The precise control that tuna have of their fins for tight turns and movement while swimming is aided by hydraulic activity of the lymphatic system, a new study reveals. Furthermore, the authors found that this specialization of the lymphatic system is associated with other fishes in the family Scombridae, suggesting that it may have evolved in response to the demand for the sophisticated maneuvering control in these high performance species. While dissecting tuna fins, Vadim Pavlov and colleagues found a chamber-like compartment, or large vascular sinus (VS), located at the base of both the second dorsal and anal fins. When they pumped fluid into the chamber, this provided finely controlled adjustment of the fin. Close video monitoring of tuna as they swam revealed that the degree of fin erection increases when tuna are engaging in behaviors that require frequent changes in movement direction, such as searching and feeding, compared to when the fish are simply cruising. Next, the researchers injected special fluid into the VS of tuna to trace fluid dynamics throughout the system. The injection was present only in a sub-section of vessels with vein-like morphology, a characteristic of the lymphatic system, which helps distribute immune cells. Analyzing fluid naturally found in the VS of tuna revealed a high portion of lymphatic cells relative to that found in blood, further suggesting that the VS is a part of the lymphatic system. Pavlov et al. also found this finely tuned VS complex in a number of other scombrids, suggesting that the VS evolved in these fish to meet their high-precision swimming needs. These results are highlighted in a Perspective by Michael S. Triantafyllou.