Biology/Natural History: It is one of the most abundant nudibranchs in Califonia. It eats hydroids, but the diet also includes small sea anemones, bryozoans, colonial ascidians (Aplidium solidum, botryllids), annelids, small crustacea, tiny clams, dead animals of any sort. Will eat other Hermissenda. In the Puget Sound, Hermissenda is the main predator of the sea pen Ptilosarcus gurneyi. Mating animals are most often found in southern California in winter, but are found year around in the Puget Sound (Washington). The egg string resembles linked pink sausages. They are commonly attached to algae and to blades of eelgrass. Each egg case usually contains one egg, but can contain up to four. Many studies have been carried out on Hermissenda, but the main area of focus is the eye. It has five cells, each about 75 um in diameter, which are large enough to receive a recording electrode. Within the cells it is suspected of containing symbiotic fungi. Hermissenda is an aggressive creature. When two individuals encounter fights will break out, which involves lunging and biting. Encounters most likely to induce a fight are those of mutual head on contact. The individual whose head is closest to the others tail or side will usually get the first bite in, this also means that they usually come out the winner. The copepod Hemicyclops thysanotus is often found adhering to the dorsal surface of Hermissenda.
The nudibranch Phidiana hiltoni may attack this nudibranch (Goddard et al., 2011)