Weekly Sea Discovery: Nudibranchs!

Commonly known as sea slugs, nudibranchs are creatures that are a mystery to many of us. These squishy invertebrates can be found in the shallow intertidal, down to the deep depths of the subtidal. Their vibrant colours and intricate shapes make them an eye-catching surprise in the murky waters of the Pacific Northwest. But how do these bright coloured slugs hide from hungry predators?

Red Sponge Nudibranch (Rostanga pulchra)

While the close relatives of nudibranchs in phylum Mollusca have shells to protect themselves (like snails and chitons), nudibranchs have a different strategy. Their bright colours are a warning sign to other species that they contain toxins and stinging cells. Nudibranchs don’t create this defense mechanism on their own; they get it from the food they eat!

These ravenous sea slugs prey on a variety of invertebrates like anemones, sponges, and hydroids, which contain stinging cells and toxins. Many other species can’t eat these toxic invertebrates, so nudibranchs play a vital role in top-down control. Grazing from nudibranchs keeps the growth of these invertebrates in check, to reduce competition and overpopulation.

Leopard Dorid (Diaulula sandiegensis). Photo: Ron Caswell

In the Pacific Northwest, there are 200 species of nudibranchs, so it can be hard to identify what species you’re looking at sometimes. While the taxonomy of nudibranchs is quite complex, there are two general groups, Dorids (Bushy-Gill Nudibranchs) and Aeolids (Bushy-Back Nudibranchs). Below are a few examples of each nudibranch group found nearby and some identifying characteristics.

The Clown Dorid (Triopha catalinae) is a Bushy-Gill Nudibranch. This suborder of nudibranchs can be recognized by their retractable gill plume (“bushy gill”) that they use to breathe, and their oval shape. Photo: Larry Taylor
This Three-lined Aeolid can have light orange to red cerata, which store the stinging cells they ingest from eating hydroids and anemones. Photo: Larry Taylor

These last three nudibranch images were photographed around Mayne Island during our Bioblitz in 2019!


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