Hooked on you: shape of attachment structures in cymothoid isopods reflects parasitic strategy

Charles Baillie,Kerry A Hadfield,Stefano Mariani,Nico J Smit,Rachel L Welicky,Robin M D Beck

doi:10.1186/s12862-019-1533-x

Charles Baillie, Kerry A Hadfield + Show 4 more

Open Access

https://doi.org/10.1186/s12862-019-1533-x

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Abstract

BackgroundParasite 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.ResultsWe 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.ConclusionsParasite 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.

Highlights

Parasite attachment structures are critical traits that influence effective host exploitation and survival
Dactylus shape variation Principal Component Analysis (PCA) clearly separates dactylus shape of externally-attached cymothoids from gill- and mouth-attaching species for both P1 and P7, where they occupy a distinct region of morphospace (Fig. 2)
Other empirical studies link the morphology of parasite attachment to various ecological factors: in relation to host size in feather lice [49]; host specialisation in platyhelminth fish parasites [50]; host thermal regulation strategy in acanthocephalans [51]; host biogeographic plasticity [52]; and, as we find here, microhabitat [53, 54]

Summary

Introduction

Parasite attachment structures are critical traits that influence effective host exploitation and survival. 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. 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. Traits for attachment function, which are imposed by this lifestyle, are critical for parasite survival and reproduction These traits are often ecomorphologically significant, segregating species between different host niches [2, 3]. Four attachment modes can be recognised within Cymothoidae: (1) mouth-attaching species (including the ‘tongue-biters’), Baillie et al BMC Evolutionary Biology (2019) 19:207

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