Moving in fast waters: the exaggerated claw gape of the new river crayfish (Cambarus chasmodactlyus) aids in locomotor performance

Humans are inherently fascinated by exaggerated and conspicuous morphological structures such as the elk antlers and the peacock trains. Because these traits are costly to develop and wield, the environment in which they are used can select for specific sizes or shapes to minimize such costs. In aquatic environments, selection to reduce drag can constrain the form of exaggerated structures; this is presumably why exaggerated morphologies are less common in aquatic environments compared to terrestrial ones. Interestingly, multiple species of crayfish possess claws with an exaggerated gape between their pinching fingers, but the function of this claw gape is unknown. Here, I describe and test the function of the exaggerated claw gape of the New River Crayfish, Cambarus chasmodactylus. Specifically, I test the hypothesis that the claw gape aids in movement throughout fast-flowing streams. I found that both claw size and gape size were sexually dimorphic in this species and that males had disproportionately larger gapes compared to females. Further, by experimentally manipulating gape size and testing crayfish locomotor performance, I found that individuals with their gape blocked were 30% slower than crayfish with a natural gape. My results highlight a unique adaptation that compensates for wielding an exaggerated structure in aquatic environments.

人类天生便对夸张且醒目的形态结构抱有浓厚兴趣，例如麋鹿的鹿角与孔雀的尾羽。由于这类性状的发育与维持均需付出高昂成本，其所处的环境会通过选择作用筛选出特定的尺寸或形态，以最小化此类成本。在水生环境中，降低拖拽阻力的选择压力会限制夸张结构的演化形式；这或许可以解释为何相较于陆生环境，水生环境中夸张形态的结构更为少见。有趣的是，多种螯虾（crayfish）拥有一对具有夸张间距的螯指——即其钳夹手指间的开口尺寸偏大，但这种螯间距的功能迄今仍未明确。本研究针对新河螯虾（Cambarus chasmodactylus）的夸张螯间距功能展开描述与验证。具体而言，本研究检验了如下假说：该螯间距有助于螯虾在急流溪流中移动。研究结果显示，该物种的螯尺寸与螯间距均存在性二态性，雄性个体的螯间距相较雌性个体比例更大。进一步通过实验操控螯间距并测试螯虾的运动性能，本研究发现，螯间距被封堵的个体其运动速度较自然状态下的螯虾慢30%。本研究结果揭示了一种独特的适应性特征，该特征可补偿在水生环境中携带夸张结构所需付出的代价。