Similarity analysis of hydroid assemblages along a latitudinal gradient in the western North Atlantic

Shallow-water (0–100 m depth) hydroid faunas reported from 26 locations along the western North Atlantic coast between the high Canadian Arctic archipelago and the Caribbean Sea were compared. Species numbers varied widely between locations, but were highest in the tropics and subtropics, lowest in arctic and subarctic waters, and intermediate in mid-latitudes. Percentages of species producing free medusae were lowest in high latitudes, intermediate in low latitudes, and highest in mid-latitudes (especially in estuaries). In a numerical analysis, similar hydroid faunas were identified at locations (i) between the high Canadian Arctic islands and the Strait of Belle Isle off western Newfoundland; (ii) between the Gulf of St. Lawrence and Chesapeake Bay; (iii) between North Carolina and southeastern Florida (south as far as St. Lucie Inlet), and including the northern Gulf of Mexico; (iv) in the Caribbean Sea, together with Dry Tortugas and the oceanic island of Bermuda. The greatest change in hydroid species composition along the coast appeared to occur around Cape Hatteras. An assemblage on the continental shelf off the middle Atlantic states of the United States appeared to be distinct because of its low hydroid diversity. Faunistically, however, its affinities were decidedly with assemblages at locations between the Gulf of St. Lawrence and Chesapeake Bay. In addition to latitudinal changes in faunal composition, dissimilarities in hydroid species assemblages were noted between certain inshore and offshore locations at comparable latitudes. Numerical classification of locations provides additional evidence for delineation of biogeographic regions along the western North Atlantic coast.