A Nuisance

Invasive colonial sea squirt continues to thrive on the gravel bottom on Georges Bank, situated between Cape Cod, Massachusetts and Nova Scotia.  The colonies are denser than in 2005 over the 88 square-mile area observed. But scientists found no colonies in nearby Canadian waters, indicating they have not spread eastward. The Georges Bank squirts are a species of the genus Didemnum.

Didemnum is a nuisance to the aquaculture industry, overgrowing shellfish in New England coastal waters.

Sea squirts, also called tunicates, have a primitive spinal cord and an outer sheath or "tunic," from which the name derives. Tunicates spread in several ways: by larvae that swim for only a few hours before settling; by colonies that hitchhike onto surfaces such as boat hulls, moorings, fishing gear, and other manmade objects and are carried to new, favorable habitats; and by fragments of colonies that are broken up by human

activities and natural events and drift until they settle elsewhere.

They expand outward by budding new millimeter-sized individuals to form circular mats up to a foot in diameter. The mats coalesce with neighboring colonies to form a tough, barren layer of intergrown colonies that attach to hard surfaces including gravel, wood, metal, and plastic. No other species is known to eat or overgrow them.

Scientists remain concerned that the infestation could threaten important fisheries in the region. Sea squirt mats could prevent fish from feeding on worms and crustaceans that live in and on the gravel bottom, reduce the shelter required for these species to avoid predators, and limit the space available for settlement of larvae of sea scallops and other species.

--LiveScience Staff

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Credit: USGS