Will the real Hermissenda crassicornis please stand up. - Merry Passage

One of our most striking nudibranchs has undergone a name change. What was once the familiar Hermissenda crassicornis is now classified as Hermissenda opalescens. (T. Lindsay, A. Vald├ęs, 2016) Historically, various nudibranch subjects from a large geographic range were classified as H. crassicornis.


Recent molecular, morphological, and populations analyses revealed that there are actually three distinct species within the genus Hermissenda. One species, H. emurai, occurs in Japan and the Russian Far East.


The two other species that were differentiated by DNA studies are also different morphologically.  They inhabit separate geographic ranges for the most part, but both species overlap between Point Reyes and Bodega Bay*.


1. Hermissenda opalescens is found from Northern California to the Sea of Cortez.

2. Hermissenda crassicornis ranges from Alaska to Northern California. It was originally described in 1831 from Sitka, Alaska.


*Nanette Van Antwerp found both species at Abalone Alley, near Tyler Bight on the backside of San Miguel Island.


In 1863, the taxonomic name, Hermissenda opalescens, was first assigned to a specimen from San Diego, and that name has been restored.  The easiest way to distinguish the two is the longitudinal white stripe on the cerata of crassicornis, while the cerata of opalescens have white tips but no white stripe.

Hermissenda crassicornis with its egg mass

Newly reclassified - Hermissenda opalescens


Hermissenda crassicornis

Composite showing the cerata of both species

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