Californian opalescent sea slugs spreading northwards to British Columbia, at least temporarily

Evolution shapes an intricate match between organisms and their environments. This, of course, has led to the massive diversity of animals, plants and other life we can observe around us. It has also lead to diversity that is harder to identify. Outwardly similar — or even identical — looking species can be adapted to different conditions but with few differences in size, shape, or colour to tell them apart. This can make things complicated for biologists when environmental conditions change and similar-looking species end up living together in the same place. By analyzing the DNA and morphology of opalescent sea slugs, we confirmed that a species previously found only in California southward was living side-by-side a similar-looking species in British Columbia.

Our research began as a student project in the summer of 2016 as part of the university educational program at Bamfield Marine Sciences Centre on the west coast of Vancouver Island, British Columbia (BC). Eventually the project grew to include collaborators from across Canada and California. We collected opalescent sea slugs from both Barkley Sound and Clayoquot Sound, near Bamfield and Tofino, BC respectively.

These sea slugs were recently discovered to not be a single species found around all coasts of the Pacific Ocean, but three separate species with similar, but not identical colour patterns. In North America, one species can be found from northern California southwards, and another in northern California northwards (the third species is found in Japan). In our study, colour patterns and DNA analysis of the animals show that both North American species, but not the species from Japan, were present in BC waters in 2016.

We are now left with a number of further questions to answer in the future. Why was the southern species found so far north? Has it always been there, and simply not been noticed?

The timing of our discovery coincided with several other reports of southern species found further north along the Pacific coast of North America. Did a marine heatwave known as the “blob” make it easier for southern species, which are adapted to warmer waters, to survive further north? Or could its presence be a result of overall warming trends in the ocean as a result of global climate change?

We don’t yet know. However, we’re fairly certain that scientists working with these animals will scrutinize their specimens carefully.

Read the full paper — Range extension for the region of sympatry between the nudibranchs Hermissenda opalescens and Hermissenda crassicornis in the northeastern Pacific by Emily M. Merlo Kathryn A. Milligan, Nola B. Sheets, Christopher J. Neufeld, Tao M. Eastham, A.L. Ka’ala Estores-Pacheco, Dirk Steinke, Paul D.N. Hebert, Ángel Valdés and Russell C. Wyeth on the FACETS website.