Locomotion is an important, fitness-related functional trait. Environment selects for type of locomotion and shapes the morphology of locomotion-related traits such as body size and appendages. In subterranean aquatic arthropods, these traits are subjected to multiple, at times opposing selection pressures. Darkness selects for enhanced mechano- and chemosensory systems and hence elongation of appendages. Conversely, water currents have been shown to favor short appendages. However, no study has addressed the variation in locomotion of invertebrates inhabiting cave streams and cave lakes, or questioned the relationship between species' morphology and locomotion. To fill this knowledge gap, we studied the interplay between habitat use, morphology and locomotion in amphipods of the subterranean genus Niphargus. Previous studies showed that lake and stream species differ in morphology. Namely, lake species are large, stout and long-legged, whereas stream species are small, slender and short-legged. We here compared locomotion mode and speed between three lake and five stream species. In addition, we tested whether morphology predicts locomotion. We found that the stream species lie on their body sides and move using slow crawling or tail-flipping. The species inhabiting lakes move comparably faster, and use a variety of locomotion modes. Noteworthy, one of the lake species almost exclusively moves in an upright or semi-upright position that resembles walking. Body size and relative length of appendages predict locomotion mode and speed in all species. We propose that integrating locomotion in the studies of subterranean species might improve our understanding of their morphological evolution.

Behavioural Ecology Group Department of Systematic Zoology and Ecology Eötvös Loránd University Egyetem tér 1 3 1053 Budapest Hungary

Department of Biology Biotechnical Faculty University of Ljubljana Jamnikarjeva 101 SI 1000 Ljubljana Slovenia

Department of Ecology Charles University Ovocný trh 560 5 116 36 Prague Czechia

Laboratory of Evolutionary Ecology of Hydrobionts Nature Research Centre Akademijos Str 2 LT 08412 Vilnius Lithuania

Scientific and Research Centre of the Slovenian Academy of Sciences and Arts Institute of Biology Novi trg 2 SI 1000 Ljubljana Slovenia

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