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Invasive Species — Didemnum Tunicate (Didemnum vexillum)

D. vex is considered an “ecosystem engineer”, capable of drastic modifications of the habitats it invades. In the majority of cases where it has been reported as a new introduction, it has grown extremely rapidly within a few years following first observation. This phenomenal growth rate can result in massive colonies that overgrow almost every other sessile species.

Didemnum vexillum colonies alter marine habitats and threaten to interfere with fishing, aquaculture, and other coastal and offshore activities. The colonies can grow on hard substrates that include dock structures and floats, wood and metal pilings, moorings and ropes, steel chain, automobile tires, polythene plastic, rocky outcrops, gravel seabed (pebbles, cobbles, boulders), and ship hulls. They overgrow organisms such as tunicates, sponges, macroalgae, hydroids, anemones, bryozoans, scallops, mussels, and oysters. Where these colonies occur on the seabed, they likely cover the siphons of infaunal bivalves and serve as a barrier between demersal fish and benthic prey. Colonies have been found at water depths ranging from intertidal to continental shelf depts. of 65m (213ft.). Invasive tunicates may result in impacts to mariculture industry, shellfisheries, ground fisheries, and to native species and habitats.