A slug rarely over 2 cm (0.8 in), green with fine white lines along its flaps, living on the flat segmented plates of Halimeda algae. The match is close enough that the practical way to find one is to search the weed rather than look for the animal.
Halimeda is a calcified alga most grazers avoid, and getting anything out of it takes a specialist: the slug punctures a cell with a single-toothed radula, drains it, and keeps the chloroplasts alive in its gut lining, where they continue to photosynthesise. What is stolen is not only food but machinery - the slug carries no genes for running chloroplasts, and how the borrowed ones keep working for weeks is still argued over. On a rubble slope with plenty of Halimeda, several of these can be sitting within a hand's width of each other, all invisible until one moves.
- tiny, under 2 cm, sitting on Halimeda segments
- green with fine white lines along the flaps
- body shape and color match the alga it feeds on
- usually found by searching the weed, not by spotting the slug
