Amusing Monday: Colorful sea slugs reveal evolutionary strategies

In conjunction with National Sea Slug Day last Monday, the California Academy of Sciences released colorful photographs of 17 newly identified nudibranch species.

Striking colors and unusual color patterns were given a special focus in a genetic study that is helping to group the nudibranch species and understand how they evolved. Hannah Epstein, affiliated with the California Academy, was the lead author on the research paper published in the Zoological Journal of the Linnean Society.

Researchers were surprised to learn that this nudibranch was the same species, Hypselodoris iba, as the one directly below on this page.
Photo: Terry Gosliner, © California Academy of Sciences

“When we find an anomaly in color pattern, we know there’s a reason for it,” said Epstein, now a researcher at James Cook University in Australia, quoted in a news release.

“It reveals a point in evolution where a selective pressure — like predation — favored a pattern for camouflage or mimicking another species that may be poisonous to would-be predators,” she noted.

Terry Gosliner, an invertebrate zoologist credited with discovering more than a third of all known sea slug species, added this:

This nudibranch and the one above are the same species, Hypselodoris iba.
Photo: Terry Gosliner © California Academy of Sciences

“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.”

National Sea Slug Day was recently established by Christopher Mah, an invertebrate zoologist at the Smithsonian Institution. To honor Gosliner, Mah chose Oct. 29, Gosliner’s birthday, for this special day of recognition for sea slugs. Check out Mah’s blog for details.

Nudibranch species Hypselodoris brycei
Photo: © Nerida Wilson

“Sea slugs have an arsenal of strategies for surviving, from mimicry to camouflage to cryptic patterns,” said Gosliner, who has described more than 1,000 nudibranch species. “We’re always thrilled to discover new sea slug diversity. Because nudibranchs have such specialized and varied diets, an area with many different species indicates a variety of prey — which means that coral reef ecosystem is likely thriving.”

Nudibranch species Hypselodoris katherinae
Photo: Terry Gosliner © California Academy of Sciences

Changes in nudibranch populations can be an early sign of changing conditions, such as seen in 2015 during a population explosion of Hopkins’ rose nudibranchs along the California Coast. The sudden shift came during a period of ocean warming. See news release, California Academy of Sciences, and the web page for iNaturalist, where volunteer observers were among the first to report the phenomenon.

Nudibranch species Hypselodoris peri
Photo: Terry Gosliner © California Academy of Sciences

The complexity of color variations is exemplified by members of the genus Hypselodoris, which inhabit coral reefs of the Indo-Pacific tropics. Coral reefs are home to some of the most astounding colors and patterns on Earth.

During the research, scientists encountered one sea slug that was lavender with a white stripe and another that was cream-colored with a lavender stripe and orange spots. They were assumed to be separate species until a diver took a photograph of the two mating. Genetic analysis revealed that they were the same species, Hypselodoris iba.

Nudibranch species Hypselodoris violacea
Photo: Terry Gosliner © California Academy of Sciences

Meanwhile, that lavender sea slug appeared to be amazingly similar to another purple species found in the region, Hypselodoris bullocki, yet the two were quite distinct.

“When two different species like H. iba and H. bullocki present in the same color, the simplest explanation is that they share a common ancestor,” Rebecca Johnson, another member of the research team, said in a news release. “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.”

Through genetic analysis, the researchers were able to show that distant relatives can evolve independently but appear quite similar to each other as each tries to cope with similar environmental conditions. This tendency is known as convergent evolution.

Some nudibranchs use their bright colors to warn away predators by advertising that they contain toxins that make them unpalatable. Other species may mimic that coloration, successfully detering predators, even though they do not contain a toxin.

For additional background on nudibranchs, including photos from the Puget Sound region, check out Water Ways, Oct. 12, 2015.

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