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 <i>et al.</i> 2012; Bartels <i>et al.</i> 2016; Caves <i>et al.</i> 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 (<i>Lepomis macrochirus</i>, 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.

诸多鱼类中均已观测到与底栖（benthic）和浮游（pelagic）生境下的觅食及运动相关的种群间与种群内性状多态性。研究人员发现，鱼类物种内不同性状的表现与摄食习性、栖息环境、水体能见度以及湖泊化学组分（如溶解性有机碳（dissolved organic carbon，DOC））均存在关联（Bentzen与McPhail 1984；Ehlinger与Wilson 1988；Drinan等人2012；Bartels等人2016；Caves等人2017）。水体中溶解性有机碳（DOC）的浓度会强烈影响湖泊底栖与浮游食物链的生产力，这表明DOC可能对这些性状施加选择压力，进而在低DOC浓度下形成典型的底栖觅食“沿岸（littoral）”型个体，在高DOC浓度下形成浮游觅食“湖沼（limnetic）”型个体。我们通过几何形态测量法（geometric morphometric）与计数分析法（meristic），对来自14座DOC浓度介于4至24 mg·L⁻¹的湖泊的蓝鳃太阳鱼（Bluegill sunfish，<i>Lepomis macrochirus</i>，日鲈科Centrarchidae）的躯体、鱼鳍、眼部及鳃耙形态进行分析，以此检验该假说。这些湖泊均位于威斯康星州北部，彼此毗邻，横跨密西西比河与劳伦琴五大湖（Laurentian Great Lakes）流域的分水岭。我们所考量的诸多性状均与DOC浓度存在显著关联，且在大多数情况下，这些关联的方向（尽管并非总能体现其强度）在两个流域中保持一致。来自高DOC浓度湖泊的个体通常躯体更宽厚，尾柄更深且更短，头部更短且更扁平，眼部或许更大。其胸鳍的附着位置通常更靠前，或许更偏向垂直方向；胸鳍长度也随DOC浓度变化，但这种影响的方向在两个流域中存在差异。高DOC浓度下，第一鳃弓上的鳃耙更长且间距更宽，但鳃耙的总数并未随DOC浓度发生变化。尽管我们量化的诸多性状均与DOC浓度存在显著关联，但这些关联的方向与预期的沿岸-湖沼形态模式并不相符。这可能是因为：[MSt3][CB4] DOC对觅食与运动性状的适合度景观存在复杂影响；高DOC浓度更倾向于选择沿岸-湖沼广适型个体而非湖沼特化型个体；抑或是沿岸-湖沼形态学二分法的普遍性与清晰性不如通常所认为的那样。