White spirals of spawn on the underside of rocks are evidence of the presence of the Grey Sea Slug , Aeolidia papillosa , which feeds exclusively on sea anemones. Like most nudibranchs it is an annual and on the occasional spawning year scores of dead bodies could litter the shore in the run of about a week when the tides are low. Grey Sea Slug feeding on a Beadlet Anemone . Photograph by Andy Horton . Sea anemones will close their tentacles when attacked and the Grey Sea Slug is not able to demolish every victim.
Sea anemones can vary in frequency from year to year.
On occasions suitable shores with rocks below the surface of shell or sand in shallow pools are carpeted with hundreds of multi-coloured anemones of the species Sagartia troglodytes . In deep limestone pools in the south-west, will be covered with the brown varieties of Daisy Anemones , Cereus pedunculatus , each specimen poking its column and span of tentacles out of narrow fissures in the rock. Every rockpooler knows that a crab has ten legs including the claws, so when you count the legs on the small porcelain crabs and you can only find eight, there needs to be some explanation. The answer is that they are not true brachyuran crabs, but anomurans like the hermit crabs, and the fifth pair of legs is rudimentary and cannot be seen. Porcellana platycheles could fit comfortably on a 50 pence coin, and the Long-clawed, Pisidia longicornis , could hide under a postage stamp.