10 Things You Didn't Know About Sea Pigs

Sea pigs are populous deep-sea dwellers, even though you'll likely never see one. As their name suggests, they look like gummy pink pigs, but with no eyes, many more legs, and near-transparent bodies. Also called Scotoplanes, the elusive marine creatures hail from the family Elpidiidae and fall into a class of animals called echinoderms, which also includes sea urchins and starfish. Secretive and mysterious as they may be, sea pigs deserve a world of praise for the vital role they play in the ocean ecosystem.

Here are 10 things you probably didn't know about these strange-yet-fascinating critters.

1. Sea Pigs Are a Type of Sea Cucumber

Scotoplanes are a subspecies of the ever-familiar sea cucumber, but they do differ slightly from their better-known relatives. Sea cucumbers, for instance, have caterpillarlike feet that remain tucked underneath their bodies while the sea pig walks on long stilts, which make it easier for them to get around in soft mud. They live in much deeper water, too, and have unique see-through bodies.

2. They Live at the Bottom of the Ocean

Although they're extremely common, you'll probably never see a sea pig in person. They live only in the deepest, darkest, coldest parts of the ocean, as far as four miles under the water's surface. Their tendency to hide out in the abyss makes the species notoriously difficult to study.

While only about four to six inches in size, they're the largest animals around in most cases. Sea pigs have been discovered in every ocean on earth.

3. If Brought to the Surface, They Disintegrate

The main reason you'll never get the opportunity to gawk at these eccentric, near-transparent creatures on land is because they can't be removed from their natural habitat. According to Ocean Conservancy, their delicate, finger-sized bodies would simply disintegrate into a pile of faux Jell-O if brought within 4,000 feet of the water's surface. They'll also break apart easily if caught in a fishing trawler.

4. They're Scavengers

Sea pigs prefer an easy meal—more specifically one that they don't have to catch. They'll congregate en masse when a dead whale or any other sort of decaying material sinks to the sea floor. Their scavenging nature is a great service for the ecosystem, too, as they act like vacuum cleaners tidying up an otherwise untouchable part of the ocean.

5. They Walk Instead of Swim

Unlike most marine animals, sea pigs don't swim—at least not in the traditional sense. Instead, they hover above the sand, using suckers on the bottom of their (notably large) tube-like feet to ground them. And those extra-long, antennae-looking tentacles protruding from their heads? Those are feet, too. They're called papillae and are primarily used to detect food. Sea pigs can dig up algae and animals from the mud with their strong mouth tentacles, MBARI says.

6. They Ward off Predators With Poisonous Skin

Sea pigs exist in such large quantities because they don't have many predators. Parasites are the only real threat that can get to them; fish won't eat them because they taste bad, and because their skin is laced with poison. The toxic chemicals in their skin are called holothurins and they're used by various species of sea cucumber as a defense mechanism.

7. Sea Pigs Have Been Likened to Earthworms

The comparison of Scotoplanes to land-dwelling creatures doesn't stop at the obvious barnyard animal, either. Marine biologist David Pawson of the National Museum of Natural History likened them to earthworms in an interview with Wired. Like the familiar land invertebrate, sea pigs increase the amount of oxygen in deep-sea mud, Pawson said, in turn making it more livable for other animals.

8. They Have Interesting Respiratory Systems

One similarity between the elusive sea pig and their relative, the sea cucumber, is their peculiar respiratory systems: They both breathe through their anuses. Scotoplanes pump water through their cloacae by expanding and contracting their bodies, extracting oxygen from it with a lunglike system called the respiratory tree. According to the Marine Education Society of Australasia (MESA), the respiratory systems of all echinoderms are "poorly developed."

9. Crabs Hitch Rides on Them

Sea pigs aren't the only bottom-of-the-ocean dwellers. Baby king crabs make a go of growing up there as well. And because they're an easy meal for predators, they often need protectors. In 2011, researchers noticed the little crabs clinging to quite a few sea pigs, a MBARI report showed. In reviewing footage of other deep-sea creatures, they witnessed this savvy survival strategy: Almost a quarter of the 2,600 sea cucumbers examined were carrying juvenile crabs, and 96% of juvenile crabs were clinging to sea cucumbers.

It isn't clear how this benefits the host and it doesn't happen everywhere. The crabs seek refuge on sea pigs only in places where sea pigs are "the largest benthic structure available as shelter."

10. No One Knows How Long They Live

Because they're so difficult to study, there is still much to be discovered about the sea pig. Scientists are still perplexed by their mating system—although it is known that they lay eggs, as many marine animals do—and have no idea how long they live. Because sedimentation is slow-going in the ocean, tracks may look fresh and be 100 years old, Pawson told Wired. The pig-resembling echinoderms living right now at the bottom of the ocean could be prehistoric, for all we know.