Unique hydraulic system allows tuna to 'fly' through sea like fast jets - study
Giant bluefin tuna possess a unique hydraulic control system that allows them to "fly" through the ocean like fighter jets, scientists have discovered.
Lymphatic fluid pumped through internal channels causes their large sickle-shaped fins subtly to change shape, aiding manoeuvrability at high speed.
In just the same way, fast jets rely on fine adjustments to their control surfaces to remain stable in the air.
Experts learned about the lymph-driven hydraulics after studying Pacific bluefin and yellowfin tuna swimming in tanks.
Dr Vadim Pavlov, from Stanford University in the US, said: "Animals are exciting sources of elegant engineering solutions in aero and hydrodynamics.”
"What we have discovered in these tunas is unlike other animal hydraulic systems. It's a musculo-vascular complex that is integrating the lymphatic system, the skeletal muscles and fin bones.”
"We've shown that in tunas and their fast-swimming relatives this complex functions to generate hydraulic pressure that provides fine adjustment of the shape of their fins.”
"By expanding or retracting their dorsal and anal fins, they alter the physical forces generated by fins, allowing for manoeuvrability."
The tuna's ability to adjust their fins quickly and precisely using the hydrodynamic mechanism may help them both to capture prey and conserve energy during long distance swims, said researchers writing in the journal Science.
The study built on observations more than a decade ago of Pacific bluefin tuna at Monterey Bay Aquarium's million-gallon Open Sea exhibit in California.
Scientists noticed that the fish, some weighing more than 300 pounds, were making fine adjustments to their pectoral, median and tail fins.
Further research led to the discovery of an unusual cavity filled with liquid beneath the base of both the dorsal and anal median fins, the two unpaired fins extending above and below a fish's body.
Muscles pressurising the liquid altered the shape of the fins to provide control while swimming, the team realised.
The link with the lymphatic system was only confirmed later. Detailed examination revealed a specialised system of large vessels and channels in the median fins through which lymph acted as hydraulic fluid.
Increased pressure in the channels appeared to alter the fins' position and stiffness to prevent the fish from rolling and yawing during active swimming. It also limited energy loss during long swimming migrations.
The primary function of lymph is to transport infection-fighting white blood cells and maintain fluid balance in the body. Its use as a hydraulic fluid has never been reported before.
Co-author Professor Barbara Block, also from Stanford University, who led the earlier research, said: "The primary examples of bio-hydraulics are in invertebrate animals like molluscs, crustaceans and jellyfish.”
"It's unusual to observe bio-hydraulic locomotion in vertebrate animals, which involves the integration of muscle, fluid and bone structures.”
"To our knowledge, this evolutionary mechanism of fishes has never before been reported and might have remained hidden if it weren't for the ability to see these fish in action in captivity.
"It illuminates how non-transparent our ocean realm is and how much is left to discover."
The tuna discovery could help in the design of "smart" shape-changing control surfaces for planes and submersibles, researchers believe.
Dr Pavlov said: "This could, for example, enhance the manoeuvrability of the air and underwater unmanned vehicles."