Two morphotypes of lacustrine threespine stickleback,Gasterosteus aculeatus, in Benka Lake, Alaska

Synopsis Within freshwater fishes, a common pattern of diversification of body form and trophic structure has recently been recognized. Two different suites of co-occurring characters appear to allow fish to efficiently forage on either benthic invertebrates or plankton. For threespine stickleback,Gasterosteus aculeatus, these suites of characters have been labeled benthic and limnetic. The forms differ in several morpholoical traits, with the limnetic having a more fusiform shape, larger eyes, longer and more numerous gill rakers, and a smaller more tubular mouth. Benthic and limnetic threespine stickleback are usually found in allopatry or parapatry, and less frequently in sympatry. Within the range of the threespine stickleback, which comprises perhaps tens of thousands of lacustrine populations, the sympatric occurrence of the benthic and limnetic forms has only been established for six lakes within a small region of the Strait of Georgia, British Columbia. We present the first evidence for the presence of sympatric morphotypes of threespine stickleback outside of British Columbia. We examine the nature and extent of this diversification, and present possible explanations for the sympatric occurrence of these morphotypes. We also explore possible reasons for the small number of documented sympatric benthic and limnetic forms of threespine stickleback despite the existence of thousands of apparently suitable lakes.