Sea hare, Aplysia fasciata caught off Dorset, now at the National Marine Aquarium

21/06/2007 00:00:00 Aplysia fasciata Aplysia fasciata is the largest and the rarest of the three species of sea hare found in the British Isles. It is an Atlantic species, found from the Channel to Angola (South-west Africa and to Brazil) and also throughout the Mediterranean.

It appears to reach its northern limit in Ireland and along the Channel coast of England. It is one of the largest sea slugs in the world. The other two British species are the relatively common Aplysia punctata variable in colour and growing to 20 cm; and Aplysia depilans with different shaped back lobes, brown or green and growing to a maximum of 30 cm. An Aplysia fasciata specimen of 30 to 35 cm and weighing 1.5 kg was caught in Poole Bay, Dorset, in October 2007.

It was brought into the National Marine Aquarium where it is on show as a ‘Feature Creature’ in the recently refurbished Shallow Waters, Hidden Depths exhibit, where it is devouring large quantities of sea lettuce.

Becoming more common
There were several recorded in the Channel Islands in the mid 1800s, but the first one in mainland Britain was found in Salcombe Estuary, Devon, at extreme low water in February 1949. Another was found at the same place in 1997. They are very rare but have also been found in Ireland and Cornwall. This year from late August to mid October, a number of individuals have been found from south west Cornwall to Dorset and in Jersey. Several were washed up on two beaches on the south Devon coast (described as ‘purple, slug-like, up to the size of a rugby ball’), and egg masses have been found in a sheltered inlet.

Sea Slugs
They are impressive animals growing to 40 cm and weighing up to 2 kg. Most found in Britain have been smaller, but the specimen from Poole was a large one of 30 to 35 cm and 1.5 kg.

While called sea slugs they are very different from garden slugs, being some of the most spectacular and beautiful of molluscs.

The sea hares have a small thin internal shell, largely covered by the large wing-like body flaps which also protect their gills. These give it a bat-like appearance when swimming. They vary from bright red to brown in colour, have a clear head, tiny eyes and have two pairs of tentacles, the larger of which look like rabbit’s ears. It is these tentacles along with its large size and rounded body shape that give it a rabbit-like look and consequently its common name. When stressed they release a purple ink into the water which is contains the toxin opaline. The animals are said to be mildly toxic but are eaten in some areas of the world. Most sea slugs feed on other animals including sea anemones, but the sea hares are vegetarians preferring seaweed.

Hermaphrodite
They come inshore to breed, usually in the Spring. Each sea hare is both male and female being a simultaneous hermaphrodite. They are known to form long mating chains, with each animal being a male to the one in front of it and female to the one behind. The penis is on the side of the head just below the right anterior tentacle. They then lay a pink to orange chain of eggs forming large spaghetti-like masses at the bottom of the shore or in shallow water. The young hatch from these, spend some time as a veliger larva in the plankton and them settle on algae as a tiny 1-2 mm sea hare. They grow rapidly reaching full size in a year, before breeding and dying.

They are a rare southern species but a combination of climatic conditions appear to have brought quite a few to our southern shores this year. This is probably a one-off occurrence, and there is no reason at present to link it to climate change, though it could be related to changes in oceanic currents.

Douglas Herdson 26th October 2007.