Abstract
AbstractHair‐like structures of the shell in some terrestrial snails have been suggested to represent an adaptive advantage. One of the recently proposed explanations was the hypothesis that the possession of hairs facilitates the snails' adherence to the leaves of the plants during foraging, when humidity levels are high. In order to obtain some reliable data to test this contact mechanics hypothesis, the present paper deals with the frictional and adhesive properties of the smooth and hairy shells of two species of terrestrial mollusks, Trochulus hispidus and T. villosulus. Additionally, actively generated traction force of the snails was measured on different substrates. The conducted experiments revealed that in wet conditions the hairiness indeed increases friction and that adhesion through the water film is high enough to retain the weight of the snail. However, possession of long hairs decreases the adhesion, in comparison with the animals with short hairs. Our data suggest that hair‐like structures require more energy expenditure during locomotion and only provide the advantage of higher adhesion on wet substrates. Obtained maximal traction forces were about 30 times higher than friction forces of the shell surface dragged over the substrate. Neither wetting condition of the substrate nor surface roughness had considerable effect on the traction forces of snails. Therefore, the function of the hair‐like structures seems not to be related with snail locomotion and adhesion.
Highlights
Hair-like structures on the surface of the shell occur in different even distantly related snail families (Pfenninger et al, 2005; Wade et al, 2007; Allgaier, 2011), suggesting that these features evolved several times independently
Shells were wetted, and water film covered the bottom side of the shell
Snail locomotion was previously investigated in the context of physiology (Lissmann, 1945; Abbott & Lowy, 1958), biomechanics (Denny, 1981; Kobayashi et al, 2003; Chan et al, 2005; Lai et al, 2010), and tribology (Kim et al, 2010; Shirtcliffe et al, 2012)
Summary
Hair-like structures on the surface of the shell occur in different even distantly related snail families (Pfenninger et al, 2005; Wade et al, 2007; Allgaier, 2011), suggesting that these features evolved several times independently. In all Trochulus species, juvenile shells are covered with hairs, in adult snails they can get lost These structures might be evolutionary expensive and should provide a selective advantage (Emlen, 2001). It was suggested by some previous researchers that shell structures of snails provide camouflage and defense against predators and parasites (Wilson et al, 2004), mechanical stability (Britton, 1995), or involved in sexual selection (Schilthuizen, 2003). These statements for hairy snails have not been tested experimentally yet. The role of mollusks’ hair-like structures still remains unknown
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