Claw shape variation in oribatid mites of the genera Carabodes and Caleremaeus: exploring the interplay of habitat, ecology and phylogenetics

Abstract

Background Claws are a commonly observed biological adaptation across a wide range of animal groups. They serve different functions and their link to evolution is challenging to analyze. While there are many studies on the comparative anatomy and morphology of claws in reptiles, birds and several arthropods, knowledge about claws of soil-living oribatid mites, is still limited. Recent research on intertidal oribatid mites has shown that claw shape is strongly correlated with microhabitat and is subject to ecological selective pressures. However, the selective constraints shaping claws in terrestrial oribatid mites are still unknown. Methods In this study, 300 specimens from 12 different species and two genera were examined. Geometric morphometrics were used to quantify claw length and curvature, and to analyze two-dimensional claw shape. In combination with molecular phylogenetic analyses of investigated populations phylogenetic signal was quantified within genera using Blomberg’s K and random replicates. Additionally, ecological information on the investigated species was gathered from previous studies and compiled into tables. Results The claw shapes of Carabodes species vary moderately, with the three species C. reticulatus, C. rugosior and C. tenuis deviating the most from the others. These three species are only found in a small number of habitats, which may require a more specialized claw shape. Our results show that there is a phylogenetic influence on claw shape in Carabodes but not in Caleremaeus. Additionally, habitat specificity and lifestyle were found to have ecological impact on claw shape in both genera. The present results demonstrate that characteristics of the claws of terrestrial oribatid mites are correlated with ecology, but this correlation is apparently weaker than in intertidal oribatid mites that are prone to strong external forces.

Multiple factors influence claw characteristics in oribatid mites (Acari)

“…However, we aim to explore this aspect more thoroughly in future studies to make clearer statements about the correlation between claw morphology and substrate. Anyway, an initial study on the claws of terrestrial Caleremaeus and Carabodes could not infer such a direct correlation but concluded that strong habitat specificity may have a considerable impact on claw shape, whereas the nature of this impact remained unsolved 27 . The present study provides further interesting insights but is also unable to infer a clear correlation between substrate and claw shape as shown in intertidal oribatid mites.…”

“…less curvature is linked with softer substrates. But if we look at the species with claw curvatures smaller than 90°, which means their claws are strongly curved, only Caleremaeus alpinus is known to predominantly occur in mosses on rocks and thus on hard substrates 27 . All the other species dwell in habitats with softer substrates, e.g.…”

“…In earlier investigations, we developed innovative methodologies for quantifying claw characteristics in mites using geometric morphometrics (GM). For a comprehensive description of our specimen preparation, photography, landmark selection, and coordinate data processing, please refer to Pfingstl et al 24 and Kerschbaumer et al 27 . In addition to claw shape, claw length, and claw curvature (γ), we incorporated the morphological parameter, claw sharpness, into our analysis.…”

“…The strong evolutionary pressure in this extreme environment even has led to the loss of a phylogenetic signal in the claws of intertidal oribatid mites 25 . However, recent initial studies on claws of a few typical terrestrial oribatid mite species 26 , 27 indicated that habitat specificity and lifestyle also have an impact on claw morphology in the terrestrial environment, but this correlation is apparently weaker than in intertidal oribatid mites which are subject to strong external forces. Moreover, a phylogenetic signal in the shape of claws could be quantified at least in one group of these terrestrial oribatid mites 27 .…”

“…However, recent initial studies on claws of a few typical terrestrial oribatid mite species 26 , 27 indicated that habitat specificity and lifestyle also have an impact on claw morphology in the terrestrial environment, but this correlation is apparently weaker than in intertidal oribatid mites which are subject to strong external forces. Moreover, a phylogenetic signal in the shape of claws could be quantified at least in one group of these terrestrial oribatid mites 27 . These recent studies on terrestrial oribatid mites provided interesting first insights but were by far not comprehensive enough to unravel the factors shaping claw morphologies in the terrestrial environment.…”