What we've identified as Goniobranchus rubrocornutus is found in at least three and perhaps four Marshall Islands atolls and is a bit different in each one. This page shows specimens from three of those atolls, Bikini, Utirik, and Rongelap. There is also a species at Kwajalein that differs primarily in having a bright yellow body. While it could be a color form, we think it is probably something different. The Kwajalein species can be found on the Goniobranchus cf rubrocornutus page. Marshall Islands specimens of the real Goniobranchus rubrocornutus most typical of the species come from at Bikini Atoll, where six specimens up to 21mm in length were found in June of 1982 under rocks in the lagoon near Bikini Island at depths of 1 to 2 meters. Two of these are pictured below. Goniobranchus rubrocornutus was first reported in the Marshalls from Bikini Atoll by Rudman (1985).
Several specimens almost typical of the species were found at Utirik Atoll on lagoon pinnacles under rocks at depths of 5 to 15 meters in June of 1983. The Utirik specimens, like the one below, had more elongate patches of submarginal purple but were otherwise very similar to those of Bikini.
Between Bikini and Utirik lies Rongelap Atoll. The sole specimen that is apparently this species found at Rongelap differed from typical specimens in that its rhinophores and gills lacked the reddish purple coloration; instead, they were pure white. Also, the white and purple submarginal bands were a bit more irregular, with some of the white patches forming small spots reminiscent of the closely related Goniobranchus galactos from Enewetak.
Created 14 December 2005
Updated 1 July 2012