Instant Genius - The science of crabs, with Peter Davie

Episode Date: July 10, 2022

Marine ecologist Peter Davie, author of Crabs: A Global Natural History, tells us about the biology and behaviour of these crustaceans. Once you’ve mastered the basics with Instant Genius, dive deeper with Instant Genius Extra, where you’ll find longer, richer discussions about the most exciting ideas in the world of science and technology. Only available on Apple Podcasts. Hosted on Acast. See acast.com/privacy for more information. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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I'm Alice Lipscomb Southwell, the managing editor at BBC Science Focus magazine. In this episode, I talked marine taxonomist and ecologist Peter Davy, who's a crab expert and honorary research fellow at Queensland Museum in Australia. His new book, Crabs, A Global Natural History, is out now. In this episode, he tells us all about crab biology and and behaviour. So Peter, just to get us started, what exactly is a crab? Are they just marine spiders? Well, actually, no, they're not spiders at all. There are things called spider crabs, but no relation to real spiders. But they all belong to the arthropoda, the same as spiders do, which is a phylum,

big phylum and contains all the insects and everything else. So they've got a skeleton and jointed legs and things like that. But crabs, particularly, specialised group of crustaceans, and they belong to a group called the Decapoda, which is mostly all the sort of familiar crustaceans that people see and eat every day. They're the prawns, the lobsters, and Decapoda means they've got ten legs, and crabs have ten legs, although people don't think of the first pair of claws as being legs, but they're derived from legs. When the crabs first evolved then? I mean, they look pretty ancient.

Have they been around for a long time? Yeah, they have. It's still a conjecture. The oldest fossils are about 180 million years ago, but genetics suggests it could be up to 220, 2.30 million. And some of the close relatives of crabs that they evolve from, there are fossils dating back to that time. Of course, the fossil records is...

a bit lucky whether you find something or not. They started during the Marine Revolution, which was around that middle of the period after the Great Permian extinction. And when the seas got to point where they could lay down calcium and chemistry and everything, all the invertebrates seemed to get going and really started to diversify.

And the crabs popped up then. And the big advantage for crabs at that time was that the trilobites, which had been the dominant crab like arthropod that swarmed over the bottom, they were all sort of did the dinosaur extinction thing at the end of the Permian. And so there wasn't a large bottom feeding group of animals that the crabs had to compete with. So they were able to really just diversify and go for it, take over all these niches that the trilobites had taken over. before that. So how many species of crabs are there then? Well, named. There's around 7,300 species, which most people wouldn't expect. There's probably another 3,000. We're probably looking at 10,000 species

all up, because there's still a lot being discovered and described every year. The genetics, we're looking at genetics now, and not just straight morphology or obvious characteristics. And we're finding that a lot of crabs that we've previously thought were common are actually assemblages of cryptic species or closely related species. So yeah, it's an interesting, it keeps me in a job and that's what I do. I like describing them and discovering new ones. So what's the biggest crab species out there that we know of? General Wisdom says it's the Japanese spider crab, giant Japanese spider crab, lives off the coast of Japan, some in semi-deep water. It gets to, I think, nearly four metres across the

leg span, weighs 19 kilograms or something like that, is very, very impressive. But there's a close runner-up in Australia, the southern, southern giant crab, and off the coast of Tasmania and southern Australia is a different shape crab. It has much smaller legs, much tidier. But a slightly wide at carapace and it comes in at only about a kilo less. It's remarkable because its claw is the size of a human forearm. So at the other end of the scale then, what's the smallest crab species? Well, I like to lay claim to describing the smallest one. Other people might disagree, but the one I described was from a, it's called a false spider crab, lives in the mud in estuaries off North Queensland, the mangrove estuaries.

And the mature female is 1.8 millimetres across the shell. So, yeah, you know, you wouldn't notice it if you weren't sieving the mud and looking hard. Because I've seen the pea crabs that sort of parasitize other animals, don't they? And I thought they were pretty small. But, yeah, that is absolutely minute that size. Yeah, yeah, less than two millimeters, yeah. Perhaps one of the things crabs are best known for is their ability to regenerate lost limbs. Can they all do this? And how exactly do they do it?

Yeah, well, they can. Crabbs tend to lose limbs by having fights or with each other or getting fish, try and bite them off. Well, they don't fall off rocks, fall off cliffs, but they actually can, if, If they're in danger, they can, a bit like a lizard's tail, they can autotomize a limb. And they have a very sophisticated, strange system that closes down the membrane, where the leg is joined to the shell and stops the crab blood from leaking out. It's called hemacyanin, which is like a blue blood. Essentially, once it's sealed, the cells inside just go into repair.

mode and initially on the next malt you'll see a little limb bud which is just like a little finger sticking out and you can often see the curled up little leg sitting inside it and then in another one or two malts it just grows out and comes back to normal so it's very cool why is it that crabs walk sideways or can they walk forwards can walk any way they like, you know. A lot of the crabs we see most commonly are shore crabs. They live in burrows.

And the thing about crabs is once, because crabs are different from other animals, because their tail, like a normal decapod has a long tail, which it uses for swimming or jumping, but it hampers what it can do. So crabs, tails tucked in under the shell. So all you see is basically the head from above, the head and thorax. So this has allowed the crabs to take a lot of different body shapes because they haven't got a tail to worry about.

So a lot of crabs that we see live in burrows. So instead of being longer than wide, they're often wider than long, and they can run quickly and they want to run down a burrow. and the quickest ways to run down sideways. So they do run sideways, but then, you know, pea crabs and soldier crabs in Australia and decorated crabs, spider crabs, ones that live on the reef, they will move whichever. They don't live in holes and they just wander around on the reef.

They don't need to go sideways. While they can run, well, while they're famous for going sideways, the ghost crab, one of the large ghost crabs and has been measured at running four metres a second sideways, which is, you know, that's why they're hard to catch sometimes. So they hold the land speed record for crabs running sideways. Now also we think of crabs as being scavengers and we've probably all seen them on the beach

where they'll be scuttling around eating any bits of detritus, bits of most shellfish that's there. But do any of them actually actively hunt? Most of them act, well, a very large number of them actively hunt. The swimming crabs, even the green European shore crab that you have in the UK is, they will actively hunt and scrunch things. But the swimming crabs are very fast. They swim, can catch fish.

And a lot of them will sit in the bottom, sit, you know, with. just under the sand and then just pounce and grab a fish as it's going by, a very sharp pointed claws designed to hunt. One of the really interesting things about crabs and their food is their claws. And the claw shape is a really good indicator of what a crab will eat. So some of them, for example, I've got very strong, powerful claws for crunching up more. shells and some of them have got very fine sharp teeth for being predators. Some of them have got scraping things for getting algae off rocks or coral and some of them have got scooped

ones for putting the sand into their mud into their mouths. So you can often look at a crab's claws and tell what it's what it eats roughly. But having said that, if you're there's a nice bit of stuff to scavenge, most crabs will scavenge it. I mean, in the UK, I don't know if it's the same over there in Australia for you, but quite often you'll see children at the seaside and they'll be fishing for crabs using little bits of bacon, because apparently the crabs love bacon, and they'll kind of grab hold of it and have a nibble and then sort of throw them back. Crabs have little, they have two pairs of antennae, the first pair called antennials,

and they have for chemo reception and detecting scents in the water. Some species can be incredibly sensitive to taste of, well, bacon is nice and pungent, but they could detect a bit of muscle meat in about, you know, 20 Olympic swimming pools. You know, they're really switched on to coming into something that's tasty. So speaking again about those claws, and you said they've all got different claw shapes, and from that we can tell what they eat. Why is it that a lot of crabs have one front claw that's bigger than the other? There's various reasons.

They don't all. Some have same sort of on both. It's called heterokely. So, for example, the box crab, which is one you don't get in Europe, but it specialises in eating gastropod snail shells. So on one claw it's got a little peg-like hook, which it hooks into the aperture of the snail and then gradually breaks away the aperture

until it gets down to the animal and the other claw is much finer and has much sharper, longer pointed fingers which it uses to reach in to get, pull the meat out of the shell. So there's good functional reasons sometimes like that. There's also males often have a one, very large claw and that can be for fighting for displaying to the females that they're handsome and in

many crabs they'll they'll actually before mating they'll hold the female with with one large claw so that it can't get away so it's like an embrace or a hug so you said there that the crabs can communicate with each other with by you know using that claw to show off that hey I'm a big man I'm you know, I'm handsome. But are there other ways that crabs can communicate with each other? Yeah, there are. I mean, not all crabs do that we know. The most common one, and it's quite widespread amongst quite a few different groups, is called stridulation, which is what cicadas and crickets and things do, where they rub one body part against another to make a distinctive sound. So that's, and that's, you know, it can be rubbing the claw against the other claw

or it can be rubbing a side of its wrist against the eye socket or, you know, it can take many different forms. It's evolved in different ways, in different species. They've recently discovered more remarkably that when crabs eat, they have what's called a gastric mill in the stomach, which is a central plate with two side plates that grind the food as it's fed in and mulch it up. So they've recently found a number of different groups that actually make stomach growls and they communicate with growling from their stomachs.

So it's almost a vocalisation. They can convey quite a lot of information like there's one group that, depending on the deepness of its voice. Other crabs will know how big it is and they'll know that it's discovered some food and they'll know where it is so they can come in. Yeah, it's really quite complex and unexpected. So on a similar note, how smart are crabs?

Do we know anything about their intelligence? We haven't really realised how smart they are up until fairly recent times. I think it varies a lot. Some crabs are smarter than others. There's been some experiments done in the UK relatively recently of the green shore crab. And they put it in a maze, pretty much like a mouse or a rat maze that they use for experimenting. And they put some quite a complicated maze.

and they put a bit of muscle clam at the end of the maze. And once a week they would put the crabs in. Initially it took them a long time to get to the end. But every time they put them in, they'd get quicker and quicker until they learned to go straight there. They then took them out and let them have a rest for a few weeks and put them back in. No food there.

Put them back into the maze. and they went unerringly straight to where the food should have been. So they can work out how to get through a complex maze and they can remember for at least weeks at a time how to get there with an anticipation rather than something. You know, they're not even smelling it. So that's pretty smart. And I think they've started to work out that crabs intertidal.

a number of intertidal crabs have a good intelligence and geographic mapping of their environment so that they can work out where the predators are and under which rocks the octopus lives and work out their environment with quite sophisticated ways. So, yeah, they shouldn't underestimate them. Now, you touched on this before when you said that the male will guard the female when they're mating. Now, the mating habits and crabs are quite interesting. because as well as the male guard and the female,

the female will also hold onto the eggs until they hatch, which I think is quite surprising because we think of crabs being invertebrates as the sort of animals that will just broadcast spawn, they'll just send their eggs and sperm everywhere. But there's actually quite a lot of care going on there, isn't there? Ah, well, they mate. They're much like other animals. They have a pair, the males have a pair of, well, the core gonobes,

which are actually modified swimmerats, which are, you know, the sorts of things you see on a prawn. And they have a pair of these under their abdomen. And they're inserted into the females, a pair of receptacles on the females underneath on her thorax. So they have some quite complex behaviour, mating behaviours. They often will only mate when the female. male has just malted so she's soft and they can detect when the female's going to malt before she does. There must be some pheromones or some hormones in the water and they will find the female

that's going to malt and they will as I said before they'll they'll give her a hug and wrap her up until she melts and then they'll roll her over and you know like a missionary style mating happens where the the male abdomen with its gonopods is inserted down under her wider flap. So it's, and she benefits because I think he's protecting her from predators too, while they're vulnerable through their mating. So that's the sort of physical process. As she lays her eggs, they're inseminated by the sperm that he's deposited.

and then they develop under the abdomen as eggs, which she tends and looks after. I mean, I think that's quite surprising because I think generally people assume that invertebrates is all external spawning and then they'll just sort of dump the eggs and leave them. But there's quite a lot of, I suppose that's quite a lot of care going into that almost, isn't it? But the female keeping hold of them is quite an interesting sort of procedure, really. And it's one of the specialties that crabs particularly have evolved is actually there's various lineages of crabs. And the more advanced, the more evolutionarily advanced crabs that have what basically turned the mating into internal fertilization,

which is so they're not just scattering things around, they're not wasting their energy. like a lot of invertebrates by just letting them go into the plankton and and running the risk that they'll be wasted. So it's been an interesting, studying the evolution of crabs and seeing that process change from the primitive crabs into the more advanced ones has been a really important discovery in crab, an understanding crab evolution. And it gets more interesting than that

because some species actually have quite remarkable maternal care with some females will actually tend to brood and even like in rainforests they'll lay the eggs into little leaf axles or water pools and bromeliads and they'll protect the growing little crabs and keep the mayflies or the dragonflies away or the spiders.

They'll put calcium, you know, they'll put little shells in the water to make it more calcium rich. And, you know, they're quite extraordinary behaviour in looking after their offspring. So once those eggs hatch out, what do the baby crabs look like? Are they a larva or are they like tiny little versions of the adults? No, they look nothing like the adults. There's a lot of different strategies. It's sort of classic what they call R&K selection.

So some crabs will produce only a few large eggs and spend a lot of energy looking after them for their survival. Others will produce millions of eggs and send them on their way into the plankton. Typically they will progress from, maybe up to five, six, seven larval stages where they just keep molting. So the classic scenario is the eggs will hatch into their first larva. They go up into the plankton.

They eat phytoplankton or the plant plankton. And as they grow, they eat other larvae from other. invertebrates and and each larval malt changes they change slightly but they still look much the same until till the last stage which is called the megalopa which is the last level stage where the where it drops out of the plankton onto the bottom and that looks at halfway between a larva and a small crab, and that starts to feed on the bottom and then mults into a juvenile crab, the first crab stage, which looks fairly much like the adult. So how long do crabs live for then? Probably the shortest lifespan, and some of the small crabs might live for less than a year.

A lot of the commercial crabs might reach maturity within a couple of years, three, two or three years, depends on where they are, how cold the water is, how much food there is and seasonal things like that. The longest, or the Japanese spider crab, the giant spider crab, is reputed to live up to 100 years old. I'm not sure if that's been proven, but it seems quite possible. They have two different strategies in school, determinate growth and indeterminate growth. The determinant growth means there's only a certain number of mults they'll have before they before they'll die. So they'll molt through to a maximum size and then they'll just get, they won't molt anymore. They'll get crusted over with barnacles and things will start growing on

them and then eventually they'll just pass away. But then we think that well we know that others just keep molting and have no terminal you know no physical count

of how many malts are going to do. They obviously age and we'll die eventually of old age but potentially the maximum size is a little bit unpredictable. And it's usually in the colder waters where they tend to live longer because they grow slower and live longer.

So you've got crabs living in cold parts of the sea, you've got crabs living in the rainforest. How have they managed to occupy all these different niches? And is there anywhere where they can't survive? Well, that's the beauty of being a crab. That's why having that crab shape has allowed them to be really elastic or plastic. in their shape, in their legs, they can get long legs, short legs. They've just been able to really explode into every niche that's available because they've sort of got that flexibility and they're living on the bottom.

So as Coral Reaves grew and got more complex, they would take up new niches in that. There's a whole swag of crabs that are specialist in, like you mentioned earlier, in symbiotic relationships with, like, pea crabs with bivalve mussels and other ones that form little gulls in coral and other ones that live inside the bottoms of sea cucumbers. And, you know, wherever there's a space and an opportunity, they seem to be able to grab it. So even, I mean, the greatest diversity is in the seawater, but even as much as 100 million years ago, they invaded fresh water,

went up through the estuaries into fresh water, and then from there they conquered the intertidal, were able to, physiologically able to adapt to the range of temperatures and being exposed and not having water. over them all the time and then into terrestrial environments so that almost all crabs still have to come back to to to the sea though to to release their larvae and release their eggs not all but this is a few families of true freshwater crabs that that have that larval development occurs directly within the egg under the abdomen and they hatch as baby crabs.

And that's because in freshwater they can't afford to let their, they need to stay where the fresh water is. They can't let it get washed away. So yeah, once they managed to breathe air physiologically, they meant they could go up into the rainforests and up mountains. And I think the only place they probably struggle with is ice. There's an Australian desert crab that lives in the desert. It can extirate with a little clutch of babies under its abdomen

for up to six, seven, eight years before it rains. The rain come down and it goes, hey, pops up. And suddenly there's baby crabs running everywhere. and they grow very quickly to take advantage of the rain period and then they dig down and they're gone again. So what sort of threats are crabs experiencing at the moment? Well, pretty much the same as everything. Global warming and issues with ocean acidification.

We often talk about coral reefs as just being about coral. You know, we're going to lose the coral, but we're not just going to lose the coral, we're going to lose huge communities of invertebrates and crabs and fish and everything that is associated with coral reefs. So there's certainly an issue with that, and potentially with the changes in ocean currents from climate change, you'll be having problems with the way the larvae circulate. and distribute. There's acidification

is causing still already been noted as causing problems with crab larvae because the larvae are particularly sensitive to chemistry in the pH and development

and the little sensory seedy and things on that they swim and detect keep alive with being damaged by the water being too acidic. Yeah there's there's a lot of potential issues and certainly with the freshwater crabs, true freshwater crabs, are very, very diverse.

One of the biggest groups changes in climate and rainfall and deserts. They're probably under the most immediate threat if you have periods where they're not adapted, where they're expecting a seasonal monsoon and it doesn't come. They're potentially very vulnerable and the number of them have been listed on the IUCN red list already. So why are crabs so important? Why should people care about this? Well, most people don't care, but they should. Crabbs play a huge role in the ecology of marine and terrestrial, but perhaps more marine.

and intertidal environments. Crab larvae, for example, like a single large mud crab, Indo-West Pacific mud crab, commercial mud crab, can have up to 6 to 8 million eggs per female. And there's a lot of them, and they're pumping all these larvae into the phytoplankton.

And the phyto plankton is where all the fish larvae are and just from that perspective, they're driving a whole ocean ecosystem or at least playing a significant role in driving marine food webs. And then if you go into specifics, there can be keystone species in the number of different environments.

In mangrove habitats, the burrowing down into the mud brings fresh sea water down into the base of the mangrove roots and they eat up to 80% of the mangrove leaf fall and turn that into into organic detritus and substances that are then available to the other invertebrates and the worms and so and then on to the to the prawns and the fish in the nursery areas that mangroves provide so they're soldier crabs that live on the east coast of Australia hundreds of thousands of them come out each tide

and sift the sand through their mouth parts and get all the little organic matter out to eat so they can turn over the surface of a beach every few days there's just so many ways And again, on coral reefs, there's a whole suite of crabs that scrape away algae. There's been a lot of studies done on fish that keep the algae from smothering the coral. But the crabs are potentially playing is just as an important role in stopping the algae from growing and smothering coral.

So, you know, wherever you look, crabs are playing a role, but very much underestimated. Thank you for listening to this episode of Instant Genius. That was crab expert, Peter Davy. To hear him tell me even more about crabs and some of the planet's weirdest crab species, head over to the Instant Genius Extra podcast now. The latest issue of BBC Science Focus magazine is out now. Pick up a copy in store or visit ScienceFocus.com. This podcast is sponsored by Name, Audio and Focal.

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