Opisthobranch of the Week Data
Elysiella pusilla is considered to be very rare on Okinawa, based on two collections (one collection with four specimens and one with a single animal). Doubtless, there are additional local populations to be found here, but due to the highly cryptic nature of these relatively small sacoglossans with their Halimeda prey, they are at best, difficult to find. The above featured 7mm animal is seen on a blade of its host calcareous green alga (Halimeda sp.). Synonyms of Elysiella pusilla include Elysia dubia Eliot, 1904., Elysia halimedae Macnae, 1954., Elysia latipes Marcus & Marcus, 1960 & Elysia macnaei Marcus, 1982. Also see a very similar appearing Atlantic species, Bosellia mimetica, which is posted on the Sea Slug Forum (Rudman, 2000). There is a rounded as well as a flattened form of the animal and Bill Rudman (1999) mentions in a Slug Forum discussion concerning Elysiella pusilla the following:
The flattened form is always well camouflaged, matching the colour of the part of the Halimeda plant on which it is living. Smaller specimens are usually a uniform bright green and are found on the smaller uniformly green growing segments of the plant. Larger animals . . . can have the same mottled appearance as the larger more heavily calcified older segments of Halimeda. In many respects this species is just a modified species of Elysia, but because of the reduced parapodia, many workers today are following Bergh and placing it in the monotypic genus Elysiella.
I've added a second page of E. pusilla illustrating the cylindrical form which the animal may assume. It's worthwhile to mention that Rudman (1999) points out, in an additional page in re the 'cylindrical form' of the species, the following:
In all reports of Elysiella pusilla the point that is noted is its flattened cryptic appearance. This is true of specimens on plants with large flattened segments . . . but if you remove animals into a dish they become much more Elysia-like in shape when they begin crawling. I have seen no previous reports of individuals from species, or forms, of Halimeda with cylindrical segments. From my observations in New Caledonia their parapodia are held more erect and the whole body is held upright. However, when similar sized animals from both plant forms are put together in a dish it is impossible to separate them in colour or shape.