Gymnodoris nigricolor is known from the Marshalls one specimen found on Enewetak Atoll and at least three from Kwajalein. They measured 9-10mm in length and were found crawling across sand at depths of 10-15 meters. The Enewetak specimen was on a lagoon pinnacle reef on 13 May 1983 and those from Kwajalein were near a lagoon interisland reef Halimeda patch. The first one below was found on 25 May 2008. Since it's hard to tell much from a photo of a black animal, my notes on the living animal are below.
The body is small, soft, and limaciform, with a very long and slender tail. Color is black, but at 12-25x this is seen as translucent gray very densely speckled with black. There are very small pustules about where mantle margin should be, especially in V shape anterior end. Foot margin is bluish white. Reproductive opening is about one-third of the way between the rhinophores and gills. Rhinophores are close set, each with about 8 oblique lamellae, and are colored bluish black. Gills are small, set in a semicircle, and colored bluish black.
The next one was found crawling on sand on a Kwajalein lagoon reef on 22 August 2010.
Various sources, including the Sea Slug Forum and Okinawa Slug Site, suggest or post photos showing Gymnodoris nigricolor is a parasite on the fins of shrimp gobies. I personally have not seen them doing this, but Marissa and Peter Brown have photographed one on the dorsal fin of the shrimp goby Ctenogobiops feroculus on the lagoon side of Kwajalein Island.
Created 1 January 2007
Updated 27 June 2020