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Species in the genus Halgerda are usually differentiated by their colour patterns. At present it is difficult to determine whether Halgerda willeyi has a very variable colour pattern or whether there is a number of very similarly coloured species. This shot shows the typical colour variant of Halgerda willeyi .
A possible dark colour variant without yellow piment on the ridges is Halgerda sp. 01.
More information on Halgerda willeyi are on Bill Rudman's Sea-Slug Forum!