Marine bivalves: Distribution of mero-planktonic shell-bearing larvae in eastern north Atlantic surface waters

Abstract

Larval shells of benthic marine bivalves occur frequently in plankton samples from temperate to tropical eastern North Atlantic waters. At many sites their abundance is higher than fifty specimens per cubic meter of surface water, thus outnumbering the other planktonic components with calcareous shells. Generally they are more frequent in nearshore water masses than far from land, but it could be shown that not only faunas on continental shelves, but also on oceanic inlands and on submarine elevations, produce larvae, which can be found far out in the ocean.Although it cannot be excluded that long-transported bivalve larvae have also been found in these samples, two arguments seem to assure that the bulk of these shells has been produced in the neighbouring shallow-water areas: decreasing abundance as well as increasing size of shells with increasing distance from the shallow-water area (this applies both to island and continental shelves).The bulk of bivalve shells is concentrated in surface waters. However, shells have also been found with decreasing abundance in water depths down to 600 m. The size distribution in the water column is qualitatively similar for water masses close to the coast and far from land; large shells occur in the upper 50 m, but average diameter decreases below this depth. However, far from land in water depths of 500–600 m they can reach sizes up to about 0.5 mm in diameter (according to one specimen). Generally, shell sizes are larger in corresponding water depth levels far from than in those close to land.The occurrence of larval shells of bivalves throughout the eastern North Atlantic also has wide paleogeographic implications. Since no specific determinations of these bivalves have been tried, it is unknown in which water depths their parental generations occur. However, it can be assumed that eastern North Atlantic island shelves and peaks of submarine guyots and other subsurface elevations which reach to within a few hundred. meters of the surface, can be populated by faunas from eastern Atlantic continental margins being transported by off-shore currents far into the ocean. Since the islands and presumably guyots, etc. as well, produce pelagic larval assemblages from their own benthic molluscan faunas, it can easily be assumed that larval shells can be transported across the ocean by surface and subsurface currents.