Pram Bug

• The female does not make her house: she builds it by hollowing out the body of a living tunicate and then living inside the transparent husk.
• She places her young on the inner wall of the barrel and keeps them there until they can swim on their own.
• Its eyes take up almost the whole head and are divided into two pairs, one facing upwards and one forwards, adapted to the gloom of the deep ocean.
• The female attacks a salp, a barrel-shaped gelatinous zooplankter, cuts out its brain, gills and stomach with her claws, eats them and leaves the tube hollow. Then she moves inside and uses it as a house.
• Technically it is not a parasite but a parasitoid: it kills its host. But the barrel retains living cells, which keeps its structure intact.
• Inside she lays hundreds of eggs, which line the walls; the young feed on what remains of the salp's tissue. Hence its common name, the pram bug.
• It swims like a child with a float: it grips the barrel with its front legs and propels itself with the rear ones. The barrel also gives it buoyancy and works out cheaper than swimming free.
• Its extraordinary double eyes, tuned to blue, have puzzled scientists for decades.
• It is often cited as the inspiration for the xenomorph in the film Alien.

Skeleton type

Invertebrate - articulated calcareous exoskeleton (crustacean)

Habitat and environment

Open water between 200 and 1,000 m, rising towards the surface at night

Litter / clutch size

Hundreds of eggs, laid inside the barrel

Sources consulted: MBARI; Smithsonian National Museum of Natural History (Osborn); Sardet (2015), Plankton