Genetic heterogeneity in euphausiid populations: Euphausia krohnii

Genetic heterogeneity of samples of Euphausia krohnii and Nematoscelis megalops collected from a 6,000-km* area in western North Atlantic Slope Water was investigated by electrophoretic assay of allozymic variability at eight enzyme loci. Both species exhibited significant among-sample heterogeneity of allozyme frequencies. Genetic distances between samples were large for intraspecific comparisons (D = 0.116 for E. krohnii and D = 0.203 for N. megalops). This heterogeneity indicates that both species are genetically structured. Zooplankton in general and euphausiids in particular show markedly heterogeneous dispersion patterns. Population patches or aggregations occur on a wide range of temporal and spatial scales, from hours to years and from meters to thousands of kilometers (e.g. Wiebe 1970; Smith et al. 1976; Haury et al, 1978; Greenblatt et al. 1982). Whether this patchiness represents persistent population structuring or ephemeral aggregations is unclear. The question is difficult to resolve by conventional techniques of timeseries sampling because of the difficulty of I This research was supported in part by NSF grant OCE 80-17248 to P. H. Wiebe. appropriate sampling strategies (repeated sampling of a single population). Allozymes (allelic variants of enzymes that differ in electrophoretic mobility) provide a means of quantifying biochemical genetic variability in organisms and populations. They can be used as unambiguous, quantitative markers of the genetic distinctiveness of conspecific populations (Nei 1972; Nevo and Cleve 1978). They can thus be used to investigate the significance of zooplankton aggregations when sampling is done on appropriate spatial and temporal scales. Allozyme frequency differences among samples collected at one time from different locations can be used to estimate quantitatively the degree of genetic differentiation associated with spatial patchiness. The degree of spatial heterogeneity will limit the extent to which temporal variability of allele frequencies (e.g. between successive samples) can be resolved, since successive collections may not sample the same population in a spatially heterogeneous species. Genetic heterogeneity resulting from spatial patchiness and temporal variability may in some cases be distinguished. Periodic changes in allozyme frequencies associated with environmental parameters may indi-