Goniobranchus fidelis is one of the most common Goniobranchus species in the Marshalls, where it lives under dead coral rocks on shallow lagoon reefs to depths of about 15 meters. We have seen them at Enewetak, Kwajalein, Rongelap and Bikini Atolls, and have additional records from Majuro. The largest of 35 specimens measured was about 22mm in length. It was first reported in the Marshalls from Enewetak Atoll by Marcus & Burch (1965).
They seem to eat a variety of sponges, including the yellow one these two are crawling across.
The specimen below is also eating yellow sponge. This one has a lot more orange in its margin.
Goniobranchus fidelis also eats the purple sponge Chelonaplysilla violacea. The colony of the sponge below seems a bit more gray than purple, but the following one is more the right color.
The bare patch in the purple sponge just above the nudibranch's rhinophore on the right side of the photo below was grazed by the nudibranch, whose mouth was over that patch when the rock was turned.
Sometimes the marginal fingers of color nearly crowd out the dorsal light yellow.
The specimen below rests between a couple of sponges, the yellow Darwinella on the left and Chelonaplysilla violacea on the right.
The specimen immediately below was observed in Majuro Atoll by Ken Cone and Beth Van Zummeren.
The specimen above and one of those below lack the magenta submarginal pigment.
Here's another specimen lacking the maroon submargin, feeding on what sort of looks like a gray form of Chelonaplysilla violacea.
Created 1 July 2012
Updated 27 June 2021