Water locomotion and survival under water in a riparian harvestman (Opiliones, Arachnida)

Abstract

Animals that live by rivers may benefit from being able to cross them, but behavioral adaptations are needed. Additionally, being able to remain submerged is also important if the animal moves under water. Here we asked whether the harvestman Heteromitobates discolor (Opiliones), that lives by rivers, (a) can propel itself across the water surface, (b) moves onto the water if disturbed and (c) can survive for long periods when submerged. Heteromitobates discolor exhibited two gaits on water, whereas a strictly terrestrial species was not able to propel itself. When experimentally submitted to simulated predator attack on a rock on the river, H. discolor walked onto the water, while a strictly terrestrial species did not. Finally, it was able to survive for 6 h under water, presumably due to the conspicuous air film that formed around its body, which was also observed in a strictly terrestrial species. Altogether, these observations suggest that the aquatic environment is not a barrier for regular activity and can be used as an extension of the terrestrial environment for H. discolor.