New Species Of Camouflaged Sea Slugs Caught Moonlighting As Toxic Algae

Seaweed or sea slug? Pictured are Caulerpa lentillifera, known more commonly as sea grapes. Sompraaong0042/Shutterstock

For more than half a century, the tiny toxic-slurping species of sea slug dependent on sea grapes was thought to be just one species. A new study published in Zoologica Scripta looks at the nudibranch in greater detail, describing not one, but four new species of the herbivorous sea slug.

Barely noticeable to the naked eye, Stiliger smaragdinus hides itself as it dines on toxic seaweed called “sea grapes.” Big bulbs on the sea slugs’ backs look similar to those in sea grapes, affording the clever slugs both camouflage and deterrent from predators. This form of mimicry is normally uncommon in sea slugs and is found namely in animals who use bright colors to ward off predators – not green leafy costumes that imitate the food they eat. This mimicry not only helps the sea slug from getting eaten – because it looks like the toxic algae that it feeds on – but the cost of blending in is low. But what is used as a survival tactic for the moonlighting sea slug presented a special challenge for taxonomists.

To describe the new species, researchers sampled algae collected with live slugs on it, which was removed once it made its way to the lab. After photographing the slug’s brilliant color, scientists took samples of their DNA to sequence and compare against other specimens found in museum collections from around the world.

Not only did they come up with four new species, but they described an entirely new genus, which they named Sacoproteus for Proteus, an old Greek sea god who was able to change his shape to whatever he desired.

Because they thrive in the toxic environment of algae, they are both chemically defended against and avoided by predators. When hungry, they sniff out their favorite seaweed entree, get up on their haunches and “dance” in the water to catch its scent, reports National Geographic

Zoologica Scripta

“Oftentimes, the habitat or food will actually trigger metamorphosis [of the slugs' larvae] into adults,” lead study author Patrick Krug, a marine biologist at California State University, Los Angeles, told the publication. “There's like a chemical dependency.”

More than just a pretty slug, Sacoproteus species represent a unique opportunity to understand marine diversity, especially as it relates to invasive species. The slug’s main food source Caulerpa has invaded oceans around the world, colonizing the sea bottom in the Mediterranean, continuing to make its way to France and beyond as it colonizes everything from rock and sand to mud and seagrass beds. Found at depths of less than a meter to more than 12 meters (40 feet), the algae smothers other algal species in the process.

It’s one of many invasive species found around the world and, as the authors note, a lack of understanding of the species that rely on these species limits our ability to predict future outcomes.

Zoologica Scripta

[H/T: National Geographic

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