Glossodoris atromarginata (Cuvier, 1804)

Glossodoris atromarginata is reasonably common in the Marshalls, but only if you know where to look for it. They most commonly live along the oceanside of windward atoll reefs, in ledges and caves up in the groove and spur system, a habitat they share with the even more common Glossodoris rufomarginata. This is an extremely rough area; we can get there only when both the winds are light and the north or east swell is very small. It is almost hard to believe that the nudibranchs can adhere to the rocks or to their sponge prey strongly enough to keep from being ripped off and washed across the reef. The sponge colonies seem to be more at risk. As noted on the Glossodoris rufomarginata page, clumps of prey sponge are sometimes ripped off the substrate and rolled across the reef into the lagoon, where they come to rest on the lagoon slope. It seems a fairly high proportion of these clumps bear nudibranch riders, who seem to make the trip across the reef without visible damage. Unfortunately, their sponge prey does not naturally occur on the lagoon slope, so once the nudibranchs finish off whatever colony they have ridden across, they have to scatter in hopes of finding another clump that has rolled across. During the windy season this appears to happen often, since clumps of the sponge are not rare. On one recent dive, seven G. atromarginata and three G. rufomarginata were found on a single lagoon slope dive; all but one were still on chunks of their prey sponge. Several other loose sponge colonies were also found without nudibranch hitchhikers. Occasionally this species can also be found on lagoon reefs or pinnacles, or even on the seaward slope of the leeward side of the atoll. Specimens we've seen have ranged up to about 35mm in length.

The 25mm specimen pictured below carried a pair of purplish commensal copepods, one with a darker colored X shaped mark. One of the copepods is right in front of the gills; the other is a bit towards the tail.The 25mm specimen pictured below carried a pair of purplish commensal copepods, one with a darker colored X shaped mark. One of the copepods is right in front of the gills; the other is a bit towards the tail.

The one below is at rest, showing the highly convoluted black line at the margin of the dorsum.

The one below had a bit unusual coloring for Glossodoris atromarginata from this area, but we think it is still the same species.

Created 3 January 2006
Updated 12 July 2008