Onchidoris muricata

(Müller, 1776)

This common species measures up to 14 mm. The colour is usually white but occasional pale yellow specimens may be found especially from northern localities. The ample mantle bears abundant rounded (pedunculate in places) spiculose tubercles which are taller and more slender towards the edge of the skirt. The rhinophores rarely bear more than twelve lamellae. There are up to eleven simple pinnate gills around the anal papilla.
This species is easily confused with Adalaria proxima , but O. muricata is generally paler in colour, the rhinophores are more pointed distally, and the digestive gland (visible ventrally through the translucent skin of the foot) does not extend so far forwards. In especially difficult individuals, it may be necessary to examine the radula.

O. muricata feeds upon a variety of encrusting bryozoans, especially Electra pilosa and Membranipora membranacea.

It occurs commonly all around the British Isles, to 15 m (Distr. O. muricata). Further distribution from Greenland and the White Sea to Cape Finistère, from Alaska, the San Juan Islands (Washington) and from Nova Scotia. There is an unconfirmed report from California.