REEF PNW Guide to Nudibranchs By Karin Fletcher

Giant Nudibranch Giant nudibranch (Dendronotus Iris) May be confused with its smaller relative the white dendronotus, Dendronotus albus. Both dendronotus can have branching, orange-tipped cerata and a juvenile Giant nudibranch can be the same size as a white dendronotus, about 2 in. or 5 cm. The distinguishing feature of the Giant nudibranch is the white line around the foot of the animal. The white dendronotus lacks this line and instead has a white line from the tip of its tail to part way up the center of its back.

Giant Nudibranch Comparison Giant Nudibranch White Dendronotus

Opalescent Nudibranch Opalescent nudibranch (Hermissenda crassicornis) Most easily confused with the White-and-Orange-Tipped nudibranch, Janolus fuscus. Many aeolids - nudibranchs that have long rows of finger-like cerata on their backs - are about the same 2 in./5cm size as the Opalescent nudibranch so it is easy to mistake any aeolid for an Opalescent nudibranch. The feature that sets the Opalescent nudibranch apart from the White-and-Orange-Tipped nudibranch and all other aeolids is the orange “racing stripe” outlined in white between its rhinophores.

Opalescent Nudibranch Comparison Opalescent Nudibranch White-and-Orange-Tipped Nudibranch

White-lined Dirona White-lined dirona (Dirona albolineata) Often abundant year-round and is the most easily recognized nudibranch on the REEF survey. It has a translucent body which can be edged in white and cerata which are also edged in a frosty white. It also has a white line connecting its rhinophores and a white line edging its frontal veil. When stressed, the White-lined dirona can shed its cerata and grow new ones, a process known as autotomy.

White-lined Dirona, common coloration

White-lined Dirona Comparison As with most nudibranchs with dorsal cerata, the White-lined dirona’s cerata can change colors depending on what the dirona has eaten. When that happens, it is possible to mistake a White-lined dirona for a Golden dirona, Dirona pellucida. No matter what color the cerata are, the White-lined dirona will always have a white line edging its frontal veil and the Golden dirona will always have white spots on its body.

White-lined Dirona Comparison White-lined Dirona, gold variation Golden Dirona

Yellow Margin Dorid Yellow Margin Dorid (Cadlina luteomarginata ) Often the most misidentified nudibranch on the REEF survey because of its resemblance to the Hudson’s dorid, Acanthodoris hudsoni. While the Yellow Margin Dorid has rounded, yellow-tipped tubercules instead of finger-like papillae, when seen from above it can be difficult to tell the two apart. The Hudson’s dorid has longer rhinophores, but rhinophore length can be difficult to measure from above. If you encounter a nudibranch you believe is a Yellow Margin Dorid, try to view it from the side where the difference between the two dorids is most noticeable.

Yellow Margin Dorid Comparison Yellow Margin Dorid Hudson’s Dorid