Specieswatch: leathery sea squirt, the knobbly invader from Korea

Seaside trippers gazing into harbours over the bank holiday might have wondered about the rather ugly creatures, about 16cm long, commonly seen attached to rocks, ropes and the hulls of boats. This is an invasive species known as the leathery sea squirt, Styela clava.

Styela clava arrived in Plymouth in 1952 on the hulls of warships returning from the Korean war and from there, they spread around the UK’s coasts.

Anchored on tough stalks, they have long club-shaped bodies, described as “rumpled” and “knobbly”. They feed on plankton and other organic material, gathering their food as they suck in seawater and squirt it out again. The Koreans eat Styela clava steamed, with watercress and bean sprouts, but in Britain they are regarded as a menace because they foul the underneath of boats, mooring lines and oyster beds.

The sea squirts need water temperatures of 15C to breed and are hermaphrodite, although more than one individual needs to be present to reproduce, as the male and female parts mature at different times to avoid self-fertilisation.

Larvae float freely for a couple of days before clamping onto a hard surface where they stay for the rest of their lives. Efforts to control their spread have so far proved unsuccessful, and they are clearly here to stay.