“Whoaaaa…what is THAT?”
“It’s gorgeous, whatever it is.”
“Dude, check this out!”
“What IS it?”
Music to a science educator’s ears, of course, thanks to the astonishing colors and reasonably viewable size of one of California’s most iconic sea slug species, the Opalescent Nudibranch (Hermissenda opalescens). Found throughout the Central California coast, these brightly-colored carnivores are often the first nudibranchs to astonish and delight the humans venturing into their intertidal world during seasonal low tides.
Photograph of Opalescent nudibranch, (Hermissenda opalescens) taken in Monterey, California. Courtesy of Robin Agarwal, under Creative Commons license via Flickr.
Nudibranchs are shell-less marine mollusks that primarily prey on live hydroids and sponges, depending on the species of nudibranch. While many nudibranchs’ diet is highly specialized, often limited to a single species, Opalescent Nudibranchs are a bit more voracious. They have been documented eating hydroids, bryozoans, small anemones, jellyfish, and even each other.
Not only are they impervious to the stinging cells—called nematocysts—of their prey, they actually absorb the nematocysts intact. These ‘stolen’ nematocysts are stored in the white-tipped “fluffs” on the Opalescent Nudibranch’s back, called its cerata, part of a specialized digestive tract that also doubles as a respiratory system. This is an excellent defensive mechanism for an animal that is otherwise relatively defenseless and slow. If a predatory fish ignores the flashy warning colors of the Opalescent Nudibranch and goes in for a bite, it gets the same sting it would as if it had bitten an anemone or a hydroid.
In case you’re wondering, these nematocysts are generally too weak to penetrate human skin, except for that one time when I received a wallop of a sting while photographing a very large Opalescent Nudibranch that had been feeding on dead jellyfish that had drifted inside a harbor. Yowza.
Given their pugnacious nature, it comes in very handy that Opalescent Nudibranchs can regenerate critical sensory body parts that are lost to predators or others of their own kind. While most Opalescent Nudibranchs that are missing an oral tentacle (food-sensing organs on either side of the mouth) or a rhinophore (chemical receptors on the top of their head) eventually will regrow it as before, occasionally you may find a nudibranch with forked (or multiple) regenerated organs.
Opalescent Nudibranch with a forked oral tentacle. Photograph by Robin Agarwal, via Flickr Creative Commons License.
Opalescent Nudibranchs have been used in laboratory settings for many years, as a model species for neurology, ecology, pharmacology, and toxicology research. However, this research may need to be revisited. In 2016, nudibranch researcher Dr. Ángel Valdés, a professor in the College of Biological Sciences at Cal Poly Pomona, pointed out that there are in fact three species of Hermissenda, not one, with two species present in California (the third is found in Japan and Russia). They are not difficult to distinguish, particularly if you have a clear photograph of the cerata. H. opalescens has a white tip on each cerata, whereas H. crassicornis has a long white line running up the front of each cerata and no white tip. Both species are about the same size—they can grow to eight centimeters, although less than five centimeters is more usual.
Video of Hermissenda opalescens (left) and Hermissenda crassicornis (right) in Monterey, California. Video courtesy of Robin Agarwal, via Flickr Creative Commons License.
Austin L. Ka’ala Estores-Pacheco, a graduate student in the College of Biological Sciences at Cal Poly Pomona, is working with Dr. Valdés to expand upon the findings of his 2016 paper. “I’m looking at the behavior of the two Eastern Pacific species—particularly to see if the species can mate—as well as doing next-generation DNA sequencing,” she said. “In the 2016 paper, Dr. Valdés discovered that the nuclear DNA was the same across all three species, and that it was the mitochondrial DNA that distinguished them. I’m expanding the sample size of the mitochondrial DNA work and looking at smaller fragments of the nuclear DNA to perhaps see differences that may be evident with a smaller fragment size.”
Whether you are an experienced biologist or a first-time tidepooler, there is always something new to be discovered—Opalescent Nudibranchs are a fascinating place to start.
Guest Author, Robin Agarwal
Lying flat on a bouncing floating dock underneath one of the biggest tourist-attraction piers in California, with my head and arms hanging over the side, I am frequently reminded of the kindness of fellow humans who think I’ve had a stroke or dropped my phone. No—just photographing sea slugs! I point, and show them a few photos on the back of my cheap underwater macro camera, and presto, another nudibranch enthusiast.
I was a tidepool kid who went astray and graduated with a liberal arts degree. In the last decade, I’ve returned to the tidepools and found a particular passion for photographing nudibranchs and other intertidal marine life. I’m co-editor of the California Sea Slugs Guide, and the Dock Fouling in California project on iNaturalist.org, where I have posted about 4,000 geotagged observations of nudibranchs, mostly along the Central California coast. Since I offer all my photos free to non-profit organizations (my way of thanking them for the work they do), you can find them all over the internet as well as Bay Nature magazine and NOAA National Marine Sanctuary informational signage. I’ve also been an enthusiastic contributor to a few scientific papers on nudibranchs, most recently Heterobranch Sea Slug Range Shifts in the Northeast Pacific Ocean Associated with the 2015-16 El Nino by Goddard et al. (2018).
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