Morphology and distribution of a new scale mechanoreceptor type in olive-headed sea snakes ( Hydrophis major )

Snakes are known for their superb sensory specialisations but are not widely appreciated for their sense of touch despite emerging evidence of tactile specialisation among sea snakes. This is partly due to the challenges in quantifying such small and numerous scale mechanoreceptors or ‘scale sensilla’ across individuals. By using a novel application of gel-based 3D profilometry (GelSight scanner) in combination with scanning electron-and light microscopy, we comprehensively quantified the morphology and distribution of scale mechanoreceptors in a sea snake, Hydrophis major (Hydrophiinae), for the first time. We discovered a new type of scale mechanoreceptor distinguished by its larger size (43.65 ± 22.57 μm height) and asymmetrical peak shape, created by a thickening in the cornified outer layers of the epidermis. Asymmetrical peaks contain a dermal papilla with central cells, indicative of meissner-like corpuscles that underlie smooth dome-shaped mechanoreceptors (31.88 ± 19.03 μm height) typically found in both sea snakes, but is positioned slightly off centre from the tallest point of the asymmetrical peak. Smooth domes are concentrated anterior-posteriorly on the head with the highest densities on the rostrum and nasal scales (9.83 ± 1.88 and 5.24 ± 2.85 per mm², respectively). Asymmetrical peaks are rarer; detected only on the dorsal and lateral sides of the head and most common on the postocular scale (mean density 1.26 ± 0.72 per mm²). We suggest functional differences in mechanosensory capabilities: asymmetrical peaks may serve primarily proprioceptive purposes whereas smooth domes are used for hydrodynamic reception.