Morphometry of Bluegill sunfish (Lepomis macrochirus) varies with lake dissolved organic carbon concentration

Inter- and intra-population trait polymorphisms related to foraging and locomotion in benthic and pelagic habitats have been observed in many fishes. Researchers have found that diet, habitat, visibility, and lake chemical components such as DOC are all associated with how different traits may present within fish species (Bentzen and Mcphail 1984; Ehlinger and Wilson 1988; Drinan et al. 2012; Bartels et al. 2016; Caves et al. 2017). The concentration of dissolved organic carbon (DOC) in the water strongly influences the productivity of benthic and pelagic food chains in lakes , suggesting that DOC might impose selection on these traits and lead to classic benthic-foraging “littoral” forms at low DOC concentrations and pelagic-foraging “limnetic” forms at high DOC concentrations. We tested this hypothesis via geometric morphometric and meristic analyses of body, fin, eye, and gill raker morphometry of Bluegill sunfish (Lepomis macrochirus, Centrarchidae) from fourteen lakes with DOC concentrations ranging from 4 to 24 mg L^-1. These lakes were in close proximity to each other in northern Wisconsin, straddling the drainage divide between the Mississippi River and Laurentian Great Lakes watersheds. Many of the traits that we considered were significantly related to DOC concentration, and in most cases the sign (though not always the magnitude) of these relationships was consistent across the two watersheds. Fish from lakes with high DOC concentrations tended to have deeper bodies, deeper and shorter caudal peduncles, shorter and shallower heads, and perhaps larger eyes. Their pectoral fins tended to be inserted more anteriorly and perhaps vertically; pectoral fin length also varied with DOC, but the direction of this effect differed between the two watersheds. The gill rakers on the first branchial arch were longer and more widely spaced at high DOC, but the number of gill rakers did not vary with DOC. While many of the traits that we quantified were significantly related to DOC concentration, the directions of these relationships did not match with the predicted littoral-limnetic patterns. This may be because [MSt3] [CB4] DOC has complex effects on the fitness landscape for foraging and locomotion traits; because high DOC favors a littoral-limentic generalist rather than a limnetic specialist; or because the littoral-limnetic morphometric dichotomy is less clear and universal than is commonly thought.