TRADE OFFS IN HABITAT-SPECIFIC FORAGING EFFICIENCY AND THE NASCENT ADAPTIVE DIVERGENCE OF STICKLEBACKS IN LAKES

Abstract

AbstractI assessed habitat-specific foraging efficiency of shallow-water and open-water forms from a morphologically variable population of threespine sticklebacks. Individuals sampled from open-water or shallow-water habitats tend to be morphologically divergent, although the morphology of the modal form is intermediate between the better-known benthic and limnetic pairs of stickleback species that coexist in other local lakes. I tested two hypotheses about this intraspecific variation. First, that heritable genetic variation in body form exists between forms. Morphological differences occurred between progeny reared under common laboratory conditions indicating that heritable genetic variation contributes to morphological variation. Second, that open-water and shallow-water forms face trade offs involving foraging on habitat-specific prey. The foraging efficiency of both forms was measured in two types of habitat (benthos and open-water) simulated in lab aquaria. Foraging efficiency was related to morphological differences between morphs. Relative morph efficiencies were reversed between habitats in a predictable fashion. More streamlined open-water forms consumed Artemia nauplii at a higher rate and with fewer bites than more robust shallow-water morphs. Conversely, shallow-water morphs required fewer bites to capture and consume more and larger amphipods than open-water forms. An asymmetry in the trade offs indicates that shallow-water types may be the more specialized form. These results are consistent with a hypothesis that trade offs in habitat use efficiency can be involved early in the adaptive divergence of sticklebacks into different lake environments. Similar trade offs exist for the coexisting pairs of stickleback species, suggesting that trade offs are more likely a cause, rather than a consequence, of speciation in sticklebacks that colonize lakes.