U.S. study discovers biological hydraulic system in tuna fins

Source: Xinhua| 2017-07-21 07:56:23|Editor: Song Lifang
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SAN FRANCISCO, July 20 (Xinhua) -- Researchers have found in a new study evidence of a biological hydraulic system in the large sickle-shaped fins centered above and below the giant blue fin tuna's body, called the median fins.

Cutting through the ocean like a jet through the sky, giant bluefin tuna are built for performance, endurance and speed. Just as the fastest planes have carefully positioned wings and tail flaps to ensure precision maneuverability and fuel economy, these powerful fish need the utmost control over their propulsive and stabilizing structures as they speed through the ocean.

"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," said Vadim Pavlov, a postdoctoral fellow at Stanford University and a lead author of the research, published in the July 21 issue of Science.

Over a decade ago, Barbara Block, a professor in marine sciences at Stanford, and colleagues from the Monterey Bay Aquarium in Northern California on the U.S. West Coast introduced Pacific bluefin tuna into the aquarium's million-gallon Open Sea exhibit.

"We were all mesmerized by watching the beauty of form and function of these majestic fish through the glass of the Monterey Bay Aquarium," Block, whose Tuna Research and Conservation Center (TRCC) partnership with the Monterey Bay Aquarium has been maintaining tuna in captivity at Stanford's Hopkins Marine Station for more than 20 years, was quoted as saying in a news release from Stanford.

During her observations of these fish, Block noticed that the Pacific bluefin tuna, which grew to be giants in the exhibit with some reaching over 300 pounds, or 136 kilograms, were making fine adjustments to their pectoral, median and tail fins. These traits became the focus of several Stanford undergraduate internship projects, which involved filming the Pacific bluefin tuna swimming and foraging at the aquarium's main exhibit.

However, it wasn't until the arrival of Pavlov at the Block lab that the mystery of the median fins was solved.

Pavlov, working with Stanford undergraduate Nate Hansen, identified an unusual sinus, or cavity, filled with liquid beneath the base of both the dorsal and anal median fins. The structure seemed enigmatic until they realized that this system of vascular channels, muscles and bones appears to be a biological analog of a canonical hydraulic system. The muscles pressurize the liquid, which helps change the fins' shape and position for swimming and maneuvering control.

"The finding was unexpected. Pavlov found this sinus area in the fin and associated structures and invited me to see if it was associated with the lymphatic system," explained Benyamin Rosental, a postdoctoral research fellow in stem cell biology and regenerative medicine and a co-lead author of the paper.

The researchers recorded videos of Pacific bluefin and yellowfin tuna swimming in the facilities at the TRCC. The footage allowed they to establish how the tuna changed the area and shape of these fins in order to execute different maneuvers. Paired with computer model simulations, the team also showed how fluid flowed across the tuna, impacting the forces generated by the fin at different swimming speeds.

To confirm that the hydraulic system was part of the tuna's lymphatic system, the team conducted detailed examinations of the pathways of the vasculature within the fins, studied the microscopic structure of the tissues and tested the cellular makeup of the fluid within this vasculature, which demonstrated it was lymph fluid. Lymphatic vessels are normally small and difficult to distinguish by the naked eye, but in tuna they are transformed into a specialized system of large vessels and channels in median fins. With lymph acting as hydraulic fluid, increased pressure in these channels affects the fin's position and, probably, the stiffness that together alters hydrodynamic properties of fins.

The capacity to rapidly adjust the fin positions affects the lift to drag forces on the fins and prevents the tuna from rolling and yawing during active swimming, limiting energy loss during long migrations.

"The primary examples of bio-hydraulics are in invertebrate animals like mollusks, crustaceans and jellyfish," Block noted. "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 nontransparent our ocean realm is and how much is left to discover."

"The natural mechanism of hydraulic control of fins could be very attractive in designing of new 'smart' control surfaces with changeable shape and stiffness," Pavlov said. "This could, for example, enhance the maneuverability of the air and underwater unmanned vehicles."