Experimental evidence for trait utility of gill raker number in adaptive radiation of a north temperate fish

North temperate fish in post-glacial lakes are textbook examples for rapid
parallel adaptive radiation into multiple trophic specialists within individual
lakes. Speciation repeatedly proceeded along the benthic
–
limnetic habitat
axis, and benthic
–
limnetic sister species diverge in the number of gill rakers.
Yet, the utility of different numbers of gill rakers for consuming benthic vs.
limnetic food has only very rarely been experimentally demonstrated. We
bred and raised families of a benthic
–
limnetic species pair of whitefish under
common garden conditions to test whether these species (i) show heritable
differentiation in feeding efficiency on zooplankton, and (ii) whether varia-
tion in feeding efficiency is predicted by variation in gill raker numbers. We
used zooplankton of three different size classes to investigate prey size
dependency of divergence in feeding efficiency and to investigate the effect
strength of variation in the number of gill rakers. Our results show strong
interspecific differences in feeding efficiency. These differences are largest
when fish were tested with the smallest zooplankton. Importantly, feeding
efficiency is significantly positively correlated with the number of gill rakers
when using small zooplankton, also when species identity is statistically
controlled for. Our results support the hypothesis that a larger number of
gill rakers are of adaptive significance for feeding on zooplankton and pro-
vide one of the first experimental demonstrations of trait utility of gill raker
number when fish feed on zooplankton. These results are consistent with
the suggested importance of divergent selection driven feeding adaptation
during adaptive radiation of fish in post-glacial lakes.