Taxonomy

Rarity

Abundance rates how many are in suitable habitat. Find difficulty rates how hard it is to locate one on a daylight snorkel or freedive through 50 ft. A higher find score means a harder search. The rarity category also reflects the kind of encounter; it is not an average of the 2 scores.

Seasonality

Location

Opalescent nudibranchs live in rocky tide pools, on pier pilings and marina floats, and on mudflats. Their main food is hydroids, the fine, branching colonial relatives of jellyfish that grow on rock and pilings. They live from the low intertidal down to about 115 feet.

The animal reaches about 3 inches. A bright orange band runs down the middle of the back from just in front of the head tentacles, usually bordered on each side by a broad white or pale blue line. The back is covered in rows of finger-like projections called cerata, orange with white tips. Young animals show orange patches edged with bright light blue lines.

Deep Dive

Until 2016 this animal shared its name with a northern relative. Older field guides treat the northern and southern animals as one species, and in 2016 a genetic study showed they are 2. The northern species carries a white stripe down the front of each of its cerata, and the southern one, this page's animal, lacks that stripe. Many older photos and accounts mix the 2.

Its diet is wide. Hydroids are its main food, but it also eats small sea anemones, bryozoans, colonial sea squirts, worms, small crustaceans, tiny clams and dead animals, and it will eat other opalescent nudibranchs.

It is also a fighter. When 2 meet head to head, the encounter often turns into a fight, with lunging and biting. The animal whose head ends up nearest the other's tail or side usually bites first and tends to win. Another sea slug, Hilton's aeolid, may attack it in turn.

Its eyes have made it useful to science. Each eye holds just 5 light-sensing cells, each about 75 thousandths of a millimeter across. That is large enough for a researcher to record from a single cell with an electrode, and the eye is the main focus of the research done on this animal.

In Southern California, mating is seen most often in winter. The eggs are laid in strings that look like linked pink sausages, attached to seaweed or eelgrass blades, with usually 1 egg in each case and sometimes up to 4.

Where to Find Them

Researched habitat and range candidates

These locations have suitable habitat within the species range. An assignment is a possible encounter, not a confirmed sighting at every beach. Season, depth, and local conditions still matter. Historical habitat without present-day evidence is excluded.