another field mark that is only reliable when present is the color of the cnidosacs (where cnidocytes or stinging cells from cnidarians like hydrozoans are stored)
H. opalescens cnidosacs are white at the tip of the cerata
H. crassicornis cnidosacs are often (not always) orange at the tip
H. opalescens cnidosacs are white at the tip of the cerata
H. crassicornis cnidosacs are often (not always) orange at the tip
Comments
it is a good additional field mark when present, but the white stripe is more reliable when distinguishing species
we get a lot of H. opalescens in Monterey & in this observation I made last year, you can see the white tipped cerata as well as the lack of white stripe on this individual: https://www.inaturalist.org/observations/223652802
vice versa, searches for H. opalescens shows images of crassicornis too! *confusing*
even on some university websites, they still incorrectly treat them as a single species
In recent years, the Hopkin's rose nudibranch & Spanish shawl had scientific name revisions too, so I get why this can make it feel confusing
plus, using tools like iNaturalist can take the pressure out of keeping track of taxonomic changes!