Biogeographical relationships of the molluscan fauna of the Ormonde Seamount (Gorringe Bank, Northeast Atlantic Ocean)

Thirty-six taxa (28 Gastropoda and eight Bivalvia) were collected at Ormonde Seamount (Gorringe Bank). Trochidae and Rissoidae, both with three species, were the best represented families. Manzonia crispa (Watson, 1873), formerly thought to be restricted to Madeira, has its range extended to the northeast. Of the 25 prosobranchs, 14 taxa have a planktotrophic type of development and 11 taxa are non-planktotrophic. Most of the species that occur at Ormonde are related to Mediterranean taxa, to those of the Portuguese continental shelf, and also to those of Madeira and the Canary Islands. The geographical position and topography of seamounts, the prevailing sets of sea-surface and depth currents, the distance to the nearest colonizing source, available area for the settlement of the larvae, mean and annual range of water temperature, larval ecology, biotic factors (interspecific competition, the history of the successive colonizers), and also anthropogenic factors are discussed, in order to provide an explanation of the biogeographical relationships of the Ormonde Seamount.