Elysia pusilla (Bergh, 1872)

Description

A small sacoglossan sea slug that lives clamped onto the calcareous green alga Halimeda. Body length is variable, ranging from a few millimetres to about 3 cm. The ground colour is a vivid grass-green, and the whole animal is sprinkled with fine white spots that are densest over the outer face of the parapodia and along their margin. There is a single pair of short, thick rhinophores, with the black eyes showing through at their base. The slug is strongly cryptic on its host: on Halimeda with broad flattened segments the body is markedly flattened, while on plants with cylindrical segments the body becomes more cylindrical, closely matching the shape of the alga.

Distribution

The type locality is Aibukit, in the Palau Islands, where the original description was based on a single specimen. The species is now known to range widely through the tropical and subtropical Indo-West and Central Pacific. In Japan it has been recorded from localities such as Wagu on the Shima Peninsula.

Etymology

The specific epithet pusilla is Latin for "very small, tiny", referring to the minute specimen — only a few millimetres long — on which the original description was based.

Remarks

A strict specialist on the green alga Halimeda, it retains functional chloroplasts from its food inside its body and uses the products of their photosynthesis (kleptoplasty). Its development is geographically variable: although a single species, different populations produce either planktotrophic or lecithotrophic larvae (poecilogony). It was originally designated the type species of the new genus Elysiella, which is now treated as a synonym of Elysia. Elysia halimedae, described from South Africa, is also a synonym of this species.

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References

• Elysiella pusilla Bergh n.g., n.sp., Bergh L.S.R. (1872). Malacologische Untersuchungen. In: C. Semper (ed.), Reisen im Archipel der Philippinen, Wissenschaftliche Resultate. Theil 1, Heft 4: 177-204, pls 21-24.
• Elysia halimedae n. sp., Macnae W. (1954). On Four Sacoglossan Molluscs new to South Africa. Annals of the Natal Museum 13(1): 51-64.
• Elysia (Elysia) halimedae MACNAE, 1954 Uchiwa-midorigai, Baba K. (1957). THE SPECIES OF THE GENUS ELYSIA FROM JAPAN. Publications of the Seto Marine Biological Laboratory. 6(1): 69-74. https://doi.org/10.5134/174573
• Elysia pusilla (Bergh, 1872), Vendetti J.E., Trowbridge C.D. & Krug P.J. (2012). Poecilogony and population genetic structure in Elysia pusilla (Heterobranchia: Sacoglossa), and reproductive data for five sacoglossans that express dimorphisms in larval development. Integrative and Comparative Biology. 52(1): 138-150.