A thick-walled green urn on rubble, and a colony of animals rather than one.
A colonial ascidian shaped like a thick-walled urn or barrel, to about 6 cm high, grey-white to olive on the outside and strongly green inside the rim and around the opening. The opening at the top is a common cloacal cavity through which the whole colony vents. The wall is firm and gritty to the eye, and colonies grow singly or in loose clusters on rubble, dead coral and man-made structure.
The green is the reason the animal looks the way it does. The tunic holds dense populations of the photosynthetic prokaryote Prochloron, which lives nowhere but in ascidians of this group, and colonies in good light are more strongly coloured than shaded ones. Each of the many small zooids in the wall draws water through its own inhalant siphon, filters it, and discharges into the shared cavity.
It is easy to mistake for a sponge at a distance. The difference is the single large opening with its ring of colour, where a sponge of that size carries either one wide osculum with a thin rim or many small ones. Recorded from the shallows to about 30 m, and commonest where there is light and some water movement.
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.