Pink sea slugs sporting soft neon spikes have been found along the central and northern California coasts, far north of their normal warm-water climates.
The University of California Santa Cruz reported last week that the species, the Hopkins' rose nudibranch (Okenia rosacea), has lately been found in significant densities from San Luis Obispo to Humboldt Counties.
Smithsonian Magazine highlighted the pink slugs with a tweet over the weekend.
"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 UCSC.
Pearse said that strong El Niños in 1998 and 1983 brought the sea slugs to the San Francisco Bay Area and even further north, but those populations receded in the intervening years.
Scientists aren't quite sure what's bringing the slugs further north in 2015, but say it could be a combination of factors.
"We have no idea whether this is part of the ongoing oscillation back and forth or if it’s perturbed by global warming; probably both," said Pearse.
Jeff Goddard, a project scientist at UCSB's Marine Science Institute, said the current population boom looks like one he observed in 1977, also during a weak El Niño. As it was then, he thinks the conditions have been brought on by the Pacific Decadal Oscillation, a period of elevated coastal water temperatures.
In addition to the pink sea slugs, scientists have also seen an increase in the populations of the Spanish shawl (Flabellina iodinea) and the California sea hare (Aplysia californica) along California's northern coastlines.
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