The Sea Hare or Aplysia punctata is related to nudibranchs (or sea slugs) and is common on many UK coasts some years and then virtually absent in others. The picture shows a mating pair but they have been observed mating in chains of up to ten individuals. ( See separate photo of a Sea Hare with its eggs) They can be found in shallow water, and even rock pools, where the feed by browsing on algae. When disturbed they can discharge a toxic cloud containing sulphuric acid which is distasteful to any fish which may think they are a tasty morsel.
North Sea. St. Abbs Marine Reserve