Thick-horned Nudibranch

Hermissenda crassicornis

The thick-horned nudibranch (Hermissenda crassicornis), also known as the opalescent nudibranch, is a brightly coloured aeolid sea slug in the family Facelinidae. Adults reach about 5 cm long and have a translucent body marked with electric-blue lines, an orange head stripe, and clusters of orange-tipped cerata. The cerata store stinging nematocysts stolen from the hydroids and anemones the slug feeds on, providing chemical defense against predators. It lives on rocky intertidal and subtidal habitats from Kodiak Island, Alaska, south through British Columbia and Washington into northern California, typically at depths of 0–35 m on rocks, pilings, kelp, and floating docks where hydroids grow. A voracious predator, it eats hydroids, bryozoans, small anemones, ascidians, fish eggs, and even other nudibranchs, and is well known in neuroscience for studies of associative learning.