From neon-blue polka dots to flame-like tails, 17 newly described sea slugs are adding to what we know about how color evolution works in the animal kingdom – and looking absolutely fabulous while doing so. In addition to describing the new slugs, researchers identified how their distant relatives evolved similar color patterns in the first-ever genetic confirmation of color mimicry across the sea slug world.
Using a combination of visual analysis and genetic testing, scientists with the California Academy of Sciences added 17 new species of the nudibranchs to the tree of life. They also defined the role color mimicry plays in the marine invertebrates’ defense against predators by creating “color trees” to see how the slugs evolved their flashy wardrobes. Further genetic analysis showed that several species from distantly related groups evolved similar color patterns in a process known as “convergent evolution”, whereby species take on similar traits because of environmental advantages or ecological niches. It's likely this happened with different-colored sea slugs that mated in order to take on another’s advantageous color traits. Their work is published in the Zoological Journal of the Linnean Society.
“Usually, most nudibranchs have distinctive color patterns that allow us to recognize distinct species. These are indicators of warning coloration in these distasteful species. Some nudibranchs do mimic each other and have similar (but subtly different) color patterns, ensuring that predators have to recognize fewer color patterns for distasteful species,” study author Terry Gosliner told IFLScience. As he notes, sea slugs have an “arsenal of strategies for surviving,” from their mimicry to camouflage to cryptic patterns. These color variations are influenced by a strong selection pressure to have similar color patterns.
“Often times those that are more divergent get eaten or damaged, so they don't reproduce. There is also strong selection pressure to evolve mimetic color patterns between species occurring in the same geographical area,” said Gosliner.
Sea slugs are part of an evolutionary group that has undergone shell reduction and loss from shelled ancestors at least seven times in different groups of sea slugs. They have instead evolved chemical defenses indicated by their vibrant color patterns. According to Gosliner, sea slugs are also seeing another bigger pressure in their marine habitat: climate change.
“[Sea slugs] feed on a wide variety of marine organisms collectively, although individual species have very specific food items, often a single species of prey. This means that if there are lots of different sea slugs, this is usually an indicator of a diverse and healthy ecosystem,” Gosliner told IFLScience.
"In degraded environments, there are fewer nudibranchs both in total numbers and in numbers of species represented. Species are often changing their known ranges in response to climate change,” he said, noting that species along the west coast of the US are being found further north than previously, likely in response to warming temperatures.