Inter- and intra-specific variation in movement behaviour of benthic macroinvertebrates from a transitional habitat: a laboratory experiment

The movement behaviour of vagile organisms has long been acknowledged as a key determinant of species distribution and biodiversity patterns. Yet, scant information is available for aquatic invertebrates; in addition, the effects of inter-specific differences in morphology and intra-specific variations in body size have been inadequately investigated. In the present study, the movement behaviour of three crustacean species [Proasellus coxalis and Lekanesphaera hookeri (Isopoda) and Gammarus aequicauda (Amphipoda)] and two gastropod species (Ecrobia ventrosa and Bithynia leachii) dominating the macrobenthic assemblage of a Mediterranean transitional habitat (Giammatteo channel estuary, SE Italy) was analysed under resource-free laboratory conditions. The average path length and speed were determined for individuals encompassing a 16-fold range in body sizes. The scale-independent fractal dimension D was used to quantify the tortuosity of their trajectories. In general, significant differences were observed in movement metrics between crustaceans and gastropods; however, species-specific effects were obscured within each taxonomic group by a strong intra-specific variability. All crustacean species were characterized by a significant negative relationship between body size and path tortuosity; specifically, breaks in the slopes of the regression curve were observed, coinciding with sexual maturation. In contrast, negligible relationships were observed for gastropod species. The results of the study suggest that body size may represent a good predictor of movement behaviour for crustaceans but not for gastropods. Specifically, crucial steps of the biological cycle of amphipods and isopods may reflect on considerable variations in movement metrics. The metabolic and ecological implications of these findings are discussed