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 Sea hare with its string of eggs wrapped around some algae (See separate photo of a Sea Hares mating) 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.
St. Abbs Marine Reserve, Berwickshire. E. Scotland