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How do animals evolve new traits? Sea robins are fish that possess specialized leg-like appendages used to "walk" along the sea floor. Here, we show that legs are bona fide sense organs that localize buried prey. Legs are covered in sensory papillae that receive dense innervation from touch-sensitive neurons, express non-canonical epithelial taste receptors, and mediate chemical sensitivity that drives predatory digging behavior. A combination of developmental analyses, crosses between species with and without papillae, and interspecies comparisons of sea robins from around the world demonstrate that papillae represent a key evolutionary innovation associated with behavioral niche expansion on the sea floor. These discoveries provide unique insight into how molecular-, cellular-, and tissue-scale adaptations integrate to produce novel organismic traits and behavior. [Display omitted] • Sea robins are walking fish that use novel leg-like appendages as sensory organs • A small clade of specialists elaborated sensory legs for locating buried prey • Sensation is mediated by mechanosensitive neurons and epithelial taste receptors • Leg papillae mediate sensory specialization across species and development Allard et al. exploit an unusual fish called the sea robin, which possesses six leg-like sensory appendages, to understand how novel traits arise and diversify to drive new behaviors. [ABSTRACT FROM AUTHOR]