Celebrating National Sea Slug Day

I’ve just discovered that my birthday, October 29, is also National Sea Slug Day, so I thought I’d look into these creatures about which I know nothing, alerted by a news release. A new 74-page paper in the Zoological Journal of the Linnean Society offers numerous photos, sketches, and electron micrographs of the animals, a type of marine invertebrate also known as a nudibranch (“naked gills”).

In the past, invertebrate zoologists have sorted out the 3,000 or so species of sea slugs by color patterns; whether the gills are elevated, vibrating, or neither; and the shapes of the jaw and the radula, a tongue-like structure with tiny teeth. The new paper adds DNA sequencing to the list of classifying criteria, which added 17 new species to the known 57 or so. All 17 are members of the vibrantly-colored genus Hypselodoris.

The researchers, from the California Academy of Sciences, compared the DNA sequences of three highly variable genes commonly used to sort out species in evolutionary studies. The DNA analysis split Hypselodoris into three subgroups.

“Nudibranchs have always been a marine marvel with their dazzling color diversity. We’re only beginning to understand the evolution of color. This is the first time we’ve had a family tree to test longstanding hypotheses for how patterns evolve,” said Academy curator and study author Terry Gosliner, an invertebrate zoologist who’s discovered more than 1,000 sea slugs species. He traces his interest back to childhood exploration of tide pools in Marin County. And his birthday is October 29 too, National Sea Slug Day designated in his honor.

Sea slugs (aka sea hares) are residents of shallow coral reefs, abundant in the shallow waters of  the Indo-Pacific. They range in length from a fraction of an inch to more than two feet. Common names reflect their multi-hued appearance and varied shapes: marigold, clown, dragon, and Spanish dancer. The animals are carnivores, eating barnacles, sponges, jellyfish, sea anemones, and tunicates, and other sea slugs, including their own species.

The new report is festooned with spectacular photos of sea slugs in all their gorgeous pinks and purples, blues and yellows, oranges and purples, with lines and spots, borders and bands. Their colors, shades, and patterns enable them to blend into the surroundings, or resemble nasty-tasting species.

The sea slugs are hermaphrodites, with both types of sex organs on their right sides. They have vaginal ducts and penial bulbs, and two sperm-storing sacs with great names: the receptaculum seminis and bursa copulatrix. A skewed size ratio of these last two – the first much smaller than the second – is an identifying characteristic of genus Hypselodoris.

Sea slugs have a curious talent. They swallow the stingers (nematocysts) of jellyfish and instead of digesting them, reroute them to their own skin, weaponizing themselves. The co-opted stingers are called kleptocnidae (jellyfish are also known as cnidarians). Sea slugs can keep the chloroplasts from swallowed algae so they can churn out their own food, a talent termed kleptoplasty. Some sea slugs ingest stinky sponges and become stinky themselves, like us eating garlic, and others produce their own odoriferous chemicals.

Perhaps the most intriguing characteristic of certain sea slugs is their Müllerian mimicry. Some species are not distasteful to predators but sport a color pattern identical to a species known to be a bitter mouthful – like the Monarch and Viceroy butterflies, and certain bees, frogs, and snakes. Predators avoid the species that superficially resembles the poisonous one. “Sea slugs have an arsenal of strategies for surviving, from mimicry to camouflage to cryptic patterns,” said Gosliner.

The addition of genetic information in the classification and identification of species of Hypselodoris sea slugs enabled the researchers to distinguish species that have the same color pattern because they independently solved the same problem (convergent evolution) from species that share a pattern because they descend from a recently shared ancestor.

Hypseledoris iba and Hypselodoris bullock, for example, have the same purple pattern. “When two different species, like H. iba and H. bullock, present in the same color, the simplest explanation is that they share a common ancestor,” said co-author Rebecca Johnson. “These two species, however, are pretty far apart on the family tree. The more likely explanation for their similar appearance is that they reside in the same geographic region where being purple is advantageous for avoiding predators either as camouflage or warning of distastefulness.”

Adding DNA to the classification of sea slugs can help in tracking changing ocean conditions as the climate warms. In 2015, for example, populations of sea slugs called the Hopkins’ Rose nudibranch along the California coast exploded in response to a warming Pacific ocean.

“Nudibranchs are more than just colorful ambassadors for the coral reef systems they inhabit. Documenting their diversity and monitoring their regional presence helps us in understanding how ecosystems are faring,” said Johnson. She co-leads the Academy’s Citizen Science Program and encourages people who find interesting sea slugs, or anything else, in tide pools to upload the images to iNaturalist.