The tuna keel is a mechanosensory structure.

Tunas are high-performance pelagic fishes of considerable economic importance and have a suite of biological adaptations for high-speed locomotion. In contrast to our understanding of tuna body and muscle function, mechanosensory systems of tuna are poorly understood. Here we present the discovery of a remarkable sensory lateral line canal within the bilateral tuna keels with tubules that extend to the upper and lower keel surfaces. Neuromast mechanoreceptor organs are found periodically along the canal lumen, enclosed within tubular ossifications surrounding the canal that we interpret as modified lateral line scales. In addition, a series of segmental, elongated skeletal elements of unknown homology support the posterior end of the keel. These observations suggest that the bilateral tuna keels act as flow sensing structures, perhaps providing information on tail beat frequency, amplitude, force, and water flow dynamics over the caudal region of the tuna body axis during locomotion.