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Hopkins' rose nudibranch. Image: Ken-ichi Ueda, Flickr

The Hopkins' rose nudibranch is a hot pink, inch-long sea slug that usually hangs out in the waters of southern California. But over the past few months, the flamboyant molluscs have been seen crowding tide pools far north of their usual haunts and rising ocean temperatures are likely to blame, scientists say. In fact, the species has not been spotted this far north, or in such large numbers, since the strong El Niños  periods of heavy rain and unusually warm ocean water that occurred in 1983 and 1998. 

Citizen scientists were among the first to raise flags about the strange population boom, tracking their observations on the website iNaturalist. Now, researchers from UC Santa Cruz, UC Santa Barbara, Bodega Marine Laboratory and the California Academy of Sciences have joined in, recording the vast cornucopias (that, apparently, is the collective noun for slugs) along the central and northern California coast, where the animals are rarely ever seen. 

"We haven't seen anything like it in years. These nudibranchs are a 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.

What makes this year's population explosion different to the spikes of decades past is that no official El Niño exists on record for 2015 leading researchers to worry that this latest bloom could be a signal of a much larger shift in ocean climate. 

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Scientists are reporting densities of up to dozens of nudibranchs per square metre. Image: Jeff Goddard, California Academy of Sciences

"While we are thrilled to see this beautiful bloom of normally rare nudibranchs, we are concerned about the long-term consequences of our changing coastal environment,” said Dr Terry Gosliner, Academy of Sciences Curator of Invertebrate Zoology and Geology.

Back in 2011, Gosliner and his team discovered a link between periods of warming ocean temperatures and nudibranch migrations. Their report, published in Limnology and Oceanography predicted that unusually warm temperatures, northward ocean currents and weak up-welling (a process that moves cold, nutrient-rich water to the ocean’s surface), if seen in combination, would cause larval nudibranchs to shift their range. "We’re seeing this exact cocktail of climate conditions in California right now,” Gosliner said. 

Unlike the young of birds and mammals, baby sea slugs are carried northward by coastal currents as microscopic larvae. Because they are fast-growing, live for a year or less and move little as adults, these nudibranchs make great study subjects for tracking rapid changes in ocean conditions – scientists can easily compare the movements and population size of each generation to that of the previous one, and gain information about current movements and upwelling strength (both key indicators of temperature change).

"Our current climate conditions are great for some of my favourite slugs, but we can’t ignore that warming seas mean less food for sea birds, and adverse impacts for all marine ecosystems ... We are tracking the trends to find out exactly what the shift means and how it might impact marine life," he added. 

The Hopkins' rose isn't the only animal that seems to be moving north: the bright purple and orange Spanish shawl (Flabellina iodinea) and the California sea hare (Aplysia californica) have also appeared in large numbers.

Though experts believe the population explosion might signal another major climate shift from cold to warm, it's too early to call just yet. But they predict that locals will see many more species from southern California appearing farther north. "California’s unique marine life can’t always adapt to so much instability," Goslinger warns.

Top header image: Ken-ichi Ueda, Flickr