(Alder & Hancock, 1842)
North Atlantic, as far south in Europe as France, & New Hampshire in North America. Very similar species Cuthona divae (Marcus, 1961) in northeast Pacific.
In hydroid colony, note egg masses. Plompe toren, de Oosterschelde, the Netherlands. PHOTO: Peter H. van Bragt
Grows to about 30mm. It feeds predominately on the hydroid Hydractinia echinata, which is often, but not exclusively, found attached to shells inhabited by hermit crabs.
• Thompson, T.E. & Brown, G.H. (1984) Biology of Opisthobranch Molluscs, Vol 2. Ray Society: London.