Despite its appearance the Sea Mouse, Aphrodite aculeata, is a type of worm and belongs to a group known as scale worms. Found all around the UK it is unlikely to be confused with any other animal. It has a convex upper surface and is sole-like below. The fringing bristles are iredescent. It is not often seen as it spends much of its life just below the surface in soft ground.
North Sea - St. Abbs Marine Reserve