Invertebrates
Sea Pig
Scotoplanes globosa · also called pig of the sea
Across the muddy plains of the deep ocean, several kilometres down, walk herds of small, plump, pink animals on rows of stubby legs, all facing the same way into the current. They are sea pigs, a kind of sea cucumber, and they are among the most abundant large animals on the deep-sea floor.
Overview
Scotoplanes globosa is a sea cucumber, an echinoderm related to starfish and urchins, living on the soft abyssal plains of every ocean, mostly 1,000 to 6,000 m deep. It is a deposit feeder, extracting nutrition from the organic-rich mud and the sludge that gathers around a sunken carcass.
A walking sea cucumber
Where most sea cucumbers crawl, the sea pig has five to seven pairs of enlarged tube feet on its underside that work like legs, lifting the body off the mud so it can walk. The tube feet on its back are stretched into tentacle-like sensors that pick up the chemical trail of a food fall and steer it there.
Herds, and hitchhikers
Sea pigs gather by the dozen or hundred at rich patches, often all lined up into the current. Juvenile king crabs shelter beneath them, using the toxic, unappetising sea cucumber as a walking bodyguard.
Status
Not Evaluated, but locally one of the most abundant large animals on the abyssal floor. Not fished, though deep-sea trawling and mining would damage its habitat.
Sea Pig: common questions
Is a sea pig actually a pig?
Not remotely. It is a sea cucumber, a type of echinoderm, which puts its nearest relatives among the starfish and sea urchins, not among mammals. The name is entirely about appearance. The body is a fat, rounded, pale pink to translucent sac, roughly the size and shape of a small potato or a piglet, and the enlarged tube feet on the underside look like stubby legs and trotters. Sailors and scientists hauling them up in deep trawls have called them sea pigs for well over a century.
How does a sea pig walk?
Like all echinoderms, it has a hydraulic system of fluid-filled canals that operate hundreds of small, extensible tube feet. In most sea cucumbers these are used for slow crawling or gripping. The sea pig has taken five to seven pairs of tube feet on its underside and enlarged them into thick, stumpy, leg-like structures, and it uses them to raise its body off the seabed and stride, slowly, across the soft mud, rather than dragging itself along. This keeps its underside out of the ooze and lets it cover ground in search of better feeding patches.
Why do sea pigs gather in herds?
They are all drawn to the same thing, which is a patch of especially rich sediment. The deep-sea floor is mostly a food desert, with nutrition arriving as a thin, slow rain of dead plankton and, occasionally, as a windfall like a dead whale or a mass of sunken kelp. Sea pigs appear to detect the chemical plume from such a food source using the tentacle-like tube feet on their backs, and many of them converge on it, which is why they are so often photographed in groups, frequently all oriented the same way into the current that carries the scent. They are not social in the sense of cooperating; they are just all responding to the same signal.
What lives on a sea pig?
Quite a few small animals treat the sea pig as mobile cover. The best-documented are juvenile king crabs, which have been found clinging to the underside of sea pigs in large numbers, using the toxic sea cucumber as a shield while they are small and vulnerable, and moving with it across the otherwise exposed plain. Small snails, amphipods, and parasitic worms also associate with sea pigs. For the crabs at least the arrangement is one-sided, a free bodyguard, since the sea pig gains nothing and loses little.