Distribution patterns of common seastars of the Middle Atlantic continental shelf of the Northwest Atlantic (Gulf of Maine to Cape Hatteras)

Twenty-five asteroid species were collected during four groundfish surveys carried out by the National Marine Fisheries Service on the continental shelf from the Gulf of Maine to Cape Hatteras. Six species groups were identified, based on similarities in geographic ranges, bottom-temperature relationships, and depth distributions. Species endemic to the boreal Northwest Atlantic include a group of middle-to outer-shelf species usually found in deeper waters north of Cape Hatteras, and a group of species with broad depth ranges whose inshore depth limits decrease from north to south (the phenomenon of submergence). Seastars found only in the Gulf of Maine in this survey include a group of subarctic species (which do not live in arctic waters), and a group of arctic-subartic species (which may live in arctic waters). The latter are wider-ranging, occurring throughout the Arctic Ocean and into the North Pacific. Seasonally variable bottom isotherms associated with Georges Bank restrict the geographical distribution of the boreal and arctic-subarctic faunal groups of seastars. Speculations are presented concerning when and where these groups originated. The possible effects of competition between Asterias forbesi and A. vulgaris on their distribution patterns are discussed.