Warm bodied fish, including species of shark and tuna, can swim more than twice as fast as other colder bodied species, according to new research. The ability of these fish to maintain a body temperature warmer than the surrounding water (endothermy) allows them to swim at cruising speeds approximately 2.7 times faster than other similar sized cold-bodied species.

The study, led by Yuuki Watanabe of the National Institute of Polar Research, Japan, involved the team gathering and analysing data from previous studies as well as collecting their own data by attaching speed sensors to sharks in Alaska, the Bahamas and the central Pacific, to compare swim speeds of warm and cold-bodied fishes.

“The physiological mechanism of keeping heat in the body is well understood. But, a more fundamental question is, why this unique evolution occurred in the first place.” says Dr Watanabe. ” In other words, what kind of advantages do the fish gain from being warm bodied?”

Dr Yannis Papastamatiou of the University of the St Andrews Scottish Oceans Institute is one of the team of researchers who made the discovery.

“Fishes are generally considered cold-bodied in that their body temperatures are very similar to that of the water they reside in,” said Dr Papastamatious. “However, amazingly a small number of tunas and sharks, including white sharks, have evolved the ability to maintain their body temperatures higher than the surrounding water – sometimes up to 20°C warmer. We found that on average, warm bodied fishes can swim almost 2.7 times as fast as cold blooded species, likely because the warmer muscle temperatures enhances power output.”

The fish species that possess this warm bodied characteristic do so through regulating the temperature of their slow-twitch, aerobic red muscle (RM) to maintain a temperature warmer than that of the ambient water. RM endothermy is exhibited by lamnid sharks (which includes the mako and the great white) and tunas, all of which share many anatomical and physiological specialisations that endow them with their impressive swimming power and speed.

In contrast to other fish where the RM is near the skin, the RM of these sharks and tunas is located near the backbone, where an elevated temperature is maintained. This specialised anatomy allows the predators to swim fast and continuously, which in turn, allows heat to be retained in the core of the fish leading to local warm-bloodedness. In most other fish, the red muscle is located close to the skin, yielding a fully cold-blooded body and only short bursts of rapid, powerful swimming. Even though the ancestors of bony tuna and cartilaginous sharks diverged more than 400 million years ago, selection pressure for high-performance swimming in each group seems to have occurred independently about 50 million years ago.

The team also found evidence that these increased swim speeds allow the warm bodied fish to migrate large distances in a relatively short period of time. The estimated cost of transport of fishes with RM endothermy is twice that of fishes without it so the team suggest that the high energetic cost of RM endothermy in fishes is offset by the benefit of elevated cruising speeds, which not only increase prey encounter rates, but also enable larger-scale annual migrations and potentially greater access to seasonally available resources.

“As such these animals can swim to distant locations and back again which may allow them to take advantage of seasonal pulses in food or other resources,” says Dr Papastamatious.

“Our study provides a potential explanation for the evolution of endothermy in fishes.”

– JK

Article Reference:

Comparative analyses of animal-tracking data reveal ecological significance of endothermy in fishes: Yuuki Y. Watanabea, Kenneth J. Goldmanc , Jennifer E. Caselled, Demian D. Chapmane, and Yannis P. Papastamatious is published in the Proceedings of the National Academy of Sciences.

Photo Header Credit: The Great White shark – Carcharodon carcharias © Raul Touzon – National Geographic