Didemnum vexillum

Didemnum vexillum, the carpet sea squirt, is a species of colonial tunicate in the family Didemnidae. Thought to be native to Japan, it has been reported as an invasive species in a number of places in Europe, North America and New Zealand. It is sometimes given the nickname 'D. vex' because of the vexing way in which it dominates marine ecosystems when introduced into new locations, however the species epithet vexillum actually derives from the Latin word for flag, and the species was so named because of the way colonies' long tendrils appear to wave in the water like a flag. D. vexillum can grow on most hard substrates, including cobble, boulders, and artificial structures including boats, dock pilings, and aquaculture equipment. It can also grow over other organisms, including sessile invertebrates, algae, and eelgrass. Its appearance has given rise to the common name marine vomit. It was first described from New Zealand as Didemnum vexillum by Patricia Kott in 2002. The species had been discovered previously in its native region of Japan. A colony of Didemnum vexillum consists of a number of sac-shaped zooids connected by a common tunic. Each zooid is about 1 mm (0.04 in) long and has a buccal siphon through which water is drawn into the interior. The water then passes into a shared cavity from which it is pumped out through an atrial siphon. The surface of the colony is smooth, leathery, and often veined in appearance; the buccal siphons appear as numerous fine pores, and the atrial siphons as a smaller number of larger holes. The colony is firmly attached to a hard surface from which it can be difficult to detach. D. vexillum has different forms in different locations. It can form a thin or thick encrusting mat, or form large or small lobes. The colour can be orange, pink, tan, creamy yellow or greyish-white and the tunic is sparsely strengthened by stellate spicules with nine to eleven rays. Where there is little water movement, the colonies may dangle in ropey masses from hard substrates, such as cables, docks, and the hulls of vessels. These stringy formations have led to it being colloquially referred to as 'Sea Cheese' in Canada. In places with stronger currents, they cover the surface of rocks, boulders, pebbles, gravel, and oysterbeds in a thin, encrusting layer. Didemnum vexillum appears to be native to Japan, where it was recorded in Mutsu Bay in 1926. It is still common there and, as well as growing on rock surfaces and seagrasses (Zostera), it grows as a fouling organism on cultured bivalves, net cages, pilings and other man-made structures. Its depth range is from the intertidal zone down to a depth of about 80 m (262 ft). The Portuguese oyster (Crassostrea angulata) cultivated off the Atlantic coast of France and Portugal was largely killed by an iridoviral disease in 1969. To replace the stock, large quantities of Pacific oyster (Crassostrea gigas) spat were imported from Japan. Some efforts were made to kill any fouling organisms that might be associated with the spat by immersing it for two periods, each of one hour, in fresh water. However, in the early 1970s, there were reports of a brown Didemnum ascidian on oyster beds in the region. Around the same time, oyster spat from Japan was introduced into the Pacific coastal regions of Canada and the USA and soon afterwards an ascidian was observed there, and later identified as D. vexillum. By 2014, this tunicate was present in the eastern Atlantic, North Sea and Mediterranean Sea in the Netherlands, the United Kingdom, Ireland, France and Italy. On the east coast of North America its range extended from New Jersey, New York, Connecticut, Rhode Island, Massachusetts and New Hampshire, to Maine. In the eastern Pacific its range extends from Alaska to California. It is also present in New Zealand, where it was first observed in 2001, its only known encroachment into the Southern Hemisphere. Because this ascidian broods its embryos and the larvae are free-swimming for only a brief time, natural dispersal of D. vexillum is limited in extent. Dispersal to new habitats in far flung parts of the world is likely to be through its accidental conveyance on the hulls of boats or other floating structures, or its introduction to aquaculture installations, perhaps with commercial oyster spat, seed mussels or aquaculture equipment. Like other colonial ascidians, Didemnum vexillum is a suspension feeder. Water is drawn in through the buccal siphon of each zooid, the plankton and fine particles of detritus are filtered out, and the water and waste products leave through a common atrial siphon.