Morphological variation in fishes along a DOC gradient

Variation in traits related to foraging and locomotion in benthic and limnetic
habitats has been observed in many fishes. Benthic and limnetic food chain
productivity in lakes is strongly influenced by the concentration of dissolved
organic carbon (DOC) in the water, suggesting that DOC might indirectly
impose selection on these traits and lead to classic benthic forms at low DOC
concentrations and limnetic forms at high DOC concentrations. We tested this
hypothesis via geometric morphometric and meristic analyses of bluegill
sunfish (Lepomis macrochirus, Centrarchidae) from 14 lakes with DOC concentrations
ranging from 4 to 24 mg/L. These lakes, located in close proximity to each
other, straddle the drainage divide between the Mississippi River and Laurentian
Great Lakes basins in northern Wisconsin, USA. Bluegill morphology was consistently
related to lake DOC concentration in both drainage basins, despite differences
in morphology between basins. Fish from higher DOC lakes had deeper
bodies and smaller heads, among other differences, though the proportion of
shape variation described by DOC was low. Gill raker length and inter-raker spacing
were positively related to DOC concentration. Although some traits were thus
related to DOC concentration, the directions of these relationships did not match
the predicted benthic–limnetic patterns. Further, no relationships were evident
between DOC and gill raker number, eye width, pectoral fin dimensions, or pectoral
fin insertion angle in univariate analyses. These variable outcomes suggest
that selection linked to DOC does not map neatly onto the classic benthic–
limnetic axis, that high DOC favors a benthic–limnetic generalist rather than a
limnetic specialist, or that the benthic–limnetic morphological dichotomy is less
clear and universal than is often suggested.