The warm ocean temperatures that brought a green sea turtle to San Francisco in September and other southern species north of their usual ranges on the Pacific coast have triggered a population explosion of a bright pink, inch-long sea slug in tide pools along the central and northern California coast.
The Hopkins' rose nudibranch (Okenia rosacea) is locally common in southern California but sporadic in central California and rare north of San Francisco. In the past few weeks, however, researchers from UC Santa Cruz, UC Santa Barbara, Bodega Marine Laboratory, and the California Academy of Sciences have reported densities of up to dozens per square meter in tide pools from San Luis Obispo to Humboldt Counties. These are the highest numbers and northernmost records of this species seen since the strong El Niños of 1998 and 1983.
"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," said John Pearse, professor emeritus of ecology and evolutionary biology at UC Santa Cruz.
Pearse and other researchers have been conducting surveys of nudibranchs at study sites along the California coast for decades. "What makes this event especially exciting for us is that in 2011 we published a paper in which we predicted that oceanographic conditions like we are now experiencing would be marked by heavy recruitment of these and other nudibranchs. It's just wonderful to see the prediction coming true," Pearse said.
The research team includes his wife Vicki Pearse, a research associate at UCSC's Institute of Marine Sciences (IMS); Jeff Goddard, a project scientist at UCSB's Marine Science Institute; Terrence Gosliner, senior curator at the California Academy of Sciences; and IMS researcher Gary McDonald. Steward Shultz, the first author of the 2011 paper in Limnology and Oceanography, is at the University of Zadar, Croatia.
Pearse noted that the study relied in part on data from Goddard's senior thesis research in the mid-1970s, when he worked with Pearse as a UCSC undergraduate. "It is always pleasing to me to see our students' senior theses end up in publications," Pearse said.
Pacific Decadal Oscillation
Unlike the last time Okenia rosacea bloomed on the central coast, there has been no El Niño to speak of this year. According to Goddard, the current population bloom is similar to the one he observed on the central coast starting in early 1977. That bloom was also during a weak El Niño, but it happened to coincide with a major climate shift in the eastern Pacific Ocean known as the Pacific Decadal Oscillation. That shift marked the beginning of more than two decades of coastal water temperatures that were elevated compared to the preceding 30 years, triggering range shifts in numerous coastal species, including gastropods, barnacles, fish, and dolphins.
During those warm decades, fluctuations in the abundance of intertidal adults of Okenia rosacea and other nudibranchs in central California were driven by regional oceanographic influences on currents and larval recruitment. In the 2011 paper, the researchers predicted high recruitment of these nudibranchs during periods of warm temperatures, northward-directed currents, and weak upwelling, and that is just what is happening now, Goddard said.
Like the previous population explosions of Okenia rosacea, the current bloom has also been accompanied by other sea slugs normally found farther south, including the bright purple and orange Spanish shawl (Flabellina iodinea) and the California sea hare (Aplysia californica). While southern species of fish, seabirds, and marine mammals have also appeared in northern California as adults, nudibranchs differ in being carried as microscopic planktonic larvae on the coastal currents northward and onshore. When the larvae have fed and grown large enough (typically over a month or two), they settle to the bottom and, if they encounter the prey of the adults, they metamorphose into juvenile slugs.
The prey for Hopkins' rose nudibranch is a rose-colored encrusting bryozoan on which it feeds during its entire benthic life and from which it derives its pink pigmentation. This perennial bryozoan is locally abundant on the Pacific coast up into British Columbia. The nudibranch's distribution, therefore, is not limited by the availability of its prey but likely by a combination of cold temperatures and ocean circulation patterns.
Because they are fast-growing, live for a year or less, and move little as adults, these nudibranchs are potentially useful in tracking relatively rapid changes in ocean conditions. The current population bloom has the researchers wondering if the nudibranchs are signaling, as a sort of zoological semaphore, another major climate shift from cold to warm coastal waters.
"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," Goddard said. "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."
Good locations to view the current bloom of Okenia rosacea in central California include rocky shore tidepools and underneath ledges at Montaña de Oro and Asilomar State Parks, Scott Creek Beach, Pigeon Point, Pillar Point, and Fitzgerald Marine Reserve. Goddard advises checking local tide charts and going during a minus tide, preferably when both the swell and wind are low, and to tread lightly, wearing shoes or boots you don't mind getting wet.