Geographical Distribution and Morphological Divergence in American Coastal-zone Planktonic Copepods of the Genus Labidocera

Résumé du livre

The combined eastern and western coastal zone of the American continents support 15 morphologically and geographically distinct populations assignable to 12 known species of temperate and tropical planktonic copepods belonging to the genus Labidocera. Morphologically, they comprise two natural and distinctive phylogenetic groups. Each group is represented by several species along the eastern and western coastal zones. Comparative phylogenetic-biogeographic study of these populations has been underway to determine (a) the nature and extent of divergent morphological characters in cephalic and thoracic appendages and body segments and (b) the geographical location and extent of the coastal zone occupied by each population. Comparison between species groups revealed (a) co-occurrences of pairs of species are common, (b) differences in feeding and swimming appendages are restricted to small details such as one more or less small seta per appendage, and (c) extensive qualitative divergence in sexually modified structures indicating essential differences in function. Within a species group (a) co-occurrence is more frequently limited to overlap at the edge of adjacent ranges or in coastal sectors experiencing pronounced seasonal changes in climate, (b) feeding and swimming appendages do not differ materially, and (c) divergence in sexually modified structures varies in proportion to the geographical extent of actual or potential overlap with proximal species. The results indicate that among coastal-zone species with similar genetic and geographical heritages there is a predominance of selection for the development of prezygotic mating barriers but little or no selection for partitioning food resources by means of morphological differentiation or body size.