Tag Archives: crabs

Episode 182: The Coconut Crab and Friends



Join us this week for some interesting crabs! Thanks to Charles for suggesting the aethra crab!

Aethra crabs look like little rocks, although some people (who must be REALLY hungry) think they look like potato chips:

A hermit crab using a light bulb bottom as an inadequate shell:

The tiniest hermit crab:

Gimme shell pls:

THE BIGGEST HERMIT CRAB, the coconut crab. It really is this big:

Show transcript:

Welcome to Strange Animals Podcast. I’m your host, Kate Shaw.

We have a bunch of crustaceans this week! I’m sorry I’ve taken so long to get to Charles’s suggestion of aethra crabs, so we’ll start with those.

There are four species of aethra crabs alive today, and they live in warm, shallow coastal waters. They like areas with lots of rocks on the sea floor, because the crabs look like small flattened rocks. They can tuck their legs under their carapace so that they don’t show at all, and often algae and other marine animals like barnacles will attach to the carapace, increasing the crab’s resemblance to a little rock. What eats rocks? Nothing eats rocks! So the aethra crab is safe as long as it stays put with its legs hidden. It lives throughout much of the world’s tropical oceans, especially around islands and reefs in South Asia, but also around Australia, Mexico, and Hawaii.

We don’t know a whole lot about aethra crabs, not even how many species there really are. There are probably undiscovered species that no one has studied yet, but we do know they used to be even more widespread than they are today. Twelve million years ago, for instance, a species of aethra crab lived in what is now Ukraine, with fossil remains only described in 2018.

Most aethra crabs only grow a few inches across, or maybe 6 cm, but the walking rock crab of Mexico can grow to 6.3 inches across, or 16 cm across. It’s light brown with lighter and darker speckles that give it a mottled appearance like a small rock.

Because they’re so flattened with rounded edges, and because some species are pale in color, aethra crabs are sometimes called potato chip crabs. I don’t like that name because it makes them sound tasty and not like little rocks. I think we have established that they really look like little rocks.

That’s just about all I can find out about the aethra crab, so if you’re thinking of going into biology and aren’t sure what subject to study, may I suggest you focus your attention on the aethra crab and bring knowledge about them to the world.

So let’s move on to a different type of crab, the hermit crab. A big part of being a crab is evolving ways to not be eaten. I mean, that’s what every animal wants but crabs have some novel ways of accomplishing it. Some crabs look like tiny rocks, some crabs hide in shells discarded by other animals.

There are hundreds of hermit crab species, which are generally grouped as marine hermit crabs and land hermit crabs. There’s also a single freshwater hermit crab that lives on a single island, Espiritu Santo, in the south Pacific, and in fact only in a single pool on that island. It was only described in 1990 and is small, less than an inch long, or about two and a half cm. It uses the discarded shells of a snail that also lives in its pool.

That’s the big thing about a hermit crab: it uses the shells of other animals as a temporary home. Like all crabs, the hermit crab is an invertebrate with an exoskeleton. But unlike most crabs, its abdomen isn’t armored. Instead it’s soft and vulnerable, but that’s okay because most of the time it’s protected by a shell that the crab wears. In most species the abdomen is actually curved in a spiral shape to better fit into most shells.

When a hermit crab finds an empty shell, it may quickly slip out of its current shell and into the new shell to see if it’s a good fit. Ideally the shell is big enough for the crab to hide in completely, but not so big that it’s awkward for the crab to carry around. If it likes the new shell it will abandon the old shell, but if it doesn’t like the new shell it will just go back to its old one. But the important thing is that it has a shell, so it spends as little time without a shell as possible. In fact, if it can’t find a shell of the right size, a hermit crab will make do with anything it can find, such as a plastic bottle, an old tin can, or other trash. But it’s safest inside a real shell. Sometimes two hermit crabs of about the same size will fight over a shell. You wouldn’t think that the ability of a hermit crab to find a good shell would be something humans can affect, but in some areas, so many shells are collected to sell as souvenirs that hermit crabs really don’t have very many left to choose from and have to use trash or pieces of driftwood instead.

Other than the freshwater hermit crab, marine hermit crabs all live in the ocean. Some species live in shallow water, others in deep water, and often around reefs. There are even a few species that are specialized to live in permanent structures on the sea floor, such as sponges or the abandoned burrows of various worms. Land hermit crabs spend most of their time on land, although they have to keep their gills wet.

People sometimes keep hermit crabs as pets, either in an aquarium for marine species, or a special terrarium for land species. Some species can live for decades if given proper care. Because a pet hermit crab is safe, it doesn’t really matter what kind of shell it wears as long as it’s comfortable, so people will sometimes give their pets imitation shells that are clear so they can see the crab’s interesting-shaped abdomen. You can also get fake shells that are shaped like skulls or tiny houses. There’s a picture that goes around sometimes online of a hermit crab using a real human skull as a shell, but that’s actually fake. Not only is the skull not real, the hermit crab isn’t real. It’s a sculpture.

The biggest species of hermit crab is the coconut crab, also sometimes called the robber crab since when it finds something that might be food, it will carry it away to investigate it. It’s not just the biggest hermit crab, it’s the biggest arthropod that lives on land. An arthropod is any invertebrate with an exoskeleton and segmented body. That includes all insects and crustaceans and arachnids, and so on.

The coconut crab has a legspan over three feet across, or about a meter. It can weigh up to nine pounds, or 4 kg. Researchers think it’s literally as big as an arthropod can grow these days and continue to live on land. It’s a bulky, strong crab that ranges in color from reddish-orange or brown to blue-gray, sometimes with white markings.

The coconut crab uses shells as protection when it’s young, but as it grows larger, it outgrows most shells available. Instead, it develops a tough exoskeleton on its abdomen. It also develops lungs, so an adult coconut crab can actually drown if it gets trapped underwater for long enough—generally about an hour. It still has gills, but they’re tiny and not very efficient.

Its lungs aren’t like those of most other arthropods. In fact it only has one lung, called a branchiostegal lung, that has traits of true lungs but also traits of gills. It doesn’t breathe like vertebrates do; instead, its lung absorbs oxygen from the air passively. To do this properly, though, the lung tissue needs to be moist. A coconut crab uses its hindmost pair of legs to dip water up and wipe it over the lung tissue, which is inside a cavity in the cephalothorax. This is the main part of the body as opposed to the abdomen. This last pair of legs is tiny compared to the other eight legs, and female coconut crabs also use these legs to tend their eggs. Usually the last pair of legs aren’t even visible, since the crab usually keeps them tucked in the lung cavity. The other legs are much larger, and the first pair of legs ends in claws like other crabs.

The coconut crab lives on lots of islands in the Indian and Pacific Oceans and used to live in Australia and on many more islands. But it’s a big crab and that means it provides a lot of food, so humans have hunted it to extinction in many areas. It’s increasingly rare in many places as a result of hunting and habitat loss. But the coconut crab isn’t helpless. If a coconut crab snaps its pincers on, for instance, a person’s thumb, it will hold on tenaciously, probably while the person flails around in panic and pain. Not only that, but sometimes a population of coconut crabs will feed on plants that contain toxins, such as the sea mango, and will retain the toxins in its body. If a person eats a toxic crab, they may get sick from the poison.

It’s called the coconut crab because it eats coconuts, but it actually doesn’t prefer coconuts. It especially likes bananas. It also eats seeds, nuts, and other plant material, but it’s an omnivore and will eat carrion, other crabs, baby turtles, and even birds. Its antennae have evolved to detect chemicals in the air instead of in the water, which means it has a good sense of smell and can track the smell of rotting fruit or meat from a long distance away.

Even the biggest crabs can climb well and will climb trees, sometimes to get away from potential predators, but sometimes to catch birds. The quickest way to get out of a tree after climbing it is just by falling, and the coconut crab often does this on purpose. Its exoskeleton is so tough that it can fall some 15 feet, or 4.5 meters, without injury. And yes, sometimes a coconut crab will use their claws to break into a coconut to eat it, but it takes a long time—sometimes days. The coconut crab is mostly nocturnal, but it will come out during the day if it’s hungry, especially if it’s raining or foggy out.

A female coconut crab glues her fertilized eggs under her abdomen and carries them around for a few months as they develop. When they’re ready to hatch, she releases them into the ocean. After they hatch, the larvae drift around for several weeks, eating tiny specks of food. As a baby coconut crab grows and develops through its juvenile stages, which generally takes several weeks, it finally settles to the sea floor and finds a shell to hide in, just like other hermit crabs do. If it can’t make it to shore on its own, it will climb onto a floating log or bunch of floating seaweed or a floating coconut, which eventually carries it to shore. It needs to be on shore because only the larvae can swim, and once it reaches its adult stage it has to breathe air.

Like other arthropods, the coconut crab has to molt its exoskeleton periodically as it grows, since the exoskeleton can’t grow. After it molts, it takes up to three weeks for the new exoskeleton to harden. During this time the crab hides in a burrow it digs, because even a gigantic coconut crab is soft and vulnerable without its exoskeleton. It lines its burrow with coconut fibers, which absorb water and helps keep the crab’s lung tissue moist while it rests. The crab will also stop up the entrance to its burrow with one of its claws, to help keep it safe and reduce the loss of moisture from the burrow. The coconut crab continues to grow throughout its life, which can be extremely long—more than sixty years. A big coconut crab’s only predator is people, and frankly I would not want to tangle with one.

Let’s finish the episode with a mystery crab. Wallowa Lake in Oregon, in the United States, is about three and a half miles long, or 5.6 km, and three-quarters of a mile across, or 1.2 km, and is 300 feet at its deepest point, or 91 meters. After gold was found in the area in the late 19th century, the Wallowa band of the Nez Perce was forced out of their ancestral home by the U.S. government, despite the treaties in place to stop that kind of thing happening. Funny how often that happens. Anyway, the gold rushers who moved in spread stories about giant crabs that lived in the lake, which would crawl out at night to grab cattle and pull them into the water to eat.

But the lake was created from melting glaciers near the end of the Pleistocene ice ages, around 11,000 years ago. It’s never been connected to the ocean and is in fact 4,300 feet above sea level, or 1,300 m. It’s also in a part of the world that experiences bitterly cold winters. All freshwater crabs are tropical or subtropical and can’t survive in cold water. Plus, of course, even the biggest coconut crab isn’t big enough to drag a cow into the water.

So the Wallowa Lake crabs are probably just tall tales. But, you know, maybe be careful if you go swimming in the lake at night, just in case.

You can find Strange Animals Podcast online at strangeanimalspodcast.blubrry.net. That’s blueberry without any E’s. If you have questions, comments, or suggestions for future episodes, email us at strangeanimalspodcast@gmail.com. If you like the podcast and want to help us out, leave a rating and review on Apple Podcasts or wherever you listen to podcasts. We also have a Patreon at patreon.com/strangeanimalspodcast if you’d like to support us that way.

Thanks for listening!


Episode 141: Zombie Animals



We’re inching closer to Halloween and it’s getting spookier out there! This week let’s learn about some animals that get zombified for various reasons. This is an icky episode, so you might not want to snack while you’re listening. Thanks to Sylvan for the suggestion about the loxo and mud crabs!

Further reading:

Zombie Crabs!

Ladybird made into ‘zombie’ bodyguard by parasitic wasp

A mud crab held by a dangerous wizard:

A paralyzed ladybug sitting on a parasitic wasp cocoon:

A cat and a rodent:

Show transcript:

Welcome to Strange Animals Podcast. I’m your host, Kate Shaw.

It’s another week closer to Halloween, so watch out for ghosts and goblins and zombie animals! Zombie animals?! Yes, that’s this week’s topic! Thanks to Sylvan for suggesting the loxo parasite, which we’ll talk about first. Brace yourself, everyone, because it’s about to get icky!

Before we learn about loxo, let’s learn about the mud crab, for reasons that will shortly become clear. Mud crab is the term for a whole lot of small crabs that live in shallow water, mostly in the Atlantic or eastern Pacific Oceans but sometimes in lakes and other fresh water near the ocean, depending on the species. Most are less than an inch long, or under about 30 mm. The largest is called the black-fingered mud crab, which grows to as much as an inch and a half long, or 4 cm. Most mud crabs are scavengers, eating anything they come across, but the black-fingered mud crab will hunt hermit crabs, grabbing their little legs and yanking them right out of their shells. It also uses its strong claws to crack the shells of oysters.

Loxothylacus panopaei is actually a type of barnacle. You know, the little arthropods that fasten themselves to ships and whales and things. But loxo, as it’s called, doesn’t look a bit like those barnacles except in its larval stages. After it hatches, it passes through two larval stages; during the first stage, it molts four times in only two days as it grows rapidly.

Then, during the cyprid larval stage, the microscopic loxo searches for a place to live. The male remains free-swimming but the female cyprid larva is looking for a mud crab. She enters the crab’s body through its gills and waits for it to molt its exoskeleton, during which time she metamorphoses into what’s called a kentrogon, basically a larva with a pointy end. As soon as the crab molts its exoskeleton, the female loxo uses her pointy end, called a stylet, to stab a hole in the crab’s unprotected body. Then she injects parasitic material that actually seems to be the important part of herself, which enters the crab’s blood—called hemolymph in arthropods like crabs. Like most invertebrates, crabs don’t have blood vessels. The hemolymph circulates throughout the inside of the body, coming into direct contact with tissues and organs. This means that once the loxo has infiltrated the hemolymph, she has access to all parts of the crab’s body.

At this stage, the loxo matures into something that isn’t anything like a barnacle, but is an awful lot like something from a horror movie. She grows throughout the crab, forming rootlets that merge with the crab’s body and changes them. Basically, the female loxo becomes part of her crab host. Eventually she controls its nervous system and molds it to her own needs. She even molds the body to her own needs, since if she’s parasitized a male crab she has to widen its body cavity so it can hold her eggs.

The crab stops being able to reproduce and doesn’t want to. It only wants to care for the eggs that the female loxo produces. She extrudes an egg sac so that it hangs beneath the crab’s abdomen, where a male loxo can fertilize it when he swims by. The crab then treats the egg sac as if it contains its own eggs, protecting them and making sure they get plenty of oxygenated water. This is true even for male crabs, which ordinarily don’t take part in protecting their own eggs. The loxo eggs hatch in about a week, and as soon as they do, the female loxo inhabiting the crab starts the process over again. While a mud crab in the wild can live for a few years, once it’s taken over by the loxo parasite it only lives around 45 days.

Most mud crab populations are reasonably resistant to the parasite, but where the loxo has been introduced to areas where it didn’t live before, it can decimate the local mud crab population. This happened in Chesapeake Bay in the 1960s in North America. The local oysters had been so over-fished that they were nearly completely gone, also nearly destroying the local oyster industry. They imported oysters from the Gulf of Mexico to replenish local stocks, but no one realized they were bringing the loxo with those oysters. These days, up to 90% of the Chesapeake Bay mud crabs are infected with the loxo parasite, while only up to 5% of the Gulf of Mexico mud crabs are infected. Researchers at the Chesapeake Bay Parasite Project are working to figure out more about how the loxo infiltrates its host and changes it genetically, and are monitoring infection rates in the wild.

If you think that’s gross, it’s not going to get any better the rest of this episode.

Next let’s learn about another zombie animal, this one a spider. A number of spiders are parasitized by a tiny wasp called Zatypota percontatoria. It lives throughout much of the northern hemisphere and prefers forested areas with plenty of web-building spiders in the family Theridiidae, also known as cobweb spiders.

Cobweb spiders are really common with around 3,000 species that live throughout the world, including the black widow, which by the way is not nearly as dangerous as people think. Some cobweb spiders are kleptoparasites, which means they steal food and other resources from another animal, in this case larger spiders. A kleptoparasite cobweb spider actually lives in the web of a larger spider, and when a small bug gets caught in the web, it steals it. Sometimes the cobweb spider will kill and eat the spider that built the web in the first place too.

But most cobweb spiders are ordinary spiders, and most are quite small, usually only a few millimeters long. Many are marked with pretty patterns in brown, white, black, and other colors. Different species build different kinds of webs, but they all eat small insects.

As for the wasp, it’s about the same size as the spider it’s trying to parasitize, and sometimes smaller. It has long wings, long antennae, and a long abdomen that in the female ends in a sharp ovipositor. The female finds a spider, usually a young spider that’s less able to defend itself, and stabs it in the abdomen with her ovipositor. Then she lays a single egg inside the spider and flies away.

The egg doesn’t bother the spider, although once the egg hatches into a larva it starts to feed on the spider’s hemolymph. Remember, that’s the equivalent of blood in the invertebrate world. At the same time, it’s releasing hormones into the spider that change its habits. Basically the wasp larva controls the spider so that it acts to the benefit of the larva, not itself.

All this takes about a month. When the larva is ready to pupate and metamorphose into an adult wasp, it secretes a final hormone that influences the spider’s behavior. This one causes the spider to spin a strong, cocoon-like web. When the web is finished, the larva bursts out of the spider’s body, killing it, and eats the spider. Then it enters the cocoon and develops into an adult wasp.

Because spiders are good at defending themselves, only about 1% of spiders end up parasitized. I’m sure the spiders think that’s 1% too many. There are other parasitic wasp species in other places, but they all act about the same as Zatypota.

Another wasp, Dinocampus coccinellae, parasitizes ladybugs. Like Zatypota, the female wasp lays one egg in the ladybug’s body. When it hatches, the larva eats the ladybug’s insides while the ladybug continues to go about its ordinary activities. But after several weeks, the larva is ready to pupate. It paralyzes the ladybug, bursts out of its body, and spins a cocoon that the ladybug sits on.

But the ladybug isn’t dead. It protects the cocoon from other insects by twitching and making grasping motions with its legs.

After about a week, the adult wasp emerges from its cocoon and flies away. The ladybug usually dies, but not always. About a quarter of infected ladybugs recover and are fine. Researchers aren’t sure how the wasp larva causes the paralysis. It may release a virus that infects the ladybug or it may have something to do with venom released by the larva.

This wouldn’t be a proper zombie episode if I didn’t talk about that disgusting parasitic fungus that affects certain carpenter ants in the rainforests in Brazil and Thailand. It completely squicks me out so I’m going to explain it very, very quickly.

Fungal spores float through the air and land on an ant, where they stick. They release enzymes that eventually break down the ant’s exoskeleton, allowing the fungus to spread inside the ant’s body. Finally it’s able to control the ant and makes it crawl up the stem of a plant and bite into a leaf vein. The ant is unable to move at this point and eventually dies. The fungus sprouts from inside the ant and grows into stalks, especially from the ant’s head. About a week later it releases spores that go on to infect other ants. Ugh. So glad I’m not an ant.

Ants can sense when one of the colony has contracted the fungus, and will carry the infected ant far away from the colony so it’s less likely to infect others. The ants also groom each other to remove any spores that may have attached. The fungus can completely destroy ant colonies, but it has a parasite of its own, another fungus that stops the first fungus from releasing spores. A related parasitic fungus also infects certain caterpillars.

Look, I’m totally over talking about fungus, so let’s move on.

So is there any chance that a parasite will turn you into a zombie? There’s not, but a behavior-changing parasite does sometimes infect humans. It’s called Toxoplasma gondii, and while its effects on human behavior has been studied extensively, the effects are so minor as to be nearly nonexistent in most cases.

Toxoplasmosis is a disease caused by a single-celled parasite, and it’s one that not only infects humans, it’s really common. I probably have it but I’m not going to think too hard about that. For most people, it never bothers them and never causes any symptoms, or only mild short-term symptoms like a lowgrade cold that takes a few weeks to clear up. But it can be more serious in people with a suppressed or weak immune system, and can cause problems for the baby if its mother gets infected while she’s pregnant.

There are estimates that up to half the people in the world are infected with toxoplasmosis but never know. The reason it’s so common is that the parasite targets cats, and can be spread in cat feces. And, you know, if you scoop out the cat’s litter box you might be exposed. That’s why pregnant women shouldn’t clean up after a cat. Infection can also result from eating undercooked meat from an infected animal, eating unwashed fruit or vegetables, drinking unpasteurized milk, and drinking untreated water.

Any mammal or bird can contract the parasite, but it can only reproduce in a cat’s digestive system. It doesn’t hurt the cat, it just wants to get inside the cat so it can reproduce. And the best way to get inside a cat is to be part of a rodent that a cat eats.

When a rat or other rodent is infected with Toxoplasma gondii, its behavior changes. Suddenly, it starts to like cats. You can probably see where this is going. Not only does it stop avoiding cats, it actually seeks them out. The cat, naturally, can’t believe its luck, kills and eats the rodent, and may become infected.

If you have a pet cat, the best way to reduce the risk of contracting toxoplasmosis is to scoop the litter box daily, then wash your hands. It takes about a day for the parasite to become active after being shed in cat poop, so if you scoop the litter box right away the risk is lower. Researchers are working on vaccines, and they’ve actually already developed a vaccine that’s now used in sheep. If you keep your cat inside, where it’s safer anyway, it’s much less likely to be exposed to the parasite in the first place.

So, take ordinary precautions but don’t worry too much about toxoplasmosis. Unless, of course, you are a rodent.

You can find Strange Animals Podcast online at strangeanimalspodcast.blubrry.net. That’s blueberry without any E’s. If you have questions, comments, or suggestions for future episodes, email us at strangeanimalspodcast@gmail.com. We also have a Patreon if you’d like to support us that way.

Thanks for listening!