A sacoglossan whose ruffled green folds hold working chloroplasts taken from the algae it eats, so it lives partly on what those chloroplasts make.
Between about 3.5 and 7.5 cm; sources disagree, and the larger figures may include a Florida form that some workers treat as a separate species. Dense ruffled folds run the length of the body and look like lettuce leaves, which is the name and is unmistakable. The ground colour is green, with white translucent spots and turquoise iridescent pigment, and the folded margins carry bands of blue, orange and white. The colour varies from green through blue to orange depending on what it has been eating.
It is a sacoglossan, not a nudibranch. It punctures algal cells with its rasping tongue, sucks out the contents, and keeps the chloroplasts alive inside its own tissue, where they go on photosynthesising for up to about four months even if the animal is starved. That is why it sits out in the open on rock and rubble in bright light in the top few metres, rather than hiding: the exposure is the point. Its prey algae include Bryopsis, Penicillus and Halimeda.
A Florida Keys mangrove form was described as a separate species in 2006, and whether it is a species or a morph of this one is still unsettled. That form is more uniformly green, has smaller white spots and lacks the dark blue band on the folds. It lives between the surface and about 12 m. Florida regulates its collection.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.
