Pink Sea Slug Boom Reported Along Central and Northern California Coast

Posted on January 31, 2015

Photo of a Hopkins' rose nudibranch

There is a boom in the population of pink sea slugs along the Central and Northern California coast. These bright pink slugs are about one inch long. The slug is the Hopkins' rose nudibranch (Okenia rosacea).

The colorful slugs get their pink color from feed on rose-colored encrusting byrozoans. The nudibranch is common in southern California but it is considered rare north of San Francisco. Lately this has been changing as researchers from the UC Santa Cruz, UC Santa Barbara, Bodega Marine Laboratory, and the California Academy of Sciences have found densities of up to dozens per square meter in tide pools from San Luis Obispo to Humboldt Counties. The scientists say this is the highest density of the pink slugs since the El Niños of 1998 and 1983.

John Pearse, professor emeritus of ecology and evolutionary biology at UC Santa Cruz, says in a release, "We haven't seen anything like it in years. These nudibranchs are mainly southern species, and they have been all but absent for more than a decade."

Pearse says they predicted this boom in the sea slugs in a research paper in 2011. He says, "It's just wonderful to see the prediction coming true."

Pink nudibranchs in a tide pool


There is no El Nino this year but Jeff Goddard, a project scientist at UCSB's Marine Science Institute, says the current bloom is similar to one in 1977 that coincided with a weak El Niño and the Pacific Decadal Oscillation, a major climate shift in the Pacific Ocean.

Goddard says, "Although the index values of the Pacific Decadal Oscillation have really jumped the past few months, it's probably still too early to say, at least until we become better at decoding the signals. However, if a decadal shift is in progress, there's a good chance the next El Niño will be a major event, on par with the 1983 and 1998 events, and bring with it myriad surprises from the south."

Photos: Jeff Goddard