Atriolum robustum is a colonial tunicate or sea squirt in the family Didemnidae. It is native to the western and central Indo-Pacific where it is usually found anchored to a hard surface in shallow water.
A colony of Atriolum robustum consists of a number of hollow, urn-shaped zooids up to 1.5 cm (1 in) long, each with a large circular terminal hole, the atrial siphon. The tunic (body wall) is firm to the touch and is perforated by a number of pore-like buccal siphons, each raised on a slight elevation,giving it a [pine cone]-like appearance. The colour is variable and may be whitish, orange or bright green. The interior contains a red pigment but this may be masked by yellowish-green cyanobacteria (Prochloron sp.) which line the cavity. This sea squirt is sometimes confused with Didemnum molle but that sea squirt has a network of internal channels and exudes mucus, making it sticky to the touch.
Atriolum robustum is found in the tropical western and central Indo-Pacific region at depths between about 5 and 18 m (16 and 59 ft). Its range extends from Madagascar to Australia and Papua New Guinea and includes Réunion and Mayotte. It is typically found in shallow sheltered habitats growing on the heads of dead coral but in New Caledonia it is known from as deep as 300 m (984 ft).
Atriolum robustum feeds on phytoplankton, zooplankton and minute pieces of detritus. Water gets drawn into the zooid through the buccal openings, the edible particles are then filtered out and the water current leaves the zooid through the atrial siphon. Sexual reproduction involves sperm being drawn into the body cavity with the inflowing water current and eggs being fertilised internally. The developing embryos are brooded at first.