The invasive colonial ascidian Didemnum sp. on Georges Bank: ecological effects and genetic identification   Email     Print   PDF

Lengyel, Nicole L.¹, Collie, Jeremy S.¹, and Valentine, Page C.^2
1Graduate School of Oceanography, University of Rhode Island, South Ferry Road, Narragansett, RI 02882, USA; ²U.S. Geological Survey, 384 Woods Hole Road, Woods Hole, MA, 02543, USA

The invasive colonial tunicate Didemnum sp. was first noted in an area of gravel habitat on Georges Bank in 2002. Since its discovery, research has focused on investigating the spread of the tunicate invasion, on evaluating the potential impacts on the benthic community, and on identifying it at the species level and determining its region of origin. Analysis of video and photo transects taken with a USGS SeaBed Observation and Sampling System (SEABOSS) in the infested areas has shown that the tunicate continued to spread annually from 2002-2006 and now is present in an area of 230 sq km in two adjacent gravel areas, one open and one closed to fishing. At some sites in the area closed to fishing, Didemnum sp. covers nearly 75 percent of the seabed.

An Analysis of Similarity (ANOSIM) test on the abundances of 120 species collected by Naturalist dredge from 1994 to 2006 indicated that Didemnum sp. has had a significant impact on the species composition of the benthic community. In particular, the abundance of two polychaete species, Nereis zonata and Harmothoe extenuata, increased significantly in areas infested by the tunicate compared to areas not infested. The polychaetes live beneath the tunicate mats, and the increased abundance of these prey species suggests they are avoiding predation by fish.