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

Hilton's aeolid lives on rocky shore, and the studies of it come from rocky intertidal reefs such as Duxbury Reef in Marin County and the Monterey area. It grows to about 2 inches. Like other aeolids, its back is covered in finger-like projections called cerata, and it closely resembles the opalescent nudibranch. The usual mark is a reddish or brownish line across the head, which is almost always present.

Deep Dive

In 1977, Hilton's aeolid began spreading north from Monterey. By 1992 it had reached Duxbury Reef, about 100 kilometers, or 60 miles, farther up the coast. Scientists from UC Santa Barbara, the California Academy of Sciences and UC Santa Cruz then asked what the newcomer was doing to the nudibranchs already living there, and published the answer in 2011.

In the wild it ate mostly hydroids, the same food many other aeolids depend on, and it also showed evidence of eating nudibranchs. In feeding trials it attacked most of the aeolid and dendronotid nudibranchs offered to it and swallowed small ones whole. Only slugs longer than about 15 millimeters, a little over half an inch, were effectively safe. At Duxbury Reef, the combined numbers of nudibranchs that it can attack fell by an average of two-thirds after it arrived, while the species it cannot attack showed no change. The researchers concluded that Hilton's aeolid likely caused the decline, both by competing for the same food and by eating the other slugs directly.

The study documents a range shift changing the community a species moves into, measured on the same reef before and after. The animal is also aggressive by reputation, biting and fighting with other nudibranchs, its own species included.

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.