Giant Sea Isopods: Deep-Sea Giants and Cousins of the Woodlouse

The giant sea isopod is a true monster of the deep - a deep-sea crustacean that looks exactly like the woodlouse in your garden, scaled up to the size of a small cat. Confirmed specimens reach 45 cm, they can survive years without eating, and they're a striking example of "deep-sea gigantism". First described in 1879, much about their biology is still a mystery. Here's what we know about these remarkable scavengers of the ocean floor - and how they relate to the isopods kept as pets.

Giant isopods belong to the genus Bathynomus, which contains around 20 known species. Typical adults of the best-known species, Bathynomus giganteus, measure 19-36 cm. The largest confirmed specimen measured 45 cm; a 50 cm animal reported from Brazil is unconfirmed, and so is a 76 cm one from the popular press. Even a typical adult is enormous for an isopod: a garden woodlouse is barely 1-2 cm. They inhabit the cold, dark depths of the Atlantic, Pacific and Indian Oceans.

A Giant Cousin of the Garden Woodlouse

For all their size, giant isopods are unmistakably isopods, sharing the same body plan as their terrestrial relatives - the pill bugs and woodlice. Their bodies are dorsoventrally flattened and armoured with a rigid, calcareous exoskeleton of overlapping segments, and like many isopods they can curl into a defensive ball to shield their soft underside. The first body segment is fused to the head, and the rearmost segments fuse into a "caudal shield" over the shortened abdomen.

They have two pairs of antennae - one long pair reaching nearly half their body length, used for sensing their pitch-black surroundings - and large compound eyes with nearly 4,000 facets, spaced wide apart to gather what little light exists at depth. Like all isopods they have seven pairs of walking legs (pereopods), the first pair modified into mouthparts (maxillipeds) for handling food. Their abdominal appendages (pleopods) double as gills, and a fan-like tail lets them swim up off the seabed and glide. Most are a pale lilac or pinkish colour.

Scavengers of the Deep

Giant isopods are the deep sea's clean-up crew, performing the same detritivore role on the ocean floor that woodlice perform in a garden - consuming the dead and recycling nutrients so new life can flourish. They're mainly scavengers, feasting on whatever sinks from above: dead whales, fish and squid, plus marine worms and other debris. They'll also take slow-moving prey such as sea cucumbers and sponges, and are notorious for raiding fishermen's trawl catches - there's even a documented case of one latching onto the face of a dogfish shark caught in a deepwater trap.

Because food is so scarce and unpredictable at depth, they've evolved an extraordinarily slow metabolism and can endure famine for astonishing stretches - captive individuals have survived over five years without eating. When a feast does arrive, they gorge themselves until they can barely move, storing huge energy reserves (in B. giganteus, lipids make up the bulk of the fat body) to see them through the lean times ahead.

Where They Live

Giant isopods are benthic, living on or near the seabed, and prefer muddy or clay substrates where they can rest and wait for food to drift down. Most live in the gloom of the continental slope and bathyal zone, generally between about 170 m and 2,140 m deep, where pressure is immense and temperatures hover just above freezing. Over 80% of B. giganteus are found between 365 and 730 m. A few species venture shallower - B. miyarei has been recorded as high as 22 m - but the genus is overwhelmingly a creature of the cold deep, and some species stop feeding or breeding if the water drops below about 3°C.

Their distribution is patchy and incompletely mapped. No single species occurs in both the Atlantic and the Indo-Pacific, and the richest diversity is off eastern Australia, with five recorded species. New species almost certainly remain undescribed. Researchers from bodies like NOAA's Office of Ocean Exploration continue to study them using submersibles and remotely operated vehicles.

Why So Big? Deep-Sea Gigantism

The giant isopod is a textbook case of deep-sea gigantism - the tendency for deep-water animals to grow far larger than their shallow-water or terrestrial relatives. The leading explanations point to the cold, which slows metabolism and may favour larger bodies, and to scarce food, where a bigger body can store more reserves to survive long fasts. Fewer predators in the deep may also allow them to reach such sizes. Biologists invoke principles like Kleiber's rule and Bergmann's rule to explain the phenomenon, though, like much about these animals, the full picture is still debated.

Can You Eat - or Keep - a Giant Isopod?

Occasionally, yes to the first. Giant isopods are sometimes cooked in parts of East Asia, including served with ramen, and their meat is likened to crab or lobster - firmer and chewier - though there's relatively little of it for the animal's size.

As for keeping one, it's a serious undertaking far removed from ordinary isopod husbandry. You'd need a large, dark, chilled marine aquarium replicating their deep, cold habitat. The change in pressure is less of a problem than you might expect: giant isopods come up alive in baited traps and fishing nets, and like other deep-sea crustaceans they have no air-filled spaces for it to damage. Warmth is the bigger risk. In a study off the Bahamas, isopods were kept for about an hour in surface water at 26 to 28°C and then returned to depth in cages; between 50 and 100% died within five days, and the researchers suspect heat stress played a part. Kept chilled, they can be flown in packed in ice and live for years in captivity, and public aquariums including Monterey Bay Aquarium in California and Toba Aquarium in Japan have had them on display, but they're an institutional challenge rather than a home pet. If you love the isopod body plan, the practical route is the popular pet species kept in terrariums and vivariums - like the hardy, beginner-friendly Dairy Cow isopod - which give you the same fascinating animal at a manageable scale.

Threats and Conservation

Giant isopods aren't immune to human impact. They're killed in large numbers as bycatch in deep-sea trawling, with post-release survival estimated to be poor due to the stress of capture. More troubling still, analysis of giant isopod stomachs in the Gulf of Mexico has found significant quantities of plastic - a stark reminder that microplastics now reach even the ocean's darkest depths, where these ancient scavengers have lived largely undisturbed for millions of years.

Frequently Asked Questions

How big do giant isopods get?

The best-known species, Bathynomus giganteus, typically reaches 19-36 cm, with the largest confirmed specimen at 45 cm. Reports of a 50 cm and a 76 cm animal are unconfirmed. For comparison, a garden woodlouse is only 1-2 cm.

Are giant isopods related to woodlice?

Yes - both are isopods and share the same body plan. The giant sea isopod is essentially a deep-sea relative of the pill bugs and woodlice found in gardens, scaled up enormously through deep-sea gigantism.

How long can a giant isopod survive without food?

Remarkably long. Their very slow metabolism is an adaptation to scarce deep-sea food, and captive individuals have been documented surviving over five years without eating.

Can you keep a giant isopod as a pet?

Not practically. They need a large, dark marine aquarium kept very cold, around 4°C, and only specialist public aquariums keep them. Hobbyists keep terrestrial isopod species instead.