Seasonal water temperature cycles and the recruitment of larvae of the colonial ascidian Didemnum sp. in New England coastal and offshore waters   Email     Print   PDF

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Valentine, Page C.¹, Carman, Mary R.², Dijkstra, Jennifer³, Blackwood, Dann S.¹, Westerman, Erica.³, Harris, Larry. G.³. ¹U.S. Geological Survey, Woods Hole, MA 02543; ²Woods Hole Oceanographic Institution, Woods Hole, MA 02543; ³University of New Hampshire, Durham, NH 03826.

  The colonial ascidian Didemnum sp. occurs in many regions of the world. It aggressively overgrows hard substrates and many sessile species and has become a nuisance to the aquaculture industry. Its extensive mat-like colonies hypothetically could alter benthic communities. The species can survive in water temperatures ranging from <-1 to >24 ^oC.In order to predict the geographic spread of Didemnum sp. and to aid in developing strategies for managing it, we attempted to determine the relationship between water temperature and the larval recruitment that is necessary for successful colonization of an area.
  Experiments using settlement plates in shallow New England coastal marine waters of up to 5 m depth showed that first larval recruits appeared at water temperatures of 19-20 ^oC in late June 2007 at Woods Hole, MA in the southern part of the region. Farther north, first recruits appeared at the same water temperatures but in late July 2006 at the Damariscotta River, ME.
  Didemnum sp. has been observed over 6 years in two adjacent areas of gravel habitat in water depths of 45-65 m on the northern part of the Georges Bank fishing grounds. The areas are characterized by maximum temperatures in the August-October period that reach only 15-17 ^oC. It is not possible to monitor larval recruitment on Georges Bank, but fully-developed larvae have been observed in colony tissues. We assume larval recruitment is responsible for the presence of dense aggregations of colonies and the species’ rapid spread in the two affected areas, which extend 14 and 30 km in their longest dimensions and encompass 230 sq km. Although tidal currents are strong in the region, the species is absent at sites only 4 to 6 km to the north that have identical gravel substrates and lie in depths of 45-55 m beneath cool water on the shelf edge, where temperatures are highly variable and generally do not exceed 12-14 ^oC.
  The shallow coastal and deep offshore sites studied here are characterized by different annual temperature ranges and rates of warming. In winter, most shallow New England coastal sites decline to temperatures of 0 ^oC and colder. Georges Bank areas lie in deeper water and are no colder than 4-5 ^oC, which we suspect may also be typical of conditions in New England deep coastal areas. Our observations suggest that the development and release of Didemnum sp. larvae is governed by response to local temperature trends, which differ between shallow and deep water areas. The failure over 6 years of Didemnum sp. on Georges Bank to expand a relatively short distance from "warm" areas to nearby "cool" areas suggests that colonies or larvae of the species likely will not successfully colonize offshore or coastal areas where water temperatures do not exceed 15 ^oC.