A Case Study Relates Ocean Currents to the Northward Distribution of Nudibranchs

Researcher:

Jeffrey Goddard
Research Biologist
Marine Science Institute
UC Santa Barbara
E.:goddard@lifesci.ucsb.edu
T. (805) 688-7041 / 893-7245

Tools:

Revised:

February 4, 2009

A Spanish shawl, Flabellina iodinea.

A Spanish shawl, Flabellina iodinea. Photo: Gary McDonald

February 4, 2009

Contact: Christina S. Johnson, csjohnson@ucsd.edu, 858-822-5334

The bright purple and orange creature (above) is a type of sea slug, known commonly as a Spanish shawl. Normally, it is rare north of Point Conception, except during El Niño conditions. Last November, however, UC Santa Barbara biologist Jeff Goddard discovered four of the striking mollusks in tidepools at Scott Creek in Santa Cruz County.

"I was amazed," he says. "We've been in La Niña conditions for the past year so their presence didn't make sense to me."

He has since been able to explain their unexpected presence to an abnormally intense, short-lived northward jet of water, recorded in early September by a network of high-frequency radar stations maintained by the Central and Northern California Ocean Observing System. The flow was also correlated to anomalously high sea levels.

"Based on their size, I figured that the nudibranchs (sea slugs) were about two months old and sure enough two months earlier, we see evidence of a strong northward current along the coast," Goddard says.

"Oceanographers tell us pulses of poleward flow are frequent in late summer off California. What is cool is that we now can make a pretty convincing case that surface currents transported nudibranch larvae around Point Conception."

Working backward from the surface current data, Goddard predicted that larvae also should have been swept to Sand Dollar Beach in southern Monterey County. This was borne out, as in December he found three more adult Spanish shawls at the study site.

"They usually aren't present at this site or anywhere in the intertidal north of Point Conception," he says.

Sea Grant is funding Goddard to see if nudibranchs, which have short life spans and are relatively immobile, can be used to better understand how climate patterns affect fine-scale intertidal ecology in California. As part of this project, he is studying the historical nudibranch record at several sites, including Monterey, Santa Cruz, Half Moon Bay and Duxbury Reef, and correlating their diversity and abundance to El Niño and La Niña cycles and the Pacific Decadal Oscillation.

El Niño episodes lead to sea slug population booms, he reports, not because of warming but because of enhanced onshore flows that sweep larvae to the coast. The reverse occurs during La Niña cycles: Sea slug numbers decline, probably because of stronger upwelling and stronger offshore flows.

Scientists have documented similar patterns with another coastal invertebrate, barnacles. "What we are seeing with adult nudibranchs is consistent with what other researchers are finding with the settlement of intertidal barnacles," he says. "The nudibranchs are like bright flags, and I feel like I'm beginning to get a handle on what the flags signify."