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Significant differences in morphology, behavior, spatial distribution, and feeding habits were found between limnetic and benthic forms of the threespine stickleback (Gasterosteus aculeatus) in a small British Columbia lake. The social behavior and feeding ability of each form were studied experimentally to provide a basis for determining the mechanisms of spatial and trophic segregation observed in the lake. The hypothesis that the benthic form was solitary, aggressive, and attracted to lake-bottom cover, and the limnetic form was gregarious, nonaggressive, and attracted to lake-surface cover was supported by the laboratory experiments. Furthermore, the hypothesis that the limnetic form could feed more successfully on zooplankton, and the benthic form on macrobenthos was supported by the experiments. The benthic form spent significantly more time in search of food in lake-bottom cover than did the limnetic form. It is suggested that the two forms of stickleback had undergone extensive adaptations to very different habitats in the lake, and therefore, their spatial and food segregation was not maintained entirely by interactions, but at least partially by inherent behavioral differences.
Gary L. Larson (Sun,) studied this question.