By Dr. Tom Niesen
The creature featured today is Berthella californica, the California Berthella or the California Side-gilled sea slug.
A specimen was recently observed at FMR in a midtidal tide pool by Tom Ciotti, Board member of The Friends of Fitzgerald Marine Reserve. Berthella spp. are easily mistaken for nudibranchs, but are classified as pleurobranchs or side-gilled opisthobranchs, because they have their large gill positioned along the right side of the foot under the overhanging mantle. As can be seen in the photo, their rhinophores are rolled and there is a thin white line along the edge of the dorsal mantle, which can vary from translucent white to brown and have scattered white dots. Berthella californica can be up to 2 inches long and ranges from Alaska to Baja California. It feeds on various invertebrates such as sea squirts and also on fish. Berthella, like all opisthobranch gastropods, is a hermaphrodite. This sea slug lays its fertilized eggs in an egg ribbon where they develop into larvae and, after several weeks, are released into the water to complete their development and settle unto the bottom.