Elysiella pusilla
Bergh, 1872

Order: SACOGLOSSA
Superfamily: ELYSIOIDEA
Family: Elysiidae

DISTRIBUTION

Tropical Indo-West Pacific.

PHOTO

Koumac Beach (=Baie de Ouanap), near Koumac, New Caledonia, 20°34'S, 164°16'E, Mixed grassbeds and algae, on Halimeda. October 1993, 7 specimens (5-32mm long alive). UPPER: Small (7mm long) animal, normally found on bright green growing segment moved onto older, whiter, segment for photograph - it was invisible on the segment on which it was found. LOWER PHOTOS: Large (32mm long) specimens photographed in situ on older segments. PHOTOS: Bill Rudman.

This extremely cryptic elysioid is found exclusively on the calcareous green alga Halimeda. When living on Halimeda with broad flattened segments, the slugs are very flattened, especially the larger individuals, but on Halimeda with cylindrical segments, the animals match the cylindrical shape of the plant, as I have illustrated on a second page.

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, as illustrated here in the lower photos, 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.

Synonyms include Elysia dubia Eliot, 1904., Elysia halimedae Macnae, 1954., Elysia latipes Marcus & Marcus, 1960 & Elysia macnaei Marcus, 1982.

See Page 2 where cylindrical form of Elysiella pusilla is illustrated.

References:
• Bergh, L.S.R. (1872). Malacologische Untersuchungen. In: C.G. Semper, Reisen im Archipel der Philippinen, Wissenschaftliche Resultate. Band 2, Heft 3: 137-176, Pls. 17-20.

• Jensen, K.R. (1992). Anatomy of some Indo-Pacific Elysiidae (Opisthobranchia: Sacoglossa [=Ascoglossa]), with a discussion of the generic division and phylogeny. Mal. Soc. London, 58: 257-96

Authorship details
Rudman, W.B., 1999 (July 24) Elysiella pusilla Bergh, 1872. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/elyspus1

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