Opalescent Sea Slug (Hermissenda crassicornis)

KINGDOM Animalia
PHYLUM Mollusca
CLASS Gastropoda
ORDER Nudibranchia
FAMILY Facelinidae


Nudibranchs are snails without shells and without the coiled body of their near relatives. One of the more obvious nudibranchs along Pacific shores, including Puget Sound, this species can be found crawling slowly in tide pools and on pilings, dock floats, and even open bottom substrates. With its opalescent body adorned with an orange midline and red, white and blue markings all over, this beautiful little animal always attracts attention.

Opalescent Sea Slugs grow to about 8 cm in length, although most encountered are smaller than that. They look as if they are four-horned, with a pair of large sensory tentacles (the simple eyes, composed of five cells each, are at the bases) followed by a pair of rhinophores. The rhinophores are specialized olfactory tentacles with corrugated surfaces to increase surface area for chemoreception.

The structures all along the body are cerata. As nudibranchs exchange gas through the entire body surface (some also have external gills), the cerata serve to increase the surface area of the body for that function. But they are also an important part of predator defense.

Hermissenda is a fierce predator on hydroids, cruising over their colonies and nipping off the polyps. It also eats other colonial animals and even scavenges on carcasses. It is aggressive toward other sea slugs of its own species, perhaps to reduce competition for prey, but it also eats them!

When Hermissenda eats hydroids, the nematocysts of its prey pass from the digestive tract through the body and enter the cerata, where they remain and are apparently quite effective in deterring predation. A potential predator that grabs the animal gets a mouthful of still active stinging cells, and it learns to leave these brightly marked mollusks alone.

These nudibranchs reproduce throughout the year. They lay strings of pink or white egg capsules that are pinched at intervals, looking like sausage links. Each capsule contains up to four eggs, which hatch after about a week into veliger larvae, which bear a ciliated swimming organ called the velum. The veligers enter the plankton and spend quite some time there before metamorphosing, by enlargement of the foot and loss of the velum, into tiny versions of the mature animals. The evolution of nudibranchs from shelled snails is shown by the development of a shell in the veliger larva, which is then lost during metamorphosis.