Early spring is the best time to look for sea slugs on British coasts. Although some of these intriguing molluscs are intertidal, most favour deeper water for much of the year making them hard to find. However, coinciding usually with the extreme (spring) tides that occur around the time of the equinox, many species move into shallow water to mate and lay their eggs.
The term ‘sea slug’ is term applied to a range of marine molluscs most of which belong to a group called nudibranchs (which literally means ‘naked gills’). Unlike most other molluscs, but in common with their terrestrial cousins, adults do not have shells. They have paired tentacles at the head end and depending on the species, a body covered in wart-like protuberances or elongate cerata. Most sea slugs can release toxins if attacked; as an added defence those that feed on hydroids accumulate their prey’s stinging cells in their cerata.
Above: The Sea Lemon Archidoris pseudargus is one of our commonest sea slugs and can reach a length of 10cm. It is found under boulders on the lower shore and feeds on encrusting sponges. Paul Sterry/Nature Photographers Ltd
Above: The eggs of the Sea Lemon Archidoris pseudargus are laid in ribbons arranged in a spiral. Paul Sterry/Nature Photographers Ltd
Above: The Grey Sea Slug Aeolidia papillosa is a large species, covered with cerata that give it a texture like a floor mop. Paul Sterry/Nature Photographers Ltd
Above: Jorunna tomentosa has a felt-like texture with two tentacles at the head end and an array of gills at the rear. Paul Sterry/Nature Photographers Ltd