Molluscan distribution patterns on the continental shelf of the middle Atlantic Bight (northwest Atlantic)
Franz, DR.; Merrill, AS.
Malacologia 192: 209-225
A zoogeographic analysis of the inner continental shelf fauna of the Middle Atlantic Bight (Cape Cod, Massachusetts, to Cape Hatteras, North Carolina, USA) is presented based on the geographical and depth distributions of 184 mollusk species collected by the R/V Research Vessel DELAWARE II in 1960 (Cruise 60-7). The Middle Atlantic Bight fauna contains fewer than 4% of species endemic to this zone, and is composed of a mixture of northern and southern species. The former comprises 2 faunal groups: an Arctic-Boreal group containing species which extend northward into arctic waters, and a Boreal group, which reaches northern limits near the south of Labrador Canada . Species of both faunal groups reach their southern limits in the Middle Atlantic zone. The faunal component of predominantly southern species is designated the Transhatteran faunal group, a term which emphasizes the capability of these species to transgress the ecological barrier of Cape Hatteras. Many arctic-boreal and boreal species exhibit submergence south of Cape Cod, i.e., they track cold isotherms into the deeper shelf waters. Species showing submergence tend to be amphi-atlantic in distribution. Endemic boreal species generally do not show submergence in the Middle Atlantic Bight, and are thus more tolerant of warm summer temperatures which characterize the inshore waters of this area. Transhatteran species do not show submergence, although they are variable in their depth distributions.