If you have ever snorkelled before, you would know that it is hard to catch a fish in the ocean. Equally, trying to collect a fish from a tank is very difficult, sometimes almost impossible. You may think that being unpredictable makes it hard to catch fish, and in the marine world, avoiding being caught can mean staying alive. Even though behaving unpredictably may appear to be crucial for survival, a recent study in Proceedings of the National Academy of Sciences has shown that predators may be able to overcome the unpredictability of their prey and adapt to outwit them.
Although it is well established that unpredictable behaviour is common among a variety of species, only few studies have been able to recreate wildlife responses in a laboratory setting. Knowing this motivated Andrew Szopa-Comley and Christos Ioannou from the University of Bristol, UK, to find out how the predatory bright blue acara cichlid fish (Andinoacara pulcher) pursue a Bluetooth-controlled robot prey determine how they reacted to the robot victim's escape attempts. First, the researchers trained the elegant blue fish to chase the roboprey in a tank. Then, once the cichlids were happy to pursue the robot, the researchers divided them into two groups: one in which the roboprey always fled along the same departure angle, selected depending on the initial direction the roboprey fled during the fish's first successful trial; and a second group where the roboprey's initial escape angle varied from trial to trial. After recording a total of 117 trials over a 3-week period, filming the fish to analyse their responses, Szopa-Comley and Ioannou determined the time the predators took to reach the roboprey, the predators’ behaviour during the approach phase and their behaviour while pursuing the fleeing victim.
Despite the roboprey's sometimes unpredictable behaviour, Szopa-Comley and Ioannou determined that the blue cichlid predator took the same amount of time to capture its target, regardless of the roboprey's escape strategy. When the roboprey consistently took the same departure angle, the fastest fish always pursued the roboprey as it fled from the fish along its forward line of sight, which is the best direction for getting furthest from the attacking predator. Conversely, when the fish were presented with a robot that darted off in various directions, they had no preferred angle at which they hit a top pursuit speed. Instead, the fish that never had the chance to learn in which direction the roboprey would take off accelerated the hardest and hit their fastest speeds when they were beside the roboprey. In addition, the attacking fish slowed as the victim moved toward them. No matter how unpredictable the roboprey's evasive strategy, the predatory fish were always able to outmanoeuvre it.
Blue acara cichlid fish are intelligent enough to adapt to any unpredictable behaviour that their prey may have, although they pursue at slower speeds than fish chasing prey that always escaped at the same angle. However, acceleration rather than approach speed allows the predatory blue fish to capture their prey. This means that small guppies might be unable to evade their big bright blue nemeses. Who knows, maybe blue acara fish are the guppy's equivalent of our great white shark under-the-sea nightmare.
