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August 2011- Didemnum vexillum
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Article Content
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| Invasive Species of the month |
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Invasive species of the month- Non-Native Colonial Tunicates- Didemnum vexillum
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| What? |
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Didemnum vexillum is a species of ancient marine animals commonly called colonial tunicates, or sea squirts , believed to have orginated from the Pacific seabeds around northern Japan. Didemnum vexillum and other colonial tunicates feed continously by filtering plankton from sea water through their siphons (thus giving them the common name squirt). They grow rapidly and, in the process, produce extensive mats of rubbery mats made of cellulose forming a “tunic” that can smother other forms of marine life. Didemnum vexillum colonies form on and foul nearly any solid or semi-soft substrate, including dock structures, floats, nets, wood and metal pilings, rope, polythene plastic, steel cable and chain moorings, tires, rock, gravel seabed (pebbles, cobbles, boulders), and ship hulls. Didemnum vexillum can overgrow tunicates, sponges, macroalgae, hydroids, anemones, bryozoans, scallops, mussels, and oysters. Where these colonies occur on seabeds, they are likely to cover the siphons of infaunal bivalves and also to prevent fish from finding benthic prey. These non-native colonial tunicates occur at depths ranging from intertidal to continental shelf depths of 65 m (213 ft). Didemnum sp. tolerates a wide range of environmental conditions including temperature, salinity, and water quality. Colonies are found in water temperatures ranging from -11 to 75 degrees Farenheit. In the United States, Didemnum sp. has tolerated daily water temperature fluctuations of 48 degrees. Colonial tunicates tend to hibernate, die off, or go dormant when temperatures are not favorable and resume growth and reproduction when favourable conditions return. Didemnum sp. have been controlled locally using wraps and acetic acid (vinegar) sprays.
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| Where? |
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Didemnum vexillum is reported from northern Europe (Netherlands, France, Ireland, Wales, Scotland), the U.S. east coast (New Jersey, New York, Connecticut, Rhode Island, Massachusetts, New Hampshire, Maine), offshore of New England on Georges Bank, the U.S. and Canadian west coasts (California, Oregon, Washington, British Columbia), Japan, and New Zealand. These non-native colonial tunicates have been reported from the Charleston Boat Basin in Coos Bay, Oregon and on some of the mooring lines and stringers of the oyster growing facility in the triangle at the mouth of the Umpqua River.
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| Lookalike? |
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Many native sponges look like non-native colonial tunicates, however, close examination reveals distinct differences. Didemnum sp. has 2 siphons, but only the incurrent one opens on the surface of the tunic, so in this species you won’t see paired openings; the excurrent siphon of each zooid opens at the side, into a spacious chamber inside the colony. Here, and there at the surface, are large round transparent openings which are the common atrial openings from which the waste products and tadpoles are released. By combining the excurrent water of many zooids, a sufficient current is produced to carry these products away. Thus on the surface one will see many randomly spaced tiny incurrent openings, and here and there a large atrial opening.
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| What can you do? |
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Touching non-native colonial tunicates can spread these invasive species to other dive sites via dive gear or by simply breaking off peiaces of it that will drift in the current and start new colonies. If you see species that may be Didemnum, try to take a photo if you can. Please report suspected invaders online at http://oregoninvasiveshotline.org/ or call the Invasive Species Hotline at 1-866-INVADER (1-866-468-2337), this number is toll free.
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