The vertebrate-catching behaviour of the fishing spider Dolomedes triton (Araneae, Pisauridae)

Abstract

This study shows that the semiaquatic spider Dolomedes triton (Araneae, Pisauridae) uses water surface waves to detect and localized aquatic and semiaquatic vertebrate prey. In rare cases, prey capture attempts were also triggered by hydrodynamic flow fields generated by a fish swimming close below the water surface. Visual stimuli alone were not used for identification and localization of prey. Concentric water surface waves are produced whenever a fish, frog, or tadpole contacts the water surface (e.g. in order to breathe or feed). D. triton could localize the centre of a concentric fish-generated wave stimulus very precisely up to a distance of at least 18 cm. However, the rate of success (fish captured) was only 9% if the stimulus triggering prey capture behaviour was a water surface wave. In the case of direct incidental contact, this value increased to 16%. D. triton could catch fish that were as much as 4·5 times heavier than the spider itself. Wave stimuli caused by fish, frogs and tadpoles had a small frequency bandwidth with an upper frequency limit rarely exceeding 30–40 Hz. In most cases these stimuli lasted less than 1 s. The mean peak-to-peak amplitude of vertebrate-generated surface waves varied between at least 100 μm (guppy) and 745 μm (goldfish). Both in the time and frequency domain, wave stimuli caused by aquatic and semiaquatic vertebrates were difficult to distinguish from abiotic noise stimuli (i.e. stimuli caused by leaf, seed or twig fall). This may be the reason why Dolomedes responded to less than 1% of fish-generated water surface waves with prey capture attempts.