Geographic Variation in the Gillrakers and Diet of European Threespine Sticklebacks, Gasterosteus aculeatus

Variation in the gillrakers and diet of threespine sticklebacks (Gasterosteus aculeatus) was evident throughout their European distribution. The gillrakers of completely-plated sticklebacks were more numerous, longer, and more closely spaced than those of low-plated sticklebacks. Among populations of either form, sticklebacks from lentic (marine, lake and pond) habitats had more gillrakers than did those from lotic (river and stream) habitats. Genetic control of gillraker number was demonstrated for one lake-dwelling and one stream-dwelling population. Lake-dwelling sticklebacks were found to consume more zooplankton than did stream-dwelling individuals although benthic invertebrates were important prey in both habitats. Variation in gillrakers could be attributed only in part to selection pressures arising from the greater abundance of zooplankton prey in lake than stream habitats. Evidence of stabilizing selection led us to conclude that conflicting selection pressures determined the habitat-characteristic patterns of geographic variation. THREESPINE sticklebacks (Gasterosteus aculeatus) are small teleosts well known for their morphological variability. They are widely distributed in the middle and high latitudes of the Northern Hemisphere where environmental instability is thought to favor the capacity for adaptive variation (Nikolsky and Vasilyev, 1973). Recent studies along the Pacific Coast of North America and in Europe have shown that variation of some morphological characters has a genetic basis, and that local selection pressures may often explain both habitat-specific and regional trends of variation (e.g., Bell and Haglund, 1978; Gross, 1977, 1978; McPhail, 1977;