Morphological Evidence for a Directional Flow Mechanoreceptor in Olive-Headed Sea Snakes (Hydrophis major)

The sense of touch (mechanoreception) in snakes is not widely appreciated despite emerging evidence of tactile specialisation among sea snakes. This is partly due to the challenges in quantifying small (< 1 mm) and numerous scale mechanoreceptors concentrated on the head. By using a novel application of gel-based 3D profilometry (GelSight scanner) in combination with histology and scanning electron microscopy, we comprehensively quantified the morphology and distribution of scale mechanoreceptors in the olive-headed sea snake, Hydrophis major (Hydrophiinae), for the first time. H. major is one of the few predators to eat eel-tailed catfishes (Plotosidae), which have venomous spines that they lock into erect positions during defence. We discovered that in addition to the radially symmetrical smooth, dome-shaped mechanoreceptors typically found in sea snakes, H. major has asymmetrical, peak-shaped mechanoreceptors that are significantly larger but rarer. Smooth domes are distributed in decreasing density antero-posteriorly on the head with the highest densities on the snout and labial scales. Asymmetrical peaks are rarer; they are detected only on the dorsal and lateral sides of the head, are most dense behind the eye, and their associated dermal papilla (that contains mechanosensitive cells) is spatially offset from the stiff peak. Based on their morphology and distribution, we suggest functional differences in mechanosensory modalities: (1) smooth domes for direct touch used in prey handling to avoid dangerous spines of catfish prey, and (2) asymmetrical peaks that create a lever system capable of amplifying directional water flow. The latter might allow H. major to detect the C-start escape response of free-swimming catfish and/or enhance kinaesthesia for the snake's perception of self-motion during foraging and predatory strikes, but physiological studies are needed to investigate these functional hypotheses further.