Climate change may be destroying coral reefs with the acidification of our ocean waters, and warming oceans have caused dolphins and tropical fish species to increase or change their ranges, but the Hopkins’ Rose nudibranch, Okenia rosacea, a beautiful pink sea slug, is loving the rising ocean temperatures.
The Okenia rosacea is a species of sea slug, a shell-less marine gastropod mollusk first described in 1905 off the coast of Southern California. It feeds on a species of bryozoans, tiny rose-colored encrusted aquatic invertebrate animals, typically about 0.50 millimeters in length. Interestingly, the individual bryozoans form a colony that mutually benefits all members in feeding and other functions. These animals also give the nudibranch its color. It is abundant off the Pacific Coast as far north as British Columbia. The nudibranch makes use of the colony not only for food, but to feed its egg mass.
Scientists from the California Academy of Sciences, UC Santa Barbara, UC San Diego and Bodega Marine Laboratory are now reporting the Hopkins’ Rose nudibranch is showing up in extraordinary numbers in tidal pools as far north as Humboldt County in California, with densities up to dozens per square meter (10.8 square feet). These numbers are the highest recorded densities and northernmost sightings of this species since the El Niños of 1998 and 1983.
The Hopkins’ Rose nudibranch is the “Canary in the coal mine”
In 2011, an important paper was published in the journal Limnology and Oceanography. It was predicting that a shift in the range of sea slug populations was inevitable when warming ocean temperatures, northward ocean currents, and weak upwellings (where colder nutrient-rich water is pushed up to the surface), overlap. The Hopkins’ Rose nudibranch has turned out to be the canary in the coal mine. With a quick growth rate and a lifespan of about 12 months, most of its life is spent on the ocean floor.
The Hopkins’ Rosa nudibranch is very useful in tracking rapid changes in ocean conditions, says Zoologist Jeff Goddard, a project scientist with the study. Pointing to the 2011 study, Goddard said, “They predicted high recruitment of these nudibranchs when warm temperatures, northward-directed currents, and weak upwelling occur—and that is exactly what is happening now.” Now, Goddard and the rest of the team are wondering if the current explosion of nudibranches is signaling another shift from cold to warmer temperatures.
The possibility of another major climate shift is still unpredictable, but at least scientists have better research and data available to them in understanding the fluctuations they are seeing. If a shift is in progress, Goddard says that there is a very good chance the next El Niño will be major, on a par with the 1983 or 1998 El Nino events.
