A colonial ascidian of many small glassy zooids, each with its own pair of siphons.
Colonies of small club-shaped or bell-shaped zooids, each one to two centimetres tall, standing close together and joined at the base by a common stolon. The body wall is glassy and transparent, so the branchial basket inside shows as a fine grid, and the pigment the colony carries is concentrated in lines: round the rims of the two siphons, along the bars of the basket, and sometimes in a band at the base. Blue, violet, white, orange and yellow all occur, and a colony reads at a distance as a cluster of small lit glasses. Numbers run from a dozen zooids to several hundred on one patch.
Each zooid keeps its own inhalant and exhalant siphon and filters for itself, which separates this genus from the other ascidians on these sites. Rhopalaea is a solitary animal of five to eight centimetres, one large body rather than many small ones, and Didemnum molle is a firm urn whose many zooids share a single common opening at the top. The tunicate biology they all share, including a swimming larva with a notochord, is set out under the sea squirt Polycarpa aurata.
Colonies grow on rubble, dead coral, sponge, wreckage and jetty structure, and they come and go quickly: a patch dense with zooids on one visit can be reduced to bare stolons a few weeks later, and colonies regenerate from those stolons. Ascidians are food for the nembrothas and for polyclad flatworms, so a colony with zooids bitten down is worth searching. Species are separated on internal anatomy and records normally stay at the genus. Recorded from about 3 to 30 m.
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.