While spitting is taboo in some human cultures, a few species have honed it to a fine art, from cobras that eject venom into the eyes of their victims to archer fish that down their prey with a precisely aimed jet of water. In addition, archer fish have implemented another weapon in their hunting arsenal – they jump out of the water to snap up prey – and Alexandra Techet, from the Massachusetts Institute of Technology, USA, explains that this alternative method of attack is more likely to be successful than spitting when other hungry archer fish are gathered round. ‘It's a competitive world out there. When they spit there is no guarantee that they'll actually get to eat the prey, but when they jump it's almost guaranteed that they will’, says Techet. With a passion for designing aquatic robots, Techet turned to the agile archer fish for inspiration.
‘We trained them by hanging bait above the tank’, says Techet, who removed the bait whenever the fish tried to spit. Once the fish were vaulting reliably, Techet and Anna Shih filmed the animals with a high-speed camera as they attempted to reach Gammarus shrimp suspended as high as 2.5 times the fish's body length above the surface of the water. Analysing the fish's tactics during the countdown, Leah Mendelson observed that once the fish had spotted the tasty treat, they hovered below it weaving their pectoral fins to and fro until ready for the launch. ‘This part of the process is the same as when they hunt by spitting’, says Techet. But as soon as the take-off was initiated, the fish began beating their tails hard from side to side while extending their pectoral fins as they surged upward and broke through the surface. Sometimes the tail continued flapping to and fro even when they were sailing through the air. Correlating the number of propulsive tail beats with the height reached, Techet was surprised to see that instead of flapping harder, the fish increased the number of tail beats to reach the highest bait, landing an impressive 70% of their catches.
In addition to determining the fish's launch technique, Techet and Shih visualised the swirling patterns generated in the water as the fish powered upward to learn more about the forces that propel the fish into the air. Although convincing the fish to jump in a plane of laser light while filming was technically challenging, Mendelson eventually discovered that the fish used their tails in combination with the anal, pectoral and dorsal fins to generate enough thrust to become airborne. And when she calculated the energetic cost of a fish launch and compared it with the amount of energy consumed during the frantic dash to capture prey after a successful squirt, the two came out about equal – ranging from 2.5 to 47 mJ – suggesting that the jump strategy may be as efficient as pursuing a victim downed by a well-aimed jet of water.
Having revealed how the fish jump out of the water from a stationary position, Techet and Mendelson are now keen to learn more about the contribution of the tail and other fins to the lift-off, and Techet adds, ‘This work serves as the foundation for our ultimate goal’, which is to produce a 3D model of the physics of a launch to design archer-fish-inspired robots that can take off smoothly from water.