Adaptive coloration and gene flow as a constraint to local adaptation in the streamside salamander, Ambystoma barbouri

Predation is an important selective force that influences animal color patt erns. Some larval populations of the streamside salamander, Ambystoma barbo uri, inhabit streams with fish predators. Other larval salamanders are foun d in shallow, ephemeral streams that are predator-free. Quantitative melano phore cell counts and estimates of percent body area pigmented indicated th at larval coloration is strongly correlated with stream type. Larvae that c oexist with fish tend to be lighter than larvae from streams that are fishl ess and ephemeral. Two approaches demonstrated that lightly pigmented salam ander larvae better match the common background in relatively permanent str eams and are less conspicuous to fish than dark larvae. First, using a mode l based on the spectral sensitivity of the fish and reflectance properties of salamanders and natural stream backgrounds, we showed that light larvae are three times more cryptic than dark larvae on rocks. Second, lighter lar vae had higher survival than darker salamanders on rocks in a predator- cho ice experiment. It is not clear why larvae in ephemeral streams are darker. Larvae in ephemeral streams should be active to feed and develop rapidly a nd reach sufficient size to metamorphose before seasonal drying. Several hy potheses may explain why larvae tend to be darker in ephemeral streams, suc h as increased thermoregulatory ability, better screening of ultraviolet ra diation tin these shallower streams), or better background matching to terr estrial predators. Among populations where salamander larvae coexist with f ish, there are differences in relative crypsis. Larvae from populations wit h fish and relatively high gene flow from ephemeral populations (where larv ae are dark) tend to be darker (with more melanophores) and more conspicuou s to predators than those from more genetically isolated populations, where larvae are lighter and more cryptic. These differences illustrate the role of gene flow as a constraint to adaptive evolution.