Tag Archives: sea slug

Episode 287: Sand Crabs, Sea Slugs, and a Mystery Octopus

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It’s INVERTEBRATE AUGUST! Thanks to Elizabeth, Richard, and Llewelly for their suggestions this week!

Further reading:

Meet Phylliroe: the sea slug that looks and swims like a fish

Hey, so these sea slugs decapitate themselves and grow new bodies

Found, Then Lost, Then Found Again: Scientists Have Rediscovered the Sand Octopus

A sand crab in the air:

Sand crabs in the water, feeding:

Phylliroe is a sea slug that looks like a fish (pictures from article linked to above):

How I used to draw snails when I was a kid, adding an extra foot because I didn’t understand that the “foot” of a snail/slug is the flat part of the body that touches the ground:

The mysterious sand octopus in mid-swim:

Show transcript:

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

It’s the first week of invertebrate August and we’re heading to the ocean for our first episode! Let’s jump right in with an episode about sand crabs, a couple of sea slugs, and an octopus mystery that was recently solved. Thanks to Elizabeth, my brother Richard, and Llewelly for their suggestions!

We’ll start with Elizabeth’s suggestion. The sand crab is also called the sand bug, the mole crab, or similar names that refer to its habit of burrowing into the sand. It’s common throughout much of the world’s oceans, especially in warm areas, and can be extremely numerous. It’s also sometimes called the sand flea, but it’s not the kind of tiny jumping crustacean that bites, also called the sand flea. This little crustacean is harmless to humans. It doesn’t even have pincers.

The sand crab isn’t a true crab although it is closely related to them. It’s gray-brown and has a tough carapace to protect it when it’s washed around by waves and to help protect it from predators. Females are larger than males and can grow up to an inch and a half long in the largest species, or about 35 mm, and an inch wide, or 25 mm. So it’s longer than it is wide, unlike most crabs, and its carapace is domed sort of like a tiny tortoise shell. Overall, it’s shaped sort of like a streamlined barrel. I saw one site that called it the sand cicada and it is actually about the same size and shape as a cicada, which it isn’t related to at all except that they’re both invertebrates. Some species have little spines on the carapace while others are smooth.

The sand crab lives in the ocean, specifically in the intertidal zone right at the area where waves wash up on the beach. This is called the swash, by the way, which is a great word. The sand crab burrows into the sand tail-first, using its strong rear legs, and during the time that there’s water over the sand, it unfurls its feathery antennae to filter tiny food particles from the water. When the wave goes out, it retracts its antennae and works on staying buried in the sand as the next wave rolls in.

In some species, males are very similar to females, but smaller. In other species, they’re tiny, barely 3 mm long at most, and even as adults they resemble larvae. The male finds a female and grabs hold of her leg, and there he stays. I tried to find out more about this, but it doesn’t look like the humble sand crab gets a lot of attention. If you’re interested in becoming a scientist who studies invertebrates and you want to spend a lot of time on the beach, the sand crab would make a good study buddy.

Lots of fish and birds eat sand crabs, and people do too. In many places they’re considered a delicacy and grilled as a snack. This isn’t surprising since they’re related to other crustaceans people like to eat, like crabs and lobsters.

Next, let’s learn about two strange sea slugs. We’ve talked about sea slugs a few times before, including in episodes 215 and 129, but there are a lot of species, with more being discovered pretty often.

Llewelly sent me a link ages ago about a sea slug that’s related to the sea bunny, which we talked about in the cutest invertebrates episode, 215. It’s called Phylliroe and doesn’t look like a little bunny or a slug. It looks like a fish.

Phylliroe grows a few inches long at most, or 5 cm, and the article Llewelly sent, which I’ve linked to in the show notes, points out that it’s about the size of a goldfish. Its rear end is shaped roughly like a fish tail, which it uses just like a fish tail to propel itself through the water. It’s probable that Phylliroe’s shape doesn’t have anything to do with disguising it, but instead is just the result of convergent evolution. A body streamlined to move through the water with minimal resistance is always going to be fish-shaped, because that’s why fish are shaped the way they are. The fish-like tail is also an efficient way to move through the water relatively quickly.

Phylliroe mostly eats tiny jellyfish, which it grabs with its small foot. It doesn’t need a big flat foot to glide on, since it doesn’t live on the sea floor like some of its relations, so over many, many generations its foot has become smaller and smaller until it’s just a little tiny foot near its mouth. It’s still sticky, though, which means jellies stick to it, which means it’s easier for Phylliroe to eat the jellies.

Phylliroe is mostly see-through, although you can see its digestive system. It also has two so-called horns, called rhinophores, that it probably uses to sense the chemical signature of its prey in the water. If you remember the sea bunny, its rhinophores look like bunny ears. Phylliroe’s look more like thick antennae or barbels. Phylliroe also exhibits bioluminescence, which is not a typical trait for a sea slug.

My brother Richard alerted me to another sea slug a while back, this one referred to as the Deadpool slug. The reason why it’s called the Deadpool slug is lost on me because I haven’t seen that movie or read the comic book, but the sea slug can separate its head from its body when it wants to, and it just grows a new body. The old body eventually dies instead of growing a new head.

The Deadpool slug is one of a type of sea slug that we talked about back in episode 129, about the blurry line between plants and animals. It eats algae and incorporates the algae’s chloroplasts into its body to use. Chloroplasts are what allows a plant to photosynthesize energy from sunlight, and the sea slug absolutely uses them for the same thing. Researchers think the Deadpool slug uses the energy from photosynthesis to regrow its body even though it has no digestive system after it separates its head from its body.

The big question is why the Deadpool slug wants to grow a new body in the first place. It doesn’t seem to be a defensive strategy if the sea slug is attacked. Instead, researchers think it often happens when the body contains too many parasites, specifically a type of tiny parasitic copepod, which is a crustacean. It might also happen after a predator bites a big chunk off the slug. Instead of hauling around a damaged body, the sea slug just jettisons the old body and regrows it.

Let’s finish with a recently solved octopus mystery that goes back almost 200 years. In 1838, the United States launched a scientific expedition throughout the Pacific Ocean and parts of the Atlantic that lasted four years. While it was mostly for exploration and mapping of places seldom or never visited by outsiders, the expedition also brought along a team of scientists and artists to document and study all the animals and plants they found. One of the things they found was an octopus.

The scientists didn’t fish the octopus up themselves. They actually bought several of them at a fish market in Brazil. It was red with little white spots all over it and not very big, although a dead octopus tends to shrink, especially when it’s out of water. The specimens were preserved in a jar of alcohol and brought back to the United States, where in 1852 they were studied by an expert on mollusks, Augustus Addison Gould. Octopuses are in the phylum Mollusca and Gould had examined lots and lots of octopuses. He decided this one was a new species and named it Callistoctopus furvus.

At some point the specimens were either lost or destroyed. Decades passed, then a century, then almost two centuries. Modern scientists thought Gould was probably wrong and that the little red octopus was one known from the Mediterranean Sea, Calistoctopus macropus. It’s red with little white spots, and has a mantle length only about 8 inches long, or 20 cm, although it has long arms and has been measured as almost five feet long, or 1.5 meters, if you include the arms. It lives in shallow water, where it spends a lot of time hunting for small animals that live in coral or in sea grass. It’s sometimes called the grass octopus.

Then a graduate student in Brazil named Manuella Dultra was studying octopuses, and part of her research involved talking to local fishers. They told her about an octopus that lived in shallow water and often buried itself in sand to hide, which is why they called it the sand octopus. They also said it was generally only seen when the wind blew from the east, and was more likely to be out and about during the new moon. Naturally Dultra wanted to find one. She asked the fishers to keep an eye out, and in 2013 she was given a freshly caught specimen.

The biologists at Dultra’s university identified the octopus as C. macropus, the grass octopus. Dultra wasn’t so sure. She noticed a lot of differences that seemed significant, and decided to do more research. She and her team gathered genetic material from specimens the local fishers caught, and sure enough, it was different from the grass octopus.

At the same time, researchers in Mexico had also found a sand octopus that they thought might be C. furvus. When Dultra compared her specimens’ DNA profile with the DNA profile from the Mexican octopus, it matched.

The discovery is still very new and isn’t accepted by all scientists yet, not until more studies are completed. The sand octopus appears to be rare, and once it’s definitely identified as its own species or subspecies and we learn more about it, we can do more to protect it.

You can find Strange Animals Podcast 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 us a rating and review on Apple Podcasts or Podchaser, or just tell a friend. We also have a Patreon at patreon.com/strangeanimalspodcast if you’d like to support us for as little as one dollar a month and get monthly bonus episodes.

Thanks for listening!

Episode 129: The blurry line between animals and plants

This week we’re looking at some really strange animals…or are they plants? Or both? We’ll start with the sea anemone, then learn about a sea slug that photosynthesizes like a plant (sort of), then learn a little about whether algae is a plant or an animal…and then we’re off and running through the wild world of carnivorous plants–including some carnivorous plants of mystery!

Thanks to Joshua Hobbs of A Degree in Nonsense for the suggestion, and to Simon for the article link I’ve already managed to lose!

A sea anemone and some actual anemones. Usually pretty easy to tell apart:

The sea onion looks so much like an onion I can’t even stand it. This is an ANIMAL, y’all!

Venus flytrap sea anemone and actual Venus flytrap. It’s usually pretty easy to tell these two apart too.


The eastern emerald elysia, a sea slug that looks and acts like a leaf:

Giant kelp. Not a plant. Actually gigantic algae. Algae is neither a plant nor an animal:

The corpse flower (left) and the corpse lily (right). Both smell like UGH and both are extremely BIG:

The pitcher plant can grow very big:

Maybe don’t go near trees with a lot of skulls around them:

Puya chilensis (the clumps in the foreground are its leaves; the spikes in the background are its flower spikes):

Show transcript:

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

This week we’re going to explore the sometimes blurry line between animals and plants. Joshua Hobbs of a great new podcast A Degree in Nonsense suggested a type of carrion flower that smells like rotting flesh to attract insects, and friend of the pod Simon sent me an article about carnivorous plants. Our very first Patreon bonus episode was actually about carnivorous plants, so I’ve expanded on that episode and added lots of interesting new content. Buckle up, folks, because we’re going to cover a whole lot of ground today!

Oh, and Joshua also says, quote, “I never had a pet growing up, but recently gained an interest in animals. Now after getting into your podcast and animal YouTube channels, I’ve got my first pet, a little corn snake named Arnold!” So welcome to podcasting, Joshua and Arnold!

Let’s start by looking at an animal that resembles a plant. The sea anemone looks so much like a plant that it was named after an actual flower, the anemone, but the sea anemone is related to jellyfish. Most sea anemones attach to a rock or other hard surface most of their lives and don’t move much, although they can creep along very slowly—so slowly that snails are racecar drivers in comparison. Many species have a body shaped like a plant stem and colorful tentacles that resemble flower petals. But those tentacles aren’t just to look pretty. The sea anemone uses them to catch prey. The tentacles are lined with stinging cells that contain venom, just like many jellyfish have. The venom contains neurotoxins that paralyzes a fish or other small animal so that the sea anemone can eat it.

So how does something that looks like a plant eat a fish?

The sea anemone has an interesting body plan. What looks like the stem of a plant is called the column, and in some species it’s thin and delicate while in other species it’s thick like a tree trunk. It sticks to its rock or whatever with an adhesive foot called a basal disc, and on the other end of the column is what’s called the oral disc. Oral means mouth. The actual mouth is in the middle of the oral disc, surrounded by tentacles. The mouth is usually shaped like a slit, which if you think about it is sort of how people’s mouths are too. The digestive system is inside the column. But there is no other opening into the body. The mouth is it. So like jellyfish, the mouth takes in food but it also expels waste, so, you know, not precisely a mouth like ours. When the sea anemone wants to eat, it uses its tentacles to push the food into its mouth.

You know the movie Finding Nemo? Nemo and his dad are clownfish, which aren’t affected by sea anemone venom. Clownfish hide among sea anemone tentacles so predators won’t bother them. In return, the sea anemone eats the clownfish’s poops. I wish I were making that up.

If a sea anemone feels threatened, many species can not only suck its tentacles into its mouth, it can retract the whole mouth inside its body. Basically, it can swallow its own mouth. A sea anemone called the sea onion retracts its tentacles and inflates its column so that it looks like an actual onion. The sea onion lives in a burrow it digs very slowly into the sediment at the bottom of the ocean, with just its tentacles sticking out.

Most sea anemones live in relatively shallow water, but there are some deep-sea species. The Venus flytrap sea anemone has been found at 5,000 feet deep, or over 1,500 meters. At first glance looks like a Venus flytrap plant, thus the name. Its body is a long, usually slender column that widens into a big oral disc on top that’s fringed with short tentacles. It mostly eats detritus that drifts down from above, which it filters from the water with its tentacles, although if a living creature strays into its tentacles it’ll eat it too.

That brings us to the actual Venus flytrap. It’s a plant that eats insects and spiders, especially crawling insects like ants and beetles. The ends of its leaves are modified into lobes that look a little like flowers because the insides of the lobes are a cheerful red while the edges and the hair-like cilia are yellow. When a bug touches the receptors inside the lobes it closes tightly. If the insect continues to move around inside, stimulating the receptors even more, the lobes seal and form a sort of stomach. Digestive enzymes are secreted and about ten days later the lobes reopen and there’s nothing left of the insect but its empty exoskeleton.

If bugs made movies, this would be the subject of every single bug horror film.

The Venus flyptrap is only found in one small part of the world, the boggy areas surrounding Wilmington, North Carolina in the United States. They’re so in demand that the plant is almost extinct in the wild due to idiots digging them up to sell. But Venus flytraps really aren’t that difficult to grow, you just have to make sure the soil you use is deficient in nitrogen and phosphorus. So you can buy Venus flytraps that were grown ethically instead of dug up from the wild. As of 2014 digging up a Venus flytrap is a felony in North Carolina.

Before we go on to talk about some other carnivorous plants, let’s discuss an animal that acts like a plant. It’s a sea slug called the eastern emerald elysia and it lives along the east coast of North America in shallow water. Even though it’s a sea slug, it will also live in fresh water. It grows to about an inch long, or 3 cm, and is green. It’s green because it photosynthesizes like a plant…sort of.

The sea slug eats algae, but it doesn’t fully digest the algae it eats. Its digestive system retains the algae’s chloroplasts, which are the parts of a plant cell that convert sunlight into energy, which is what photosynthesis is. The sea slug keeps the chloroplasts in its digestive system and keeps them alive for months, living off the energy the chloroplasts produce. Researchers aren’t sure how the sea slugs keep the chloroplasts alive.

This is pretty amazing, but it’s not the only sea slug that photosynthesizes in this way. The blue dragon sea slug, that lives along coasts around the Indo-Pacific Ocean, doesn’t just keep chloroplasts alive to produce chlorophyll energy. It gets even more complicated about it. The blue dragon eats tiny animals called hydrozoa, which are related to jellyfish and include the freshwater hydra, although since the blue dragon only lives in the ocean it doesn’t actually eat the hydra. The blue dragon eats hydrozoa that themselves contain a type of microscopic algae that live in a lot of animals, like giant clams, some jellyfish, even some sea anemones, and exchange energy from photosynthesis for protection from predators by living in or on its host. So when the blue dragon eats the hydrozoa containing these algae, it retains the algae and keep them alive. So basically it gets to eat its prey and steals its prey’s symbiotic algae.

Speaking of algae, most algae photosynthesize, and in fact many seaweeds, like kelp, aren’t plants but are giant plant-like algae. But algae, technically, aren’t plants. They’re not animals either. Researchers and taxonomists are still working out the ways various algae are related to each other and to other organisms, but most algae are considered more closely related to plants than to animals without actually being plants. They’re usually grouped with plants above the kingdom level of taxonomy, but since at that level animals like humans and fish and worms and mosquitoes are grouped with fungi, this is a really broad category.

And that brings us, in a roundabout way, to the rotten meat smelling plant suggested by Joshua. There are several plants that attract flies and other insects to pollinate their flowers by smelling of rotten meat. Some of these have freakishly large flowers, like the corpse flower. It lives in rainforests in parts of Sumatra and Java and is actually related to the calla lily. It’s a weird-shaped plant and hard to describe. You know how a calla lily has a pretty white petal that wraps around a yellow spike thing? The corpse flower is like that, only it can be ten feet high, or 3 meters. The thing that looks like a petal is actually a specialized leaf and the yellow spike is called the spadix. The yellow part is made up of tiny flowers, so a calla lily isn’t a single flower, it’s lots of flowers that look like one. Well, the corpse flower is like that, although its flowers are actually only at the bottom of the spadix. The petal-like leaf is dark red inside. The top of the spadix is where the rotten smell comes from, and it’s incredibly stinky—something like rotting meat and rotting fish with some extra smell like dung on top of it. It releases this stink mostly in the evenings and the top of the spadix actually grows hot to better disperse the smell.

The largest single flower in the world is sometimes called the corpse lily and it can grow over three feet across, or about a meter. It’s dark reddish-brown with white speckles and five fleshy petals, which look like meat. It smells like rotting meat too. Flies are attracted to the flower, which pollinate it. The flower can take an entire year to develop but only blooms for a few days. If it’s successfully pollinated, the flower produces a round fruit full of seeds that are eaten by tree shrews, which later poop the seeds out and spread them.

But the corpse lily isn’t any ordinary plant. It doesn’t even have roots or a stem or leaves. All it has is the flower, which grows directly from the roots of the corpse lily’s host plant. That’s right, the corpse lily is a parasitic plant, but it’s no ordinary parasite. It grows not on or around its host plant, but inside it. The host plant is a type of vine called Tetrastigma, related to the grape vine. When a tree shrew poops out a seed, the seed germinates and if it happens to germinate on a Tetrastigma vine, it develops tiny threadlike filaments that penetrate the vine and grow inside it.

The corpse lily lives only in the rainforests of Borneo and Sumatra, and it’s rare and getting rarer since so much of the rainforests in those areas are being destroyed. Fortunately, the corpse lily is actually a tourist attraction since it’s so rare, so spectacular, and so stinky. People who have corpse lilies growing in their yard sometimes protect the flower buds from harm and charge tourists to come look at them, which helps the people of the area and the plants.

There are literally hundreds of carnivorous plant species, with carnivorous habits evolving probably nine different times among plants that aren’t related. Different species use different methods to catch insects. For instance, the pitcher plant has modified leaf that forms a slippery-sided pitcher filled with nectar-like liquid. When an insect crawls down to drink the liquid, it falls in. The insect drowns and is dissolved and digested.

Some carnivorous plants have leaves lined with sticky mucilage, which traps small insects. The sundew has tentacles lined with hair-like structures beaded with mucilage. When an insect becomes trapped in the mucilage, the tentacles bend toward the insect and stick onto it, sometimes quite quickly—in seconds, or in at least one species, a fraction of a second. Generally you don’t think of plants as moving that fast.

Almost all known carnivorous plants are pretty small. The largest are pitcher plants. Two species of big pitcher plants grow in the mountains of the Philippines. Attenborough’s pitcher plant was discovered in 2007 and described in 2009, and is a shrub with pitchers that can hold nearly two liters of fluid. An even bigger pitcher plant was discovered in 2010. But the biggest pitcher plant known is from a couple of mountains in Malaysian Borneo called Nepenthes rajah. It’s been known to science since 1858 and its pitchers can hold over 2 ½ liters of digestive fluid. The biggest pitcher ever measured was over 16 inches tall, or about 41 cm, and the plant itself is a messy sort of vine that can grow nearly 20 feet long, or 6 meters. Mostly pitcher plants just attract insects, but these giant ones also trap frogs, lizards, rats and other small mammals, and even birds.

There’s always the chance that even bigger pitcher plants have yet to be discovered by science, although probably not much bigger than the ones we do know about. The larger an animal, the more likely it is to damage the pitcher while trying to escape. Insects and the occasional small animal are fine, anything bigger than that could just bust through the leaf.

But there have long been rumors about plants that eat much larger animals, even humans. In the 1870s, a German explorer named Karl Liche claimed he’d witnessed a tribe in Madagascar sacrifice a woman to a carnivorous tree. His account is not very believable. He describes the tree as about eight feet high with a thick trunk. A coat of leaves hang down from the top of the tree, leaves about twelve feet long with thorns. At their base is a flower-like receptacle with sweet liquid inside, with six ever-moving tendrils stretching up from it. When the sacrificial woman was made to drink the liquid, the tendrils wrapped around her and the tree’s long leaves folded up and over her. After ten days, the leaves relaxed, leaving nothing but a bleached skull at the base of the tree.

Later expeditions to Madagascar never found any plant that resembled Liche’s. In fact, everyone who’s researched Liche, the tribe he mentioned, and the tree in question haven’t found any evidence that any of them ever existed. It turns out that the account was a hoax from start to finish, written by a reporter named Edmund Spencer for a newspaper called the New York World in 1874.

A 1924 book called Madagascar: Land of the Man-Eating Tree describes a more realistic-sounding carnivorous plant that was supposed to be from India. Its blossoms have a pungent smell that attracts mice and sometimes large insects, which crawl into a hole in the blossom that turns out to be a bristly trap. This sounds a little like the corkscrew plant that lives in wet areas of Africa and Central and South America. It has ordinary leaves aboveground but modified leaves that grow underground. The modified leaves are traps with a stalk lined with hairs pointing in one direction. Tiny water animals, especially single-celled protozoans, stray into the leaves but can’t get out because of the hairs. They’re digested and absorbed by the leaves. But there are no known corkscrew plants or anything like them that trap larger animals or animals that live aboveground.

An 1892 article describes a friend of a friend of a friend’s encounter with a tangle of thin, willow-branch-like vines covered with an incredibly sticky gum. This was supposed to have happened in Nicaragua in Central America. A Mr. Dunstan’s dog was ensnared by the plant but was rescued by Dunstan, who managed to cut the vines with his knife. In the process, both man and dog suffered blistered injuries from the plant, as though it had been trying to suck their blood. The article also says that natives of the area say the plant will reduce a lump of meat to a dried husk in only five minutes.

From these sorts of factual-seeming accounts, it’s a short step to plants of folklore like the Japanese Jubokko tree that grows on battlefields and drinks human blood. It captures people who pass too close to it, sticks its branches into them, and sucks out their blood. If someone cuts into the tree’s bark, blood comes out instead of sap.

Another carnivorous plant was supposedly encountered by a French explorer in 1933 in the jungles of southern Mexico. He doesn’t describe the plant in his 1934 magazine article, just says it’s enormous, but he does say that when a bird alighted on one of its leaves, the leaf closed and pierced the bird with long thorns. The expedition’s guide called it a vampire plant.

A similar story supposedly of a plant found in South America and Central Africa is of a short tree with barbed leaves that grow along the ground, and if an animal or bird steps on the leaves they twine around it and stab it to death, then squeeze the blood out to absorb.

There may actually be a real plant that these stories are based on. It’s called the Puya chilensis and it lives in Chile in South America, on dry hillsides of the Andes Mountains near the ocean. It’s an evergreen plant that only flowers after it’s some 20 years old, with a flower spike that can grow over 6 ½ feet high, or up to 2 meters. The flowers are pollinated by birds. But its leaves are long, edged with hooked spines, and grow in clumps that can be up to six feet wide, or nearly two meters.

Those hooks along the leaves give the plant its other name, the sheep-killer. Sheep and other animals can become entangled in the leaves, which are so tough that locals use the leaf’s fibers to make rope. If the animal can’t escape, it dies and its body decomposes, adding nutrients to the soil around the plant. Yum.

You can find Strange Animals Podcast online at strangeanimalspodcast.blubrry.net. That’s blueberry without any E’s. We’re on Twitter at strangebeasties and have a facebook page at facebook.com/strangeanimalspodcast. 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!