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Most species of Rostanga are orange or red, and apparently take up colour pigments from their sponge food so that their colour exactly matches the sponge they are feeding on. Individual animals show a close resemblance to their food sponge. This means that colour can vary within a species quite considerably if the sponge they feed on is itself variable in colour, or if the species feeds on a number of species of sponge. External differences can be found in the shape of the rhinophores and in biological characteristics such as nature of egg mass, size of eggs, and food sponge.
Internally, the radular morphology is the most useful character.
As long as I am not sure I label it Rostanga sp. 03.
More information on Rostanga spp.are on Bill Rudman's Sea-Slug Forum!