomedical industry, which harvests the blood of horseshoe crabs to create what is called Limulus ambhisite ly sate or L A L, which is used to test

medical devices and pharmaceutical drugs for indotoxins. And this is because their blood contains potent amida sites which function like white blood cells, so enzymes are instantly released when they come into contact with bacteria, which is observable and less than one part per per trillion, So just a tiny drop of the blood can help spot contamination. So it's important for drug implant and environmental safety tests, and this

also includes space exploration applications as well. If you want to see some footage of horseshoe crab blood harvesting in process, you should definitely check out the nat GEO documentary series One Strange Rock. We've heard us talk about this in the past on the show excellent documentary. Wonderful footage features a number of astronauts and Will Smith narrating all of this,

but they have a section on there. It's on Disney Plus right now, so I highly recommend you check it out while you're mainlining all of the Mandalorian goodness there. In fact, it was while watching One Strange Rock that my with my family that I was reminded that this would be a great topic because my son was watching this and they were showing all these these horseshoe crabs being lined up and bled and he just immediately did

not like it. And he just gets this very stern look out his face and he says, human beings are the worst and uh, and I had to reassure him, no, Uh, these horseshoe crabs are going to be fine. Uh, you know that we're not. They're not draining them to death. They are draining them and then uh a certain portion of their blood and then releasing them into the wild. So there is some mortality. Yes, yeah, I was looking

around on this. Uh there's a paper from the Department of Fisheries and Wildlife Sciences and Horseshoe Crab Research Center. This is Virginia Polytechnic Institute and State University from LINKA Hurton. Uh, this is a this is a small study, but it it looked at them the morality the mortality rates for horseshoe crabs. It's gonna vary depending on the sex of the of the horseshoe crab and the amount of blood drawn. And also not every specimen that is collected is ultimately

deemed suitable for a draw. So the source I was looking at place male mortality rate anywhere between zero percent at ten percent of blood drawn to thirteen points six percent at thirty percent blood drawn, while females it range from zero at zero to fifteen point four percent at blood drawn. And wait, hold on, no, no, go ahead, Well wait why did they ask zero percent drawn? This comes down to the fact that it was such a small study that there were zero that word, that had

zero percent drawn on this particular study. So, like I say, the very small sample size makes these numbers, you know, not the gospel, but they give some idea of of what you're looking at here. For the females, the more standard blood draw was came with a mortality rate of ten point three, So thirty percent of a of a crabs blood is generally extracted before it is returned to its natural environment within seventy two hours and their placed

further out, usually to prevent repeat capture and draining. However, this is still a physically stressful situation. You know, not to anthropomorphize the creatures experience or anything, but the crabs take three to seven days to regain their blood volume and up to four months for those amata sites to return to baseline levels. And they're also usually harvested during spawning periods because this is when their easiest to catch.

A June nineteen study published in the University of Chicago Press looked at the stress placed on the crabs following their bleeding and how it might be uh impacting their re productive potency um so owings at All found the following. First of all, overall, biomedical bleeding may impact the reproductive output of female horseshoe crabs during the season in which

they were bled. Week one following the bleeding, bled animals appeared to spawn less than the controlled animals, and they also found that control females appeared to spawn on average four point eight times uh the rate of bled females,

which spawned on average just two times. They also found that bled animals tended to stay clear of shallow zones places that they actually needed to be for breeding purposes, and this might this might be due to disorientation in the animal following the blood draw or it also just might all of this might come down to weakness, like the creature is going to be weakened for a state of you know, a week or even months following of what's gone on here, and that may impact their reproductive

health as well. So all of this can ultimately alter the sex ratio at those breeding areas that we talked about at the top of the program, which is then going to impact reproduction overall for the species in these

areas where blood harvesting is taking place. So the harvesting of horseshoe crab blood probably has saved thousands or millions of lives over the years that it's been done, but it's still not good to be hurting these populations like this, it is, you know, and again like this is a study from just last year, so you know, we're still learning more and more about the impact as we are also looking for ways to get better, ways to get away from the use of horseshoe crab blood because there

are some synthetic solutions now, right, Yeah, there are, and we're generally we're looking at a future where we're going to depend less on it. Another thing to keep in mind is this is not the only risk factor for horseshoe crabs. It would be one thing if it were, but uh, they are also harvested for their eggs, so they can be used as bait for eels as well

as features known as welk. Yeah, forty was talking about this in his book, and the welk is some kind of conk like animal that people fishing for it have used horseshoe crabs as bait. So you know, these are two different human practices that are having varying degrees of impact on their reproductive health. And we have to come back again to the fact that this is not just some mirror outsider species. It's just left over from a

bygone age. They are keystone species. Their eggs are important food source for a number of organisms, again including migratory sea birds. So the future promises new biomedical tests as well as hopefully alternative baits for the fishing industry, and hopefully all of this will come together to ensure the long term survival of the horseshoe crab. I hope it is not the sixth extinction that wipes them out. But that's the thing, isn't it. The great sixth mass extinction

event is proving to be the human occupation of the planet. However, the one thing we have going for is is that this extinction event is largely self conscious, or at least it's self consciousness is growing. I'm going to be an optimist, and it can do things like curb it's uh, it's fishing practices. It can do things that are self reflective and hopefully sustainable. Yeah, save the limuli folks, they're scuttling masses are precious, absolutely all right. So there you have

at the horseshoe crab. Obviously, we'd love to hear from everybody, especially those of you who have you know, any firsthand experience with the horseshoe crab. If you have ever eaten the row of horseshoe crabs, let us know. We'd love to hear about that as well. In the meantime, you can find us wherever you get your podcasts. Go to stuff to Blow your Mind dot com and that'll redirect you to a place where you can find the episodes.

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