Biology
Elysia chlorotica is a “solar-powered” marine sea slug that sequesters and retains photosynthetically active chloroplasts from the algae it eats and, remarkably, has incorporated algal genes into its own genetic code. It is emerald green in color often with small red or white markings, has a slender shape typical of members of its genus, and parapodia (lateral "wings") that fold over its body in life. This sea slug is unique among animals to possess photosynthesis-specific genes and is an extraordinary example of symbiosis between an alga and mollusc as well as a genetic chimera of these two organisms.
To obtain algal chloroplasts Elysia chlorotica slugs use their radula (tooth) to pierce a filament of the alga Vaucheria litorea and suck out its contents. The ingested algal cytoplasm and nuclei move through the gut but algal chloroplasts are trapped and concentrated in vacuoles along branches of the digestive tract. While inside an algal cell, functional chloroplasts use proteins encoded by their own genes as well as others encoded by genes within the algal nucleus. Within a sea slug, however, isolated chloroplasts can not receive proteins from the algal genome. Remarkably, these chloroplasts remain functional anyway because the slug genome includes the algal genes necessary for plastid function. Elysia chlorotica probably gained these algal genes through lateral (or horizontal) gene transfer. One possible vector is a virus that infects the sea slug and carried pieces of algal DNA (Pierce et al., 2003).
Original Description of Elysia chlorotica: A. A. Gould, 1870 (Report on the Invertebrata of Massachusetts): "Animal emerald green, dotted with white and red spots; sleder, tapering behind, with broad, lateral expansions, folded and overlapping each other on the back when the animal is in motion; tentacles two, lanceolate, folded beneath; head distinct, obtuse, slightly emarginate; anterior angles of foot widely produced, triangular.
Actæon, Agassiz, Proc. Bost. Soc. Nat. Hist, iii. 191 (1850)
Actæon chloroticus, Agassiz, in MSS.
Animal emerald green, finely dotted with opaque white interspersed with red specks. Body slender, tapering backwards with very broad lateral expansions or wings, which, when folded as they are when the animal is crawling, overlap each other on the back in a roof-like manner, and the while animals has then a lance-shaped form generally acutely pointed behind, but in some attitudes obtuse; when expanded, they have a broad ovate form, like a leaf with the border more or less undulating, and this resemblance is further carried out by the vein-like folds or canals which ramify on its surface from the heart which forms a globular or bulbous eminence in front; the expansion begins at the anterior part of this bulb. In front of this is a well-marked neck and head, on which latter are two delicately lanceolate tentacles, which are furrowed or folded beneath. The eyes are placed a little behind the tentacles. The head is obtuse and slightly emarginate. The organs of generation are just behind the right tentacle, and the male organs is very often protruded, of about the same form and nearly as large as teh tentacle. The anterior angels of the foot are widely produced, of a recurved triangular form, as if another pair of tentacles. Length, about one inch, sometimes an inch and a half; breadth, when folded, about one fifth the length, and height equal to breadth, when fully expanded, equal to three fourths the length.
Found in great numbers in brackish water, on the Cambridge marshes, in the spring of 1848 (Agassiz)."
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