The tunic is strengthened by the presence of many spherical calcareous spicules which are particularly concentrated round the neck of the urn.

The tunic contains mycosporines which filter out ultra-violet radiation and protect the symbiotic cyanobacteria from damage in strong sunlight.

In shallow water the colonies are small, the colour is more intense and there are more spicules present making the external surface bright white.

[2] It is very similar to Atriolum robustum in appearance, especially young colonies, but can be distinguished by the mucus secretion and the network of canals connecting the thorax internally to the atrial siphon.

A new bud will form and begin to actively feed while the zooid from which it emerged gradually regresses and is eventually re-adsorbed.