The sea pig is one of those deep-sea animals that looks like it was designed as a joke. Pale, translucent, sausage-shaped, with multiple rows of stumpy inflated legs and a cluster of sensory tentacles at one end, it walks slowly across the abyssal floor in the darkness 4,000 metres below the surface. Hundreds at a time. Seafloor cameras often show herds of them, all facing the same way, across the mud. On soft abyssal bottoms they are among the most commonly seen large animals, in a place most people never think about.

Why is it called a sea pig?

The name comes from the body shape — a pale, rounded, somewhat rotund form with multiple stubby legs, resembling a cartoon pig when viewed from the side.

The comparison is loose but evocative enough that it stuck. Scotoplanes are not the only sea cucumbers called sea pigs — a few other deep-sea holothurians get the name — but Scotoplanes globosa is the most commonly referenced species. Its inflated tube feet are the most pig-like feature: they look like short, fat legs compared to the slender tube feet of most sea cucumbers.

How do sea pigs move?

Sea pigs walk on inflated tube feet — locomotory appendages that are hydraulically inflated with seawater to create rigid, stumpy legs that push against the soft sediment.

They also have papillae — sensory projections on the upper body surface — that are not used for locomotion. Movement is slow but directional. Sea pigs are often seen facing into the prevailing current rather than randomly. This is thought to position them to intercept food particles — mainly marine snow and organic detritus — carried in the bottom current. When food-rich sediment arrives from above — often following a whale fall or a bloom sinking from the surface — sea pigs aggregate in enormous numbers at the food source.

What do sea pigs eat?

Marine snow — the slow rain of organic particles from the surface ocean — and organic-rich sediment. Sea pigs process sediment by ingesting it and extracting organic compounds, then expelling the processed material. This makes them critical recyclers in the deep-sea ecosystem, releasing nutrients locked in sediment back into bioavailable form. Some evidence suggests they are also attracted to whale falls, where an enormous pulse of organic material creates a temporary bonanza in an otherwise food-poor environment.

How do you identify a sea pig?

Look for a plump, pinkish, see-through sea cucumber about 10 to 15 cm long, walking on six pairs of stubby legs, with antenna-like papillae sticking up from its back.

Sea pigs belong to the genus Scotoplanes in the family Elpidiidae, order Elasipodida. Three species are currently recognised: S. globosa, S. angelicus and S. mutabilis. S. globosa is the one in most photos and videos. The body is bilaterally symmetrical, which is unusual for an echinoderm. It has six pairs of enlarged tube feet, largest at mid-body and smallest toward the rear, and ten buccal tentacles around the mouth that shovel in sediment. On its back are elongated papillae that look like antennae or ears. Scientists don’t use them for walking, but they are built the same way as the feet, each with a muscular water canal running through the middle.

The most common look-alike on the same seafloor is Peniagone, another elpidiid sea cucumber. On Pacific seafloor video, Scotoplanes can be told apart by its two pairs of antenna-like papillae, whereas Peniagone has a single pair.

How does a sea pig work inside?

It is a hydraulic animal: the legs are moved by water pumped through the skin.

In most sea cucumbers, each tube foot is powered by a small internal bulb called an ampulla. Elpidiids replaced those bulbs with cavities in the skin. That lets the feet grow much larger, so they work as legs and hold the body up on very soft mud. As a sea pig walks, it ploughs through the top layer of sediment and disturbs the small animals living there. When disturbed, some sea pigs can swim short distances using lobes at the front and rear of the body, and a few species spend a lot of time just above the bottom.

Like other sea cucumbers, sea pigs have a simple nerve network without a central brain, and their skin contains holothurin, a toxic saponin that deters many predators. Unlike most echinoderms, they have a single gonad, so each female has one ovary and each male one testis. Studies have found active egg and sperm production in both sexes, which suggests a different breeding strategy from many related deep-sea cucumbers. They are adapted so completely to cold, high-pressure water that specimens brought to the surface quickly fall apart. That’s why you won’t see a living sea pig in a public aquarium.

What do sea pigs eat, and how do they find it?

They are deposit feeders that pick the freshest organic matter out of the mud, and they can smell a rich meal from a distance.

All deep-sea holothurians feed by swallowing sediment and digesting the organic particles in it, but sea pigs are picky about it. Studies of S. globosa show it strongly prefers fresh phytodetritus, the remains of plankton blooms that have just sunk from the surface, over older and poorer mud. Experiments also show that sea pigs use smell to locate rich food falls such as whale carcasses, and they gather around them in large numbers. On video, groups of sea pigs are often seen all pointing the same way, facing upstream. That position probably helps them pick up the scent of food carried along by bottom currents.

Because they gather where food is, sea pigs are patchy. Early trawl collections brought up groups of 300 to 600 individuals at once, and seafloor cameras often show hundreds within view. Aggregations may also bring mates together.

Where do sea pigs live, and who lives on them?

On abyssal plains in the Atlantic, Pacific and Indian Oceans, mostly from about 1,200 to 5,000 m, often with hitchhikers.

Scotoplanes occurs on deep soft-bottom plains in all three major oceans, and related elpidiids reach Antarctica. Off Oregon and Washington, the Ocean Observatories Initiative cameras have filmed them from about 1,230 m on the continental margin to about 2,600 m at the base of Axial Seamount. They share these habitats with brittle stars, other sea cucumbers, sea pens, grenadiers and hagfish.

Sea pigs are small ecosystems in their own right. Parasitic snails attach to and feed on their body wall, and tiny tanaid crustaceans also live on them. In 2016, the Monterey Bay Aquarium Research Institute filmed juvenile king crabs (Neolithodes diomedeae) at about 1,260 m sheltering beneath sea pigs. Researchers concluded the crabs gain protection from predators by hiding there.

The genus was named by the Swedish zoologist Hjalmar Théel in 1882, in his report on the holothurians collected by the HMS Challenger expedition of 1872 to 1876, the voyage that founded modern oceanography. Sea pigs haven’t been assessed for the IUCN Red List. Researchers see their dependence on undisturbed soft sediment as a reason for caution about deep-sea mining and bottom trawling on the abyssal plains.

Related on TheSea

• Red Handfish
• Red-Lipped Batfish
• Colossal Squid