Researchers in California have recorded a sharp increase in Okenia rosacea, also known as the Hopkins’ rose nudibranch or pink sea slugs much farther north than they normally venture. The appearance of the slugs normally coincides with warmer sea temperatures except this time there’s nothing to count for the temperature increase except for climate change.

Warmer temperatures are causing a sudden uptick in invasive species as animal and plant populations readjust to find the temperatures that suit them. This is as much true at sea as it is on land. The pink sea slugs are common in southern California, uncommon but not unusual in central California and normally very rare north of San Francisco.

Recently surveys from UC Santa Cruz, UC Santa Barbara, Bodega Marine Laboratory, and the California Academy of Sciences have reported dozens of the animals per square meter in tide pools in California’s north.

The last time Okenia rosacea appeared in these kind of numbers was during the strong El Niño years of 1983 and 1998. This year however, there has been no El Niño to speak of.

“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. 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,” John Pearse, professor emeritus of ecology and evolutionary biology at UC Santa Cruz, told the university’s newscenter.

The research that Pearse is referring to was first published in the 2011 paper “Climate-index response profiling indicates larval transport is driving population fluctuations in nudibranch gastropods from the northeast Pacific Ocean“ published in Limnology and Oceanography.

According to Jeff Goddard, a project scientist at UCSB’s Marine Science Institute, an uptick in population of the sea slugs was seen from 1977 until 1998, after which the population declined again.

This period marks the Pacific Decadal Oscillation, a two decade period of warming in the pacific, which triggered habitat shifts in a number of coastal species.

The slowing of the Pacific Decadal Oscillation coincides roughly with the start of the ‘global warming hiatus’. The return of the sea slugs could be one of the first signs that thet ‘hiatus’ is coming to an end or, at the very least means that temperatures appear to be climbing again with the assistance of El Niño.

“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,” said Goddard.

Researchers consider the Okenia rosacea, which is fast growing and lives for less than a year, a good indicator of sudden changes in ocean conditions.

The surveys also noted population increases in other sea slugs including the California sea hare (Aplysia californica) and Spanish shawl (Flabellina iodinea). The appearance of species of fish, marine mammals and sea birds which normally remain in the south has also been noted.