No abstract

In a previous study we showed that nocturnal piscivorous catfish track the wake left by a swimming prey fish to locate it, following past locations to detect the present location of the prey. In a wake there are hydrodynamic as well as chemical signatures that both contain information on location and suitability of the prey. In order to determine how these two wake stimuli are utilised in prey tracking, we conducted experiments in catfish in which either the lateral line or the external gustation was ablated. We found that a functional lateral line is indispensable for following the wake of swimming prey. The frequency of attack and capture was greatly diminished and the attacks that did occur were considerably delayed when the lateral line was ablated. In contrast, catfish with ablated external taste still followed the wakes of their prey prior to attacking, albeit their attacks were delayed. The external taste sense, which was reported earlier to be necessary for finding stationary (dead) food, seems to play a minor role in the localisation of moving prey. Our finding suggests that an important function of the lateral line is to mediate wake-tracking in predatory fish.

Swimming fish leave wakes containing hydrodynamic and chemical traces. These traces mark their swim paths and could guide predators. We now show that nocturnal European catfish (Silurus glanis) locate a piscine prey (guppy, Poecilia reticulata) by accurately tracking its three-dimensional swim path before an attack in the absence of visible light. Wakes that were up to 10 s old were followed over distances up to 55 prey-body lengths in our setup. These results demonstrate that prey wakes remain sufficiently identifiable to guide predators, and to extend considerably the area in which prey is detectable. Moreover, wakes elicit rear attacks, which may be more difficult to detect by prey. Wake tracking may be a common strategy among aquatic predators. When an animal swims, chemical and hydrodynamic stimuli persist in its wake for some time after it has left the vicinity (1, 2). Previous studies on predatory fish using olfactory or mechanosensory cues to localize moving prey are few in number and indirect (3,4). Most studies on predator-prey interactions in fish have used predominantly visual predators under well lit conditions (5). The niches of many piscivorous fish, however, require them to hunt at night or at depths where the limited penetration of solar, sidereal, or lunar illumination limits the utility of visual senses (6). We hypothesized that in these circumstances, wakes left by prey fish are used by predatory fish to detect and track their prey in three dimensions, analogous to the way in which dogs or snakes follow the two-dimensional tracks left by their terrestrial prey (7,8). To test this hypothesis, we analyzed the predatory strategy of a nocturnal catfish (Silurus glanis) as it found and attacked swimming prey fish (guppies, Poecilia reticulata). The use of a prey's wake can be distinguished from visual, acoustical, and electrical tracking of prey by path analysis. In all but wake tracking, the predator perceives the instantaneous location of its prey and will approach it directly or in an arc, intercepting the prey's path (9, 10). An indication, therefore, of wake tracking is the similarity of the paths of prey and predator through space with a significant time lag. Materials and MethodsWe used a video-based infrared-illuminated system, maintaining both prey and predator in visual blackout conditions and allowing us to make three-dimensional evaluations of their swim paths. This system consisted of a glass test aquarium (120 cm ϫ 60 cm, filled to a height of 40 cm) illuminated by infrared back lighting. The infrared used was in the 810-950-nm range (maximum at 870 nm), which is outside the range of absorption of the visual pigments of fish (11). Catfish conditioned to react to visual stimuli do not react under infrared illumination, confirming that they cannot see in infrared (K. Pohlmann, personal observation). Fish behavior was recorded on digital video by using two IR-sensitive cameras from different directions. The two recordings were synchronized accurately to the frame. Guppies (total lengths...