Bluefin tuna can stretch 3 meters, weigh a metric ton, and reach speeds of 60 kilometers per hour. Yet they still turn on a dime when hunting, thanks in part to their lymphatic systems. Unlike humans, who use their lymphatic systems to produce and transport white blood cells, tuna use theirs to move two of their fins, researchers report today in Science. When scientists dissected the nimble fish, they discovered empty cavities at the base of their second dorsal and under-belly fins. These vascular sinuses are connected to a network of vessels that extend into the fin, between the bones that make up the fins’ rays. When the team pumped a saline solution into the vascular sinuses of recently deceased tuna, the solution flowed into the vascular channels and increased the fins’ internal pressure. This caused the fins to rise and stand erect from the body. The fish make such changes in their fin position when they make a series of sharp turns, especially during hunting, the team found. The researchers suspect that this fin movement offers extra stability during tuna’s tight maneuvers so the fish don’t have to sacrifice their speed for balance.