Invertebrates
Giant Isopod
Bathynomus giganteus · also called giant isopod, deep-sea isopod
The giant isopod is what happens when a woodlouse grows to the size of a house cat. It lives in the cold dark of the deep ocean floor, waiting for something large to die and sink, then eating until it can barely move — and then, if it has to, not eating again for years.
Overview
Giant isopods (genus Bathynomus, about 20 species) live on the deep continental slopes of the world’s oceans, mostly between 150 and 2,500 metres down, on soft mud and clay. They are true isopods — the same order as the garden woodlouse and pillbug — with an armoured segmented back, seven pairs of legs, and the pillbug’s trick of rolling into a ball. They are just vastly larger: commonly 20–30 cm, and up to about 50 cm and 1.7 kg in the “supergiant” species.
Deep-sea gigantism
The giant isopod is a leading example of deep-sea gigantism — the tendency of certain deep-ocean animals to dwarf their shallow-water kin. The likely drivers are the cold (slow growth, long life, large final size), high pressure, and above all the scarcity of food: a big body stores more energy and can travel further and wait longer between meals.
Feast and famine
The deep seabed is a food desert broken by rare windfalls — a dead fish, a squid, or a whole whale carcass. Giant isopods smell these from a distance, converge, and gorge until their bodies are swollen taut. Then they drop into a very low metabolic state and wait. A famous individual at Japan’s Toba Aquarium ate nothing for over five years before it died.
Built for the dark
The isopod has large, reflective compound eyes with about 4,000 facets to gather the faintest light, and long antennae for feeling its way across the bottom.
Status
Giant isopods have not been assessed for conservation. They are widespread and locally common, turn up as deep-trawl bycatch, and are eaten as a novelty in parts of Taiwan, Japan, and Vietnam, where the leg and tail meat is likened to crab.
Giant Isopod: common questions
Is the giant isopod just a huge woodlouse?
Essentially, yes. It belongs to the order Isopoda, the same group as the garden woodlouse, pillbug, and the sea slaters you find under rocks on the shore, and it has the same basic body plan — a segmented, armoured back, seven pairs of legs, and the ability to curl into a protective ball. It is simply enormously scaled up, from around a centimetre in a woodlouse to 30 or even 50 centimetres in the largest "supergiant" isopods. It is not closely related to trilobites, which it superficially resembles; that likeness is coincidental.
Why is it so much bigger than a normal isopod?
It is a classic case of deep-sea gigantism, a pattern in which some animals — isopods, amphipods, squid, sea spiders — reach far larger sizes in the deep ocean than their relatives in shallow water. The reasons are debated but likely include the cold temperature (which slows metabolism and growth but allows a longer life and a larger final size), the high pressure, the scarcity and unpredictability of food (a bigger body stores more energy and can go longer between meals), and higher dissolved oxygen in cold water. A large body also loses heat and water slowly and can range further to find a carcass.
How does it survive on the deep-sea floor with so little food?
The deep seabed is mostly a food desert, punctuated by occasional bonanzas — a dead fish, squid, or, jackpot, a whale carcass. Giant isopods are built to exploit exactly that. They can detect a carcass by smell from a distance, converge on it, and gorge until their bodies are visibly distended, sometimes so full they can barely move. Then they metabolise very slowly and simply wait. One giant isopod kept at Toba Aquarium in Japan famously ate nothing for over five years. In between big meals they also take slow live prey and detritus.
Are giant isopods eaten by people?
Yes, in a few places. They are brought up as bycatch in deep-water trawls, and in parts of Taiwan, Japan, and Vietnam they are sold and eaten, usually steamed or fried, with a taste compared to crab or lobster (the meat is in the legs and tail). They are a novelty item rather than a major fishery. Live specimens also attract attention at public aquariums, where their size, prehistoric look, and enormous appetite followed by years of fasting make them popular exhibits.