The Two-Foot-Wide Sea Star That’s Basically a Bear Trap

Labidiaster annulatus kinda looks like a regular sea star fell off a skyscraper and splatted.

Few oceanic creatures are as iconic and beloved as the sea stars (which, fun fact, are collectively known as “asteroids,” which only makes them that much cooler). They’re just so simple, so quaint, so … star-like. Hell, slap a pair of googly eyes on one and it’d be downright cute. Yet these are actually voracious scavengers, hoovering up whatever meat they happen upon on the seafloor. And a few species even go one step further: They’re hunters—sluggish but accomplished hunters.

Reigning among them is Labidiaster annulatus, a two-foot-wide titan that menaces the waters around Antarctica. Not content with just scavenging, and not even content with just hunting creatures on the seafloor like other sea stars, it can actually pick off fish swimming in the water column. Sounds far-fetched, I know, but stick with me here. Or don’t. I’m not your father.

These things are really, really successful down there in Antarctica. “They form this sort of Lovecraftian benthic landscape,” says marine biologist Christopher Mah of the Smithsonian, who’s the guy to talk to about sea stars, “because they tend to crawl up onto perches or sponges or any kind of elevated perch and hold their arms up in the water to take advantage of the food. So that could be anything natural such as a sponge to seismic sensors.” Mah knows this, by the way, because a geologist once sent him an email asking what the hell a giant sea star was doing on top of one of his devices.

Now, the classical notion of sea stars is that they’re supposed to have five rays, and indeed they all start out with five. Clearly, though, Labidiaster annulatus scoffs at this rule. As it grows, it adds ever more rays in between those original five until it ends up with more than 50. That’s 50 weapons to wield against its prey.

So, it perches itself on a sponge or seismic sensor or what have you and puts its hands in the air like it just don’t care, usually the majority of its rays at any given time. There it waits for hapless swimmers—crustaceans like krill, say—to come along and bump into it, sticking fast to the many jaws that coat the sea star. Think of it like how a jellyfish hunts, only much more toothy.

“[The claws] are very nasty and formidable looking,” says Mah. “They look like bear traps. And they function very much the same way. There are these big, jagged teeth, and as soon as a krill impacts upon them the claws just snap shut and grab them.”

Oh hey look a thing that will stalk my dreams for the next decade.

These claws are formally known as pedicellariae, and the sea star is positively covered in them. Running along its arms are fleshy bands called annuli, each loaded with as many as 80 pedicellariae, so there aren’t many places on the creature that aren’t capable of snagging prey. When it does grab something, it’ll bend in other arms to help subdue the victim, kind of like an octopus (it’s unusually flexible for a sea star—maybe it’s into yoga, I dunno). Then, using little structures called tube feet, the sea star starts conveyor-belting the victim into its mouth, where it’s swallowed whole and very much alive.

Other sea stars, such as the ochre star, have pedicellariae as well, but nothing has pedicellariae as big as Labidiaster annulatus. “I’ve had experiments where you’ll take some big hairy guy,” says Mah, “and you put a sea star arm against their forearm and you pull back and the sea star is attached like velcro to the guy’s arm hair. And so [Labidiaster annulatus] would be worse.”

“If they were five feet across we’d be running from them,” he adds, “because they’re not very friendly looking animals when you look at them up close.”

A closeup of the pedicellariae. They kind of look like the jaws of the bugs in Starship Troopers, only, you know, they’re not computer-animated.

As long as the prey’s the right size, there isn’t much the sea stars won’t take on. Judging by stomach contents, they seem to be particularly fond of the aforementioned krill, but will from time to time tackle fish and even run down their own kind. One Labidiaster annulatus that scientists hauled aboard a ship disgorged brittle stars on deck, alive and well save for a missing limb here or there.

Thus does Labidiaster annulatus dominate the Antarctic ecosystem. Really, its echinoderm kin as well as other invertebrates dominate the ecosystem as a whole. And this is a bit goofy. A tropical reef, say, is ruled by vertebrates: sharks and other fish. Fish in particular are bad news for sea stars—Labidiaster annulatus waving its long arms around in a reef would just be asking to get amputated. But fish are in short supply in these frigid waters—save for a very special variety that has antifreeze for blood—so sea stars and other invertebrates have ascended the throne. (Labidiaster annulatus is joined here by another giant, the ribbon worm, which grows to six feet long and has a similarly unrestrained diet. Researchers testing what it would eat started to run out of ideas, so they gave it tomato sauce. It liked that too.)

“This is what’s called a Paleozoic-style ecosystem, one where the invertebrates are actually the dominant ones,” says Mah. “This is in reference to the Paleozoic, before fish evolved, and you have this system dominated by soft bodied organisms such as cephalopods and worms and so forth.”

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