Genetic Variability in the Pelagic Environment: A Paradox?

The pelagic environment is often regarded as rather homogeneous by comparison with shallow—water benthic environments. Species of krill of the pelagic genus Euphausia (Crustacea) display a trend in genetic variability from low in high latitudes to high in low latitudes, closely similar to trends displayed by shallow—water benthic invertebrates. We interpret the trends as genetic strategies, with few functionally—broad alleles in high latitude species or in species that range widely ecologically, and numerous functionally—narrow alleles in low latitude species that are narrowly restricted ecologically. The functionally—narrow alleles are maintained by forms of balancing selection and permit a high degree of specialization. Therefore the tropical kill species is highly sensitive to spatial heterogeneities within the pelagic zone, which does not appear homogeneous to selection.