Pelvic Reduction and Its Directional Asymmetry in Threespine Sticklebacks from the Cook Inlet Region, Alaska

Bell, M; Francis, R; Havens, A

Variation of the pelvic complex and number of dorsal spines in populations of Gasterosteus aculeatus from lakes near Cook Inlet, Alaska is described. The frequency of specimens with pelvic reduction and the degree of reduction varies among samples but all samples exhibit strong directional asymmetry in the degree of development of pelvic vestiges. Based on samples from this and other studies, there appears to be a strong tendency among isolated populations for pelvic vestiges to be larger on the left side in G. aculeatus. Dorsal spine number, which is reduced in all other populations known to exhibit pelvic reduction, is not reduced significantly in any of the samples. The lakes from which these sticklebacks come are low in dissolved calcium and also lack predatory fishes, two environmental factors hypothesized to favor evolutionary loss of the pelvic complex. However, failure of the dorsal spines to be reduced in number may be inconsistent with the hypothesis that absence of fish predation and the concomitant increase in insect predation favor evolution of pelvic reduction.

Pelvic Reduction and Its Directional Asymmetry in Threespine Sticklebacks from the Cook Inlet Region, Alaska

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