This would appear to be Elysia degeneri Ostergaard, 1955. It is fairly variable, but always has an orange band on the inside margin of the parapodia. The first specimen below was photographed at Kwajalein Atoll.
The specimen below was found at Enewetak Atoll. It measured 14mm and was under a rock on a lagoon reef at a depth of 5 meters. Just between the closed parapodia, you can see traces of the orange marginal band that runs along the inside of the parapodia. Submarginal to the orange band is a band of dark green, not visible in the photo. The small green oval with the white band at the left side of the Elysia is a small species of marginellid snail.
The specimen in the next three photos was found on Halimeda algae at a depth of 8m on 1 September 2008. It measured 14mm in length. Barely visible in the photos below is an orange band on the inner surface of the parapodia.
The next two specimens were found on 8 March 2010. The upper crawled out from some Halimeda algae placed in a bucket and the other was under a rock on the same seaward reef. The first one appears to have a deformed right rhinophore.
Hawaiian specimens can be seen on the Sea Slugs of Hawaii site.
Created 5 February 2007
Updated 9 March 2010