Coral seas are very crowded in comparison to the open ocean. Many reef fish manoeuvre slowly around the coral by holding their bodies rigid, and flapping their median or paired fins (MPF swimming). Fish that cruise around the oceans rarely need to make tight turns at low speeds, they tend to swim for long periods by flexing the body and tail fin (BCF swimming). Some coral fish are so well adapted to their cluttered environment that they can only power BCF swimming for short bursts. But looks can be deceptive! John Steffensen's team from Helsing[UNK]r, has proved that one coral reef specialist, the Piccaso triggerfish, is equally capable of both styles(p. 1253).
Terrestrial animals change gait to reduce energy costs as they move faster. But no one had ever compared the cost of slow gait MPF swimming with faster BCF swimming to see if fish used the same energetic trade-off to speed up. In fact, it wasn't even clear if an MPF specialist could even switch to the high-speed gait for sustained periods of time. Most metabolic measurements had been made on sleek BCF swimmers. Steffensen decided that he needed to measure the metabolic rate of some MPF specialists to settle the matter, but the shores of Denmark are not covered with tropical coral reefs teeming with MPF swimmers.
Steffensen, Keith Korsmeyer and Jannik Herskin relocated their lab to tropical Okinawa for a month. Once they'd reassembled the swimming respirometer in Japan, they collected triggerfish and parrotfish from the reefs, ready to measure the fish's metabolic rates. The team gave the fish 12 hours to adjust to swimming in the confines of the respirometer before testing the fish's swimming performance. As Korsmeyer increased the water speed every 30 minutes, the fish speeded up to match the increasing waterflow. They filmed the fish's movements, watching for changes in the fish's swimming style, while measuring their metabolic rate as they swam faster.
At slow speeds both species used their median or paired fins exclusively,but as the water flowed faster the two species opted for different strategies.
The parrotfish kept on determinedly flapping with their pectoral fins until they were finally forced to switch to BCF swimming for a short burst, before they became exhausted. The triggerfish, on the other hand, began alternating their MPF gait with a few flicks of the tail at relatively low speeds. The triggerfish easily speeded up, and began swimming strongly using a BCF stroke that they used to top up their median fin thrust. Most of the triggerfish cruised happily at 75 cm s-1, which is four body lengths per second. One remarkable fish swam vigorously at 90 cm s-1 for half an hour!
The other surprise came when Korsmeyer looked at the triggerfish's metabolic rate as they switched to the high speed gait. Instead of dropping,the fish's cost of transport rose.
At first sight this is confusing because terrestrial animals switch gait to save energy, not use more. But fish have to work harder than animals that move through air to push themselves through the viscous liquid. Korsmeyer suspects that the cost of transport goes up for the triggerfish because they top up median fin propulsion by recruiting an extra set of powerful high endurance muscles to power the undulating BCF gait.