A snail that looks like a clam. Under 8 mm (0.3 in), it lives inside a translucent green shell of two hinged halves that open and close exactly as a bivalve's do - and yet the animal inside is a gastropod, a sea slug relative that drains algal cells with a single-toothed radula.
The family carrying this arrangement was known from fossils and empty shells long before anyone found a living one, and the shells were filed as small clams for decades; the discovery that they belonged to a snail rearranged a chunk of the textbook. The shell grows as two valves from a single larval coil, so the resemblance is convergent rather than inherited. It lives on Caulerpa, color-matched to the frond, and at this size on green weed it is effectively invisible - most records come from sorting algae in a tray rather than from spotting one underwater.
- a snail inside a two-part shell, like a tiny green clam
- shell translucent green, matching the weed
- under 1 cm, on Caulerpa fronds
- hinged shell opens like a bivalve
