Cavanilles Institute for Biodiversity and Evolutionary Biology, University of Valencia, Calle Catedratico Jose Beltran Martinez 2, Paterna 46980, Spain.
School of Earth Sciences, University of Bristol, Life Sciences Building, Tyndall Avenue, Bristol BS8 1TQ, UK.
Proc Biol Sci. 2022 May 25;289(1975):20220093. doi: 10.1098/rspb.2022.0093. Epub 2022 May 18.
The evolution of grouping behaviour involves a complex trade-off of benefits and costs. Among the latter, an increase in the risk of parasitic transmission is a well-documented phenomenon that has likely promoted the evolution of defensive mechanisms in aquatic vertebrates. Here, we explore the relationship between grouping behaviour, parasitic richness (∼parasitic pressure), and the evolution of potential defensive traits in the squamation of sharks through phylogenetic, standard and zero-inflation regression models. Our results demonstrate that sharks that frequently aggregate show increased parasitic pressure, which may constitute an agent of selection. Accordingly, their squamation is characterized by large-scale crown insertion angles and low-scale coverage, which are interpreted as traits that compromise parasite attachment and survival. These traits are less evident in regions of the body and ecological groups that are subjected to high abrasive stress or increased drag. Thus, the squamation of sharks responds to a compromise between various functions, where protective and hydrodynamic roles prevail over the rest (e.g. ectoparasitic defence and bioluminescence aiding). This work establishes a quantitative framework for inferring parasitic pressure and social interaction from squamation traits and provides an empirical basis from which to explore these phenomena through early vertebrate and chondrichthyan evolution.
群居行为的进化涉及到利益和成本的复杂权衡。在这些成本中，寄生虫传播风险的增加是一个有充分记录的现象，这可能促进了水生脊椎动物防御机制的进化。在这里，我们通过系统发育、标准和零膨胀回归模型来探索鲨鱼的群居行为、寄生虫丰富度（~寄生虫压力）和潜在防御特征的进化之间的关系。我们的结果表明，经常聚集的鲨鱼表现出更高的寄生虫压力，这可能构成选择的一个因素。因此，它们的鳞片具有大尺度的冠插入角度和小尺度的覆盖，这被解释为影响寄生虫附着和生存的特征。这些特征在身体的其他区域和生态群体中不太明显，这些区域受到高磨损压力或增加的阻力。因此，鲨鱼的鳞片对各种功能之间的权衡做出了响应，其中保护和流体动力作用比其他作用更为重要（例如，外寄生虫防御和生物发光辅助）。这项工作为从鳞片特征推断寄生虫压力和社会互动建立了一个定量框架，并为通过早期脊椎动物和软骨鱼类进化来探索这些现象提供了一个经验基础。
