Didemnid colonies take various forms: 1 where the current is weak, they hang from hard structures in long stringy lobes resembling ropes, and 2 where the current is strong, they grow in dense flat mats. Like other tunicates, the colonies are formed of individuals called zooids. Colonies are tan, cream-colored, white or yellow and can grow up to a diameter of 5 ft. This tunicate is more of a threat than the colonial tunicate Botrylloides violaceus because it is an aggressive invader and a threat to marine life including commercial shellfish fisheries.
Studies have shown the tunicates can directly affect the Atlantic cod and haddock by reducing the habitat available to deposit their eggs. It has no natural predators in North America, since it produces metabolic toxins, and grows rapidly in size it is able to take over and smother out other native species.
Didemnum vexillum a filter feeder that feeds on zooplankton, phytoplankton, and detritus is hermaphroditic and breeds sexually by broadcast spawning and asexually by budding. Breeding occurs when water temperatures are higher, usually early spring and fall. Didemnum vexillum does not grow in the winter, and in fact dies back a bit during that time period.
A new colony can form from fragmentation when a lobe of the colony breaks off and drifts to different location. Such stray pieces may attach to a new substrate within six hours of arrival. Introductions were likely due to shipping either via hull or sea chest fouling. It was first officially documented in Maine in , but there are suggestions that it has been in New England waters since the s.
Since its discovery in , it has been documented off the coast of Washington state at Puget Sound in and was confirmed off of the coast of Oregon in Habitat: Sub-tidal zone, attached to rocks, boulders, and artificial structures such as pilings and floating docks.
