06/18/2026
My biggest surprise during this week’s low tides was spotting a true Opalescent Nudibranch, Hermissenda opalescens.
If you check a nudibranch field guide or search online, you’ll find that this species is generally reported from Northern California south to Mexico. However, iNaturalist tells a more nuanced story. There are observations outside that traditional range, including 18 records from Washington State. I’m still learning, but of those Washington observations, only two others besides the one I observed with Luan Roberts appear convincing to me. Oregon, by comparison, has 201 observations, and most of those look solid. As the climate changes, this species may be slowly expanding northward, pushing beyond the range we once allotted to it. TBD.
The easiest way to distinguish H. opalescens from H. crassicornis is the white line running along the cerata. H. crassicornis has this white stripe, while H. opalescens does not. That said, the stripe isn’t always obvious on H. crassicornis—lighting and viewing angle can make it difficult to see. When the two species are viewed side by side, however, their differences become much more apparent.
H. crassicornis is the designated northern species and common in Washington state. It was referred to as the "opalescent nudibranch" until genetics determined there were 2 different species -H. opalescens was the southern one and H. crassicornis the northern. We now call our northern species the "horned" or "thick horned" nudibranch.
I am still on my own learning journey, so if you have more information about either species you would like to add, please weigh in!
