There are several common threads in the studies that have been summarized above. The zooplankton species studied exhibited molecular diversities in the average range, despite enormous census population sizes and geographic distributions. This result is contrary to theoretical predictions of high genetic diversity in large populations. One reason for the finding of lower-than-predicted levels of genetic diversity is the highly skewed haplotype frequency distribution for mitochondrial genes. Many species have one predominant, nearly ubiquitous haplotype and numerous unique ones. Also, because unique haplotypes are not geographically informative, the genetic diversity was not usually geographically resolved on spatial scales smaller than the mesoscale (100s to 1000s of km). A number of zooplankton species are genetically structured on larger spatial scales, including meso- to megascales (1000s of km). Populations may be delimited by gradual changes in the frequencies of genetic characters rather than by abrupt changes in the genetic makeup of the population. Exceptions to this principle are cases of subspecies or cryptic species, between which there may be sharp genetically-defined boundaries. The most probable explanation is that ocean mixing is usually sufficiently frequent and effective to obscure boundaries. Molecular population genetic analyses of zooplankton have practical applications in the prediction of spatial and temporal patterns of secondary production in marine ecosystems. Implementation is most likely through incorporation of molecular genetic data into coupled biological/physical models of ocean processes. The long-term goal of these studies is to understand the biological and physical determinants of the productivity of marine ecosystems, especially those that are commercially exploited.
Acknowledgments. Numerous people have collected plankton from many different regions for the genetic analyses discussed in this paper: E. Durbin (University of Rhode Island), B. W. Frost (University of Washington), R. G. Lough (NMFS Northeast Fisheries Laboratory), C. B. Miller (Oregon State University), J. Montoya (Harvard University), M. M. Mullin (Scripps Institution of Oceanography), J. Runge (Institute Maurice LaMontagne), P. H. Wiebe (Woods Hole Oceanographic Institution), and R. Harris and R. Williams (both at the Plymouth Marine Laboratories). N. J. Copley (Woods Hole Oceanographic Institution) and B. W. Frost (University of Washington) assisted with copepod identifications. T. C. LaJeunesse and J. Conroy (University of New Hampshire) provided technical assistance. D. Medeiros-Bergen and C. C. Caudill (University of New Hampshire), P. H. Wiebe (Woods Hole Oceanographic Institution), and two anonymous reviewers provided constructive criticism of the manuscript. This work was supported by an NSF Research Fellowship in Biotechnology and Ocean Sciences (Grant No. OCE-9018528), the U.S. GLOBEC Program (NOAA Grant No. NA36GP0298-01), and the Oceanic Biology Program, Office of Naval Research (Grant No. N00014-93-1-0178).