Check Out the Weird Asian Sea Slugs Invading SF Bay

The San Francisco Bay is being invaded by translucent, multi-colored East Asian sea slugs that have already been spotted creeping along docks on the peninsula.

There’s no reason to panic, although we’ll admit that the throbbing, alien appearance of these Japanese mollusks is a little alarming, looking as they do a bit like those mind controlling brain slugs from "Futurama."

But while these visiting nudibranches (technical name: dendronotus orientalis) may be harmless invertebrate tourists or an ecological disaster in the making, they have no interest in your central nervous system either way.

As the orientalis part of the name suggests, these little buggers are native to East Asian coastlines around Japan, China, and the Philippines, and have never before been sighted outside of their home waters. They almost certainly came to California by hitching an unwitting ride in the tanks of freighters, according to the California Academy of Sciences.

Robin Agarwal, an amateur marine wildlife buff from Redwood City, first spotted the new additions last month, after finding them clinging to a dock on the peninsula. She uploaded some video of them to the site iNaturalist so that other DIY biologists could figure out what they were.

It’s exciting that something new has popped up in local waters. It’s also potentially worrisome, since they are an invasive species. In a short video released on Twitter, the Academy of Sciences says it will be a while before we know if nudibranches are dangerous for bay ecology. They mostly eat hydrozoans, which are like jellyfish, although small enough for the nudibranches, which are themselves only a couple of inches long.

If nothing else, they’re a treat for local slug enthusiasts. "The name slug hardly does them justice," wrote one iNaturalist naturalist. "Many people feel that nudibranchs are the most beautiful organisms on Earth."

Now that you mention it, they are rather pretty, in a sluggy sort of way. But these things are in the eye—or in this case, the chemical-sensing, waving, frond-like rhinophores—of the beholder.

• Discovery of roving nudibranch [Cal Academy Science]
• Observation of the Week [iNaturalist]