LOSS OF NUPTIAL COLOR IN THREESPINE STICKLEBACKS (GASTEROSTEUS ACULEATUS).

Loss of conspicuous nuptial color in Gasterosteus aculeatus (threespine stickleback) has been reported from several localities in western North America and is ascribed to increased rates of predation or to convergent threat displays. I have examined 66 populations of G. aculeatus from the Queen Charlotte Archipelago and found extensive variation in the expression of red nuptial color. Males in 31 of the populations lack red throats, while males in five populations have major expression of red nuptial color. I test two hypotheses for nuptial-color loss: that the loss results from increased predation rates and that it involves differences in water spectra (relative transmission at 400 nm). Results, which are consistent with the second hypothesis, show that the greatest expression of red pigment occurs in habitats with the highest water clarity, while loss of red nuptial color is generally found in heavily stained waters. There is no correlation between nuptial-color loss and presence or absence of vertebrate predators. Two new hypotheses for these associations are proposed: signal-masking in spectrally restricted habitats and carotenoid deficiencies in the diet. Previous studies of red nuptial color and its loss in Gasterosteus merit additional attention, given the associations with underwater spectra.