A genetic and morphometric comparison of Helisoma trivolvis and Gambusia holbrooki from clean and contaminated habitats.

Genetic and morphometric data from freshwater snail (Helisoma trivolvis) and mosquitofish (Gambusia holbrooki) populations from a relatively clean and a severely contaminated habitat were compared. Within the clean habitat, snail genetic patterns may have been more influenced than those of mosquitofish by site-specific selection because of the lesser likelihood of gene flow among snail subpopulations. Distinct genetic patterns within the contaminated habitat, combined with data from other published work, suggest that selection for tolerant genotypes may have occurred in both species. Body size in both species was associated with glucosephosphate isomerase allozyme genotype. In snails, apparent selection for a particular allele in the contaminated habitat may be related to its contaminant tolerance and body-size plasticity. In mosquitofish, a particular genotype associated with small body size appears to have been favored in the contaminated environment.