Didemnum molle (Urn ascidian)


A colony of Didemnum molle is composed of a number of linked zooids adhering to the substrate. Each individual zooid has a plump, urn-shaped body with a diameter of up to 10 cm.
The colony is white, brown or green, or some combination of these colours. This hue is due to the symbiotic blue-green algae of the genus Prochloron present in the tissues.
In shallow water the colonies are small, the colour is more intense and there are more spicules present making the external surface bright white. At greater depths the colonies are larger, there are fewer spicules and the colour is more greyish or brownish.


Has not yet been assessed for the IUCN Red List.


Native to the western Indo-Pacific region where it is common and widely distributed. It grows on rock or dead coral on shallow reefs. In New Caledonia there is a green variant that grows direct onto sand.


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

Sexual reproduction also takes place. Each zooid is hermaphrodite and fertilisation takes place in the body cavity when a clutch of eggs is fertilised by sperm drawn in with the water stream.
The embryos are brooded for a few days before the larvae are expelled into the water column. These quickly settle and cement themselves to suitable surfaces and start new colonies.


Individual zooid body: diameter of up to 10 cm.

Prey / Predation

It is a suspension feeder. Water is drawn into the body of each zooid through the numerous buccal siphons, phytoplankton, zooplankton and fine organic particles are filtered out and the water is exhaled through the atrial siphon.

Special features

It is commonly known as the tall urn ascidian, the green barrel sea squirt or the green reef sea-squirt.