There are few things as important to a growing zebrafish (Danio rerio) as finding food, and as part of their normal foraging behaviour they show spontaneous turns that help them find nourishment in the water column. Given that turning is a vital zebrafish behaviour, Nicole Danos and George Lauder from Harvard University wondered how it changes as the fish grow(p. 3374). To find out, the team analysed the kinematics of turning fish, measuring 0.38–1.97 cm from snout tip to the centre of their tail forks, by filming them at 1000 frames per second. They used a technique called digital particle image velocimetry, a tool normally used for quantifying fluid motions, to track and measure the motions of the body, such as how fast the fishes moved their bodies and fins during turning.
The team found that smaller fish turned more frequently and beat their tails many times out of a turn, while larger ones tended to have one strong tail beat out of their less frequent turns before coasting to a halt. They also found that the different components involved in turning changed in different ways as fish grow. In fish up to 1 cm long, the angle of the turn and the angular velocity got smaller, however when the fish were more than 1 cm long, these measurements became bigger with body size. This transition coincides with major morphological changes to the muscles and fins. It implies that in order for the fish to develop properly, ensuring that they can maintain turning performance as adults, some aspects of turning get worse before they get better as the fish develop.
The velocity of the pectoral and caudal fins and the time taken to turn were all larger in bigger fish, probably because the muscles in larger fish are better developed. However, how much the body curved during turning got smaller as fish increased in size, probably because the skeleton is slightly stiffer in bigger fish. The team conclude that it is the development of a zebrafish's fins and skeleton that are important in maintaining its turning performance as it grows up.