PHENOTYPES AND GENOTYPES IN CLADOCERAN POPULATIONS.

Recently Kerfoot (1975) discussed the divergence of adjacent populations of Bosmina longirostris in response to two different kinds of predators in Union Bay, Seattle, Washington. Sampling Union Bay during the summer of 1972, he noted that animals taken along a transect from littoral to pelagic waters differed from each other with respect to egg and neonate sizes, carapace thickness, body pigmentation, and eye diameter and that, while the variation was essentially continuous, the frequency of phenotypes fell within three modes. Near the lily pads the Bosmina populations consisted almost entirely of animals with short body features, the "short-featured" form. This population also contained a few animals possessing moderately long body features, the "intermediate-featured" form. Immediately beyond the lily pads and across the shallow portions of the bay there coexisted a bimodal mixture of animals with very long body features, the "long-featured" form, and "short-f eatured" Bosmina (See Fig. 1). In the deeper offshore waters Kerfoot found only long-featured animals. Further sampling done at 26 sites near or within the marginal lily pads disclosed similar spatial and seasonal fluctuations in phenotypic character and abundance (Kerfoot, 1975). The magnitude of the spatial differences and the abruptness of seasonal changes in phenotype frequencies suggested that strong selective forces were working to maintain this segregation of phenotypes. Previous work had revealed the interaction of two feeding guilds with this Bosmina population (Kerfoot, 1977). In the offshore, blindly grasping copepods fed upon Bosmina, while visually searching fishes, as well as copepods, fed upon Bosmina in the inshore. As described by Kerfoot (1975), in the offshore where Bosmina suffer injury or death in encounters with predatory copepods the long-featured phenotype is at an advantage, the long tail-like mucrones and anterior antennules rendering the Bosmina difficult to grasp and handle. In t1ie inshore, fish are the more important predators upon Bosmina, for they selectively remove the more visible predatory and herbivorous invertebrates, including the more conspicuous long-featured phenotype of Bosmina. One important facet of Kerfoot's work is the selective removal of intermediates in both inshore and offshore waters. Although abundant at various times of the summer in the lily pad region, the intermediates are never as abundant in the far inshore and offshore as the two extreme phenotypes (Kerfoot, op. cit.). Thus, the Bosmina population in the bay appears to be undergoing predator-mediated disruptive selection, intermediates being selected against in both the pelagic and littoral zones. This pattern of combined selection against intermediates provides an ideal situation in which to investigate the origin of the littoral and pelagic phenotypes and the genetic nature of the intermediatefeatured form. Are these animals phenotypically diverse because they are members of separate species and thus do not interbreed or because they are members of a single species which has diverged in response to differing selection pressures in the bay? The Union Bay population also permits a more general examination of cladoceran population structure. Because cladocerans are cyclical parthenogens, each iteroparous female giving birth to several cloned offspring, selection is acting on ani-