John Metcalfe was CityLab’s Bay Area bureau chief, covering climate change and the science of cities.
Dendronotus orientalis nudibranchs likely hitched a ride to California in ballast water.
“[B]ut is it a good thing or bad that it’s here,” is the reasonable question posed by one viewer of this video that the California Academy of Sciences posted this week.
If San Franciscans start waking up with dozens of these things latched to their bodies, gently palpating the flesh while extracting vital fluids, we’ll know the answer is “bad.” But until then everything’s in the air, because this is reportedly the first sighting of Dendronotus orientalis ever recorded outside Asia.
Robin Agarwal and her daughter found the inch-long nudibranchs, or sea slugs, clinging to a dock in the San Francisco Bay near Redwood City. She uploaded images of her catch to the iNaturalist community, which helped make the identification. It’s unclear how the exotic species got to California, but the reigning theory is they stowed away in ships’ ballast water—the same form of transport that brought invasive zebra mussels to the Great Lakes.
Here’s more on nudibranchs from iNaturalist:
One of the most active and encouraging members of the iNaturalist community is Susan Hewitt (@invertzoo), a “serious amateur” malacologist, who tells me that nudibranchs are a diverse group of sea slugs, “although the name ‘slug’ hardly does them justice! Many people who are familiar with them feel that nudibranchs are the most beautiful organisms on Earth, their colors and forms rivaling or exceeding even those of butterflies.” Not much is known about this particular species, but Susan points out its most distinguishing feature, which is that the sheaths of its rhinophores (chemoreceptive structures protruding from the body) extend to well past the body length of the animal. And like many other nudibranchs this species eats hydrozoans, tiny predatory (and often colonial) animals related to jellyfish and corals. Some nudibranchs even sequester the stinging cells of hydroids and use them for self-defense!
Chalk up the nudibranch’s arrival to just one more sign that gelatinous ocean creatures love San Francisco. In 2015, bloated “sea hares” were spotted crawling along the bottom of Lake Merritt in Oakland, and before that "By-the-Wind Sailors" transformed area beaches into seeming repositories for thousands of neon-blue, deflated condoms.
Here are stills from the Cal Academy video; the full production is posted below.