Along the central and northern California coast, a certain rose has been blooming like crazy—the Hopkins’ rose nudibranch, a type of sea slug. Adorned with a spray of bright pink appendages, the slug, which can grow to about an inch, looks like a “frilly piece of bubblegum,” says Jeff Goddard, a project scientist at the University of California, Santa Barbara’s Marine Science Institute, who has been studying nudibranchs for several decades. (One of the characteristics that differentiates nudibranchs from other sea slugs is that they’re carnivorous.)
The species—which is harmless to humans—is more widely found in tide pools along the Orange County and San Diego coasts. But over the past couple of months, it’s been cropping up in numbers rarely seen in the upper half of the state. “It’s a population explosion,” says Goddard. Other species infrequently found in those areas have also started appearing, including the California sea hare and another nudibranch aptly called the Spanish shawl. (For more sea slug news, click on the audio above.)
Goddard started noticing signs of a sea slug surfeit this past fall, when he was surveying a couple sites in central California. For instance, last spring at a location in Montana de Oro State Park, Goddard found only one Hopkins’ rose nudibranch, but in November, he spied dozens. And a couple weeks ago, the site boasted hundreds, if not thousands, he says. Other colleagues have been reporting similar flowerings.
What’s going on? The answer is blowing in the wind. Since July, “the wind’s been coming from the south a lot more than we usually see,” says Nate Mantua, a climatologist at the Southwest Fisheries Science Center, part of NOAA’s National Marine Fisheries Service. As a result, the current along California’s coast has shifted from flowing southward and away from the shore, as it typically does, toward the north and onto the shore.
“As far as we know, the [sea slug] bloom is resulting from their larvae being carried northward and onshore, into the tide pools,” says Goddard. Indeed, most sea slug larvae exist as zooplankton, riding ocean currents. When they grow large enough, they settle on rocky surfaces, where a food source signals their metamorphosis into juveniles.
For the Hopkins’ rose, that food is generally a type of bryozoan, a marine organism that lives in colonies of interconnected individuals called zooids. “The slug has a built-in muscular pump” that it uses to suck up one zooid at a time, says Goddard. The organisms have pink tissues and confer the slug with its rosy hue.
The last two times Hopkins’ rose nudibranchs occurred in such large numbers were during strong El Niño events in 1998 and 1983, according to Goddard. But we’re currently experiencing a weak El Niño.
The current sea slug spike instead “reminded me of the bloom that I saw in 1977,” which was also a weak El Niño year, says Goddard. The increase coincided instead with a shift in the Pacific Decadal Oscillation (PDO), which is like a long-lived El Niño event and can have dramatic effects on climate and marine life—both in the Northeast Pacific as well as on land. “That made me wonder if the nudibranchs right now are signaling that another climate shift like that—a decadal shift—is in the works,” he says.
Indeed, a shift in the PDO started brewing last year, according to Mantua, although it’s unclear how long it will last. If it persists, it could have a profound effect on currents, water temperatures, and nutrients available to marine organisms, with repercussions up and down the food web.
For Goddard’s part, he’s curious about what other sea slugs the wind might blow in. “What I want to do is see what other species show up north of here, and I also want to see just how far north,” he says, and whether they establish a gastropod-hold. He’s already planning a trip to check things out.