Sea Slugs

Sea slugs are marine gastropods, which just like their land equivalents, evolution has led to the reduction of the shell to a small internal remnant or is completely lost. Without the protection of a shell, sea slugs have had to evolve alternative defence strategies. Depending on the species these vary but they can include, giving off a repulsive liquid or using stinging cells.

Many species of sea slug that eat sea anemones or hydroids, do it without activating their prey’s stinging cells. However some species do not even digest them, instead these stinging cells (nematocysts) are utilised for the sea slug’s own defence. There are protuberances on their back the cetera, here the acquired nematocysts are stored close to the surface. If provoked, these nematocysts can be fired.

Sea slugs tend to live offshore but can be found on the shore in summer where they congregate to breed. They are hermaphrodite, containing both male and female reproductive organs. When a pair mates both fertilise each other. There are many species that come to breed on Cornish shores, but those commonly found are the Sea Lemon Archidoris pseudoargus, Common Sea Hare Aplysia punctata and the Common Grey Sea Slug Aeolidia papillosa.

Sea Lemon Archidoris pseudoargus

The sea lemon can reach over 5 inches in length. Sea lemons have a ring of feathery gills at their rear, and two antenna at the front. It is commonly yellow hence the name, but the colour does vary. It feeds on sponges and sea squirts.

Sea Hare Aplysia punctata

Aplysia punctata is a red to brown coloured sea hare that can reach 8 inches in length. The sea hare has two pairs of tentacles and two large flaps (parapodia). The upper pair of tentacles are said to resemble hare’s ears, hence the common name.

It is not a true sea slug but a tectibranch as it possesses a shell. This fragile, transparent shell is located internally within the animal.

During the summer they congregate on shore in large numbers to breed, laying pink ribbons of spawn. If provoked it produces a purple slimy liquid.

Grey Sea Slug Aeolidia papillosa

The grey sea-slug is coloured grey tinged with brown or lilac. There are rows of cetera on its back, these are primed with nematocysts from its prey of anemones and hydroids. It can reach six inches in length.