Sea Slugs in WA—Diverse and Unknown

November Meeting Report

Our speaker was Nerida Wilson, a research scientist at the WA Museum. As head of the museum’s Molecular Systematics Unit, Nerida’s interest is global biological diversity and resolving evolutionary relationships between taxa. Sea slugs are of special interest to her, and she has been on seven expeditions to Antarctica to study them in that extreme environment. Four species have been named after her.

Slug-like molluscs are found worldwide and, according to genetic studies, appear to have evolved independently from several groups of shelled gastropods. Some of their closest relatives have complex shells. Sea Hares are not nudibranchs; they are on a different branch of the sub-class Heterobranchia. Nudibranchs are what we would consider to be ‘true sea slugs’. There is a diverse and largely unknown assemblage of nudibranchs in Australian waters. Some groups are brightly coloured and patterned—some beautifully and some garishly. This is puzzling, because nudibranchs can’t form visual images. The purpose of the colouring (which may be obtained from the sponges they eat) seems to be a warning to potential predators that they have a very nasty taste. Nerida has observed fish mouthing nudibranchs and then spitting them out.

It was the stunning beauty of nudibranchs that lured Nerida into a real passion for them, especially the family Chromodorididae, which she considers the most beautiful. She followed this passion with much snorkelling and diving and, inspired partly by the book Sea Slugs of Western Australia, this led to an Honours degree and a PhD.

There are two broad groups of nudibranchs: those with a small bunch of gills on the rear dorsal surface and those with no specific gills but which respire through their skin. Both have a pair of rhinophores (chemo-receptors) at the front.

The task of classifying the genus Chromodoris was found to be rather daunting, as the similar patterns between species can be confused with variation within a species. Mimicry of related species complicates the issue too. And there are hybrids as well! However, one clue was that some laid their eggs in a planar pattern and some in an upright pattern. Then, with the help of a PhD student, mitochondrial DNA was used to clarify this complex issue and it showed them to consist of 39 species. Later, nuclear DNA was used to draw up a family tree of the species and clarify the phylogeny, showing evolutionary relationships. (Mitochondrial genes have a faster mutation rate but nuclear genes give a truer understanding of relationships.)

This research improved the understanding of the range and distribution of the various species, and it showed up trends such as spotted patterns being ancestral to striped ones.

Nerida is interested in sea dragons too, but that is another story. Perhaps one day she will be able to share with us her infectious enthusiasm for those beautiful marine creatures as well.

Mike Gregson