The significance of meristic variation in Benthosema glaciale (Pisces, Myctophoidei) and of the species distribution off northwest Africa

Abstract

A comparison of meristics (vertebrae, AO photophores, and gill rakers) between populations of the lanternfish Benthosema glaciale from the eastern North Atlantic north of the Gulf of Cadiz, the Canary Current Upwelling Region, and the Mediterranean reaffirms the distinctions made by Tåning (1918, Report on the Danish Oceanographical Expeditions 1908–1910 to the Mediterranean and adjacent seas, 2, 1–154) between Northern Atlantic and Mediterranean forms and furthermore shows that the Upwelling and Mediterranean populations are indistinguishable on the basis of these characters. Although the current systems through the Straits of Gibraltar are such that the potential for contact between the Northern Atlantic and Mediterranean populations exists, no clinal relationship of meristics between them was found in the area of Mediterranean Outflow. This, and observed changes in interspecific relationships held by the Northern population and also the different relationships held by the Northern and Mediterranean populations point strongly to their genetic separation. Consideration of other species showing distribution patterns analagous to those of B. glaciale populations suggests that whilst gene flow between the Mediterranean and Canary Current Upwelling regions must be strongly maintained, that between the Mediterranean and Northern Atlantic is much weaker. Moreover, the evidence suggests that a subpolar-temperate and temperate mesopelagic fish fauna may exist in a pseudoceanic capacity, extending southwards of the Gulf of Cadiz along the northwest African slpe area in the “Mauritanean Upwelling Region”. Within the northern sector of this region these species have their centres of abundance over the slope rather than off it, whereas species of other basic distribution patterns (northern and tropical) have their centres further offshore, at least in late winter and early spring. The onslope fauna breeds and thrives around upwelling conditions, whereas the situation regarding northern species of the offslope fauna may be analagous to that on the western seaboard, where tropical species can be maintained in abundance in temperate climes by the Florida Current-Gulf Stream system. It is suggested that the hydrographic and circulatory characters off northwest Africa would maintain the core abundances of these two faunas as separate entities.