A small flattened sea squirt with something no other ascidian here has: a hinged lid. The front of the tunic forms a rigid flap that swings shut over both siphons, sealing the animal inside a closed box, and a disturbed individual does exactly that within a second or two.
At about 4 cm (1.6 in) across, with a translucent tunic that usually collects sand grains, it is easy to overlook among algae and rubble - and once shut, it looks like nothing at all. Why this genus evolved a door when its relatives get by on contraction alone is not settled; the likely answer is predation by small fish that pick at exposed siphons. Feeding is standard for the group: water in, mucus net, water out, several litres an hour through an animal the size of a coin.
- flattened body with a hinged lid that closes over both siphons
- translucent tunic, often with sand grains stuck to it
- closes up completely when disturbed
- width up to 4 cm
