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A tentacled snake (Erpeton tentaculatum) under the scanning electron microscope revealing the unusual pair of scaled appendages on the snout. Tentacled snakes are fully aquatic and prey almost exclusively on fish. In this issue Catania and colleagues investigate the function of the tentacles and find they are sensitive mechanoreceptors that respond to water movements. Responses from the tentacles project to the optic tectum, suggesting mechanosensory information about water movements is integrated with vision when the snakes hunt.
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Summary
We investigated the function of the tentacles in aquatic, piscivorous tentacled snakes (Erpeton tentaculatus) by examining anatomy, peripheral innervation, and the response properties of primary afferents. We also investigated visual and somatosensory responses in the optic tectum and documented predatory strikes to visual stimuli and under infrared illumination. Our results show the tentacles are sensitive mechanoreceptors that respond to water movements. They are innervated by rami of the maxillary and ophthalmic branches of the trigeminal nerve and contain a dense array of fine terminal neurites that cross the interior of the tentacle orthogonal to its long axis. The optic tectum contained a retinotopic map of contralateral receptive fields with superior fields represented dorsally in the tectum, inferior fields represented laterally, nasal fields represented rostrally, and temporal fields represented caudally. Large somatosensory receptive fields were identified in deeper layers of the tectum and were in approximate register with overlying visual fields. Tentacled snakes struck accurately at a simulated digital fish, indicating that visual cues are sufficient to guide strikes, but they also captured fish under infrared illumination, suggesting water movements alone could be used to localize prey. We conclude the tentacles are mechanosensors that are used to detect fish position based on water movements and that visual and mechanosensory cues may be integrated in the tectum to enhance localization when visual cues are reduced.
Keywords: optic tectum, brain, mechanosensory, behavior, touch vision
Function of the appendages in tentacled snakes (Erpeton tentaculatus)
