School of Biological and Environmental Sciences, Liverpool John Moores University, Liverpool, L3 3AF, UK.
School of Aquatic and Fishery Sciences, University of Washington, 1122 NE Boat St, Seattle, 98105, USA.
BMC Evol Biol. 2019 Nov 8;19(1):207. doi: 10.1186/s12862-019-1533-x.
Parasite attachment structures are critical traits that influence effective host exploitation and survival. Morphology of attachment structures can reinforce host specificity and niche specialisation, or even enable host switching. Therefore, it is important to understand the determinants of variation in attachment structures. Cymothoid isopods are striking ectoparasites of fishes that include the infamous 'tongue-biters.' They are known to parasitise hosts in one of four qualitatively distinct anatomical regions. Here, we quantify variation in cymothoid attachment structures - hook-like appendages called dactyli - and test whether differences in dactylus shape are correlated with parasite mode (where they attach), allometry, or both, using multivariate ordinary least squares regression. We also assess the influence of shared ancestry on shape using a molecular phylogeny to weight our models using phylogenetic generalised least squares regression.
We find clear differences in shape between externally-attaching and internally-attaching cymothoids but also between anterior and posterior dactyli across various species with the same attachment mode. Allometric effects are significant for anterior but not posterior dactyli. Mouth-attaching species show greater shape variability than gill- and mouth-attaching species. We find no evidence that there are clade-specific patterns of association between parasite mode and dactylus shape.
Parasite mode appears to be the main driver of attachment morphology. This likely reflects several components of parasite ecology including feeding and functional demands of attachment in different microhabitats. Geometric morphometric approaches to the quantification of shape variation of simple structures is an effective tool that provides new insights into the evolvability of parasite attachment.
寄生虫附着结构是影响有效宿主利用和生存的关键特征。附着结构的形态可以增强宿主特异性和生态位特化，甚至可以实现宿主转换。因此，了解附着结构变异的决定因素非常重要。海胆是鱼类的一种显著的外寄生虫，包括臭名昭著的“咬舌者”。它们被认为寄生在四个定性不同的解剖区域之一的宿主上。在这里，我们量化了海胆附着结构的变化——称为钩状附肢的指状突——并使用多元普通最小二乘回归测试了指状突形状的差异是否与寄生虫附着模式（它们附着的位置）、异速生长或两者都相关。我们还使用分子系统发育评估了共同祖先对形状的影响，使用系统发育广义最小二乘回归为我们的模型加权。
我们发现，在具有相同附着模式的不同物种中，外部附着和内部附着的海胆以及前、后指状突之间的形状存在明显差异。前指状突的异速生长效应显著，但后指状突则不然。口附着的物种比鳃和口附着的物种具有更大的形状变异性。我们没有发现寄生虫附着模式与指状突形状之间存在特定进化枝模式的证据。
寄生虫附着模式似乎是附着形态的主要驱动因素。这可能反映了寄生虫生态学的几个组成部分，包括在不同小生境中附着的觅食和功能需求。对简单结构形状变异的几何形态计量方法是一种有效的工具，可以为寄生虫附着的可变性提供新的见解。
