Translucent violet-blue tunicates in clusters, and food for several nudibranchs.
Solitary ascidians of the genus Rhopalaea, each a translucent club or vase five to eight centimetres tall, attached by a narrow base to rubble or dead coral and usually growing in loose clusters of several to a few dozen. The colour is a deep violet-blue to blue-green, strongest around the two siphon rims and around the internal structures, with the body wall clear enough that the gut and the branchial basket show through. The two openings are the identification: water is drawn in at the terminal siphon and expelled from the second, set a little below it. Species in the genus are separated on internal anatomy and need a dissection, so a photograph settles the genus and no more, and this entry stops there. The other blue animal it is confused with is the encrusting colonial Clavelina, which is made of many small zooids sharing a common base rather than one large body.
It is a filter feeder, and the biology it shares with the rest of the tunicates — a larva with a notochord, and so a closer relationship to vertebrates than to sponges — is covered under the sea squirt Polycarpa aurata. The reason it is worth stopping at is what it feeds. Ascidians are the food of the nembrothas, and a cluster of Rhopalaea is one of the more reliable places to find Nembrotha chamberlaini and its relatives, often with a bitten-down zooid beside the animal and a spawn coil nearby. Polyclad flatworms graze the same clusters. Small ghost gobies of the genus Pleurosicya sometimes rest on the outside of the body, transparent against a transparent host, and are easiest to see against the coloured siphon rim.
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.