Eastward Extension of the Distribution of the Marine Cladoceran ( Penilia avirostis ) in the Northwest Atlantic: a Case of Ecesis?

Abstract

Data are presented on the distribution and abundance of Penilla avirostris Dana in continental shelf waters, Cape Hatteras to Nova Scotia, during 1980 and 1981. Penilia occurred in the plankton samples during approximately 6 months of the year (June-July to December) During periods of maximum abundance. Penilia occurred throughout the Middle Atlantic Bight and in isolated patches on Georges Bank, which represents an eastward extension of its previously-reported distribution. This eastward spread corresponds to the previously-documented direction of range expansion, but it is at variance with the westerly direction of residual drift. The absence of Penilia in the Gulf of Maine and seaward of the shelf break (outside 200-m isobath) indicates that some minimum level of bottom water temperature, which is necessary to initiate the development of resting eggs ( similar to 10 degree C), may be a determinant of the observed distributional patterns.