Get a grip—evolution of claw shape in relation to microhabitat use in intertidal arthropods (Acari, Oribatida)

Abstract

Claws may be the most common biological attachment devices in animals but relatively few studies have examined the ecological and evolutionary significance of their morphology. We performed the first geometric morphometric investigation of arthropod claws using 15 intertidal oribatid mite species from two different families living in three different habitat types to determine if claw shape is correlated with ecology. Our results show that species living on rocky shores show remarkably high and strongly curved claws while species from mangrove habitats show significantly lower and less curved claws. Euryoecious species are able to dwell in a wide range of habitats and show an intermediate claw type. These results indicate a strong relationship between claw shape and microhabitat and the best predictors of microhabitat use seem to be claw height and curvature. Claw length varied to some degree among the species but without any noticeable ecological pattern. A comparison with terrestrial and freshwater aquatic oribatid mite species, on the other hand, confirms that their claws are only half as long as that of intertidal mites and it is suggested that tidal flooding and wave action strongly selects for long claws. In this microarthropod group which occupies a vast array of microhabitats, claw morphology may play an important role in niche separation and hence demonstrate the importance of ecomorphological studies.

Testing for phylogenetic signal in claws suggests great influence of ecology on Caribbean intertidal arthropods (Acari, Oribatida)

“…A phylogenetic signal is defined as ‘a tendency for related species to resemble each other more than they resemble species drawn at random from the tree’ 18 . In the case of intertidal oribatid mites, on the other hand, certain congeneric species clearly clustered in different ecological groups 12 indicating that ecology is the most important factor in shaping claws and that claw shapes contain little phylogenetic information. Nevertheless, the authors of this study 12 did not specifically test for a phylogenetic signal in their dataset, therefore, it remained uncertain if claw shape is mainly a result of ecological adaptation or just a result of closely related species showing similar ecologies.…”

“…Büscher and Gorb 11 performed a sophisticated study on the attachment devices of stick insects and demonstrated a complex complementary effect of claws and adhesive pads and that these structures correlated with substrate surface and hence with the ecology of the animals. Apart from this work, there was only a single study 12 demonstrating a possible correlation between claws and ecology in arthropods where these authors investigated mites. Mites represent the smallest arthropods and they occupy a wide range of ecological niches, which means the landscape microstructure is highly diverse for the different ecological groups 10 .…”

“…Therefore, investigating their claws may provide important insights into the correlation between morphology and ecology. Indeed, the above-mentioned study 12 investigated the shapes of intertidal oribatid mite claws by means of geometric morphometrics and revealed a significant difference between species dwelling on hard substrates, like rock, and species dwelling on soft substrates, like mangrove roots and litter. The former show higher and stronger curved claws while the latter exhibit lower and less curved claws and species occurring on a wide range of substrates show an intermediate claw type.…”

“…Intertidal oribatid mites inhabit the zone between low and high tide, which means they are subject to strong forces caused by tidal water movement. This tidal flooding causes strong selection in terms of movement and attachment resulting in the remarkable correlation between claw shape and used substrate 12 .…”

Habitat differences filter functional diversity of low dispersive microscopic animals

“…These traits are chiefly specialised claws ( Figure S1d, S1e), which might aid in clinging to the leaf's surface and thereby withstand turbulence (e.g. Pfingstl, Kerschbaumer, & Shimano, 2020; but see Pugh, King, & Fordy, 1987) and a larger body size ( Figure S1g). In contrast, the assemblages in the matte consist of species bearing these traits, as well as species with more slender bodies ( Figure S1i) and a longer and pointier gnathosoma ( Figure S1j).…”