Didemnum vexillum (Colonial sea squirt)

Animal Tunicate/Sea-squirt
Family: Didemnidae
Classification: Unlisted Invasive Species

Colonial tunicates (Didemnum lahillei) attached to mussel cages in British Columbia. Photo by Gordon King

Colonial tunicates (Didemnum lahillei) attached to mussels in Puget Sound. Photo by Gretchen Lambert

State aquatic managers ask the public to report sightings of tunicates and other aquatic invasive species. To report sightings contact:

Allen Pleus
A quatic nuisance species coordinator for WDFW
(360) 902-2724

Pam Meacham
WDFW Assistant aquatic nuisance species coordinator
(360) 902-2741


Assumed to be native to the waters around Japan, and introduced to Pacific Northwest waters, Didemnum vexillum is a colonial tunicate consisting of many small individuals called zooids.  Each zooid pumps water through its own siphon, filters out oxygen and feeds on small organisms such as phytoplankton, zooplankton, oyster and mussel larvae, and other suspended organic materials. It then discharges the filtered water and waste into a common space that is shared within a system of other zooids where it is discharged through a small pore.  D. vexillum’s clear, firm, fleshy matrix consists of several systems of zooids linked together to form a contiguous mass colony that looks like a network of darkish leaf-like veins with pores.  Each zooid will have a consistent color with the rest of the colony. The color of a colony can range from yellow to orange to reddish brown.  As the colonies grow they can smother out other stationary wildlife and filter feeders, including mussels and oysters. D.vexillum exudes a toxic substance onto its surface, preventing other species from fouling its exterior and discouraging predators.

D.vexillum prefers protected marine waters and reproduces rapidly, fouling ship hulls, docks, piers and shellfish aquacultures.  It can be found on any substrate, including gravel, seabed, metal, plastic, rope, fiberglass, wood, and even eelgrass and seaweeds.  Mature colonies in calm waters can appear as long, rope-like drippy, mats extending downward from where they are attached.  It has been found from intertidal to over 200 feet depth.  They do well in polluted waters.

Unlike the solitary tunicates, the colonial D. vexillum broods its larvae within the colony’s matrix, and then releases the planktonic larvae.  The larvae are very short lived, likely settling onto a firm surface and near the point of origin, where it metamorphoses into a zooid an then buds off asexually to form a colony.  However, fragments from a colony can survive indefinitely, reproducing asexually while drifting, or traveling within ship ballast water to colonize new areas.

See the Tunicate information page for more information.

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