Previous studies of the population genetics of marine copepods using allozymes have demonstrated that there is considerable genetic variation within planktonic species. The genetic variation appeared to lack geographic pattern at coarse (10s of km) to mesoscales (100s of km) [ Bucklin and Wiebe, 1986], but geographic structure was evident at meso- to megascales (1000s of km) [ Bucklin and Marcus, 1985; Bucklin et al., 1989; Bucklin, 1991]. In some cases, it has been possible to infer zooplankton disperal through parallel analysis of physical structure. For example, significant differences in allozymic frequencies that were observed between coastal and offshore samples of the calanoid copepod, Metridia pacifica [ Bucklin, 1991], were obscured during coastal filament activity, presumably as a result of cross-shelf transport [ Bucklin et al., 1989].
Restriction fragment length polymorphisms of
mtDNA have been assayed
for a number of organisms (see review by Avise et al. [1987]). Among
marine invertebrates, several have been shown to exhibit population genetic
structuring on meso- to megascales, including: the horseshoe crab,
Limulus [ Saunders et al., 1986], the oyster, Crassostrea
[ Reeb and Avise, 1990], the mussel, Mytilus [ Edwards and
Skibinski, 1987], and the copepod, Calanus pacificus [ Bucklin and
Kann, 1991]. Mitochondrial gene sequences have revealed considerable
intraspecific molecular variation and some evidence of population structure in
marine fish [ Carr and Marshall, 1991a, 1991b; Finnerty and
Block, 1992]. For invertebrates, the molecular variation did not resolve into
significant population structure at the spatial scales studied for the sea
urchins, Strongylocentrotus pallidus [Palumbi and Kessing, 1991]
and Heliocidaris tuberculata [ McMillan et al., 1992], or for the
penaeid shrimp, Penaeus stylirostris [ Palumbi and Brand,
1993]. However, populations of the copepod, Acartia tonsa, differed
significantly in mtDNA sequence among estuaries of the eastern seaboard of the
U.S. [ Caudill and Bucklin, 1994].
Two general principals emerged from the various studies: first, that both marine fish and invertebrates are moderately to highly variable in biochemical and molecular genetic traits, and second, that geographic patterns of genetic variation are evident for some of the species, usually at spatial scales larger than the oceanographic mesoscale. (See Ovenden [1990] for a review of marine stock assessment using molecular techniques).
The results of three recent studies of planktonic copepods in different oceanographic domains will serve to illustrate how population genetic analysis of marine zooplankton may be used to address questions regarding the dispersal of zooplankton in the ocean. The questions are: are there identifiable source regions that drive secondary production in planktonic ecosystems?; how effective are major current systems in transporting zooplankton?; and, are there definitive boundaries between populations inhabiting different oceanographic regions? The copepod species used in each study are (respectively): Calanus finmarchicus in the western North Atlantic; Nannocalanus minor in the Gulf Stream; and> Calanus pacificus in the California Current.