The sense of touch underwater
Our sense of touch (e.g. the texture and shape of objects) is conveyed by ‘mechanoreceptors’ in the skin. For land animals like humans, we need to come into direct contact with something to touch it. In the water, however, water currents are deflected off on objects, or generated by animals, allowing many aquatic animals to sense things underwater by the ‘indirect’ motion of water. A well-studied example of ‘hydrodyanmic reception’ is harbour seals, which use their whiskers to track the hydrodynamic trials generated by a fish swimming, even while blind-folded!
Marine snakes are thought to also be able to detect water motion (i.e. hydrodynamic reception) and pressure changes (i.e. hydrostatic baroreception) using purported scale mechanoreceptors known as ‘sensilla’, but these hypotheses had not been explicitly tested.
To better understand how scale sensilla have evolved in sea snakes, we quantified sensilla traits using morphological techniques (quadrate sampling, scanning electron microscopy) that accounted for the effects of allometry (head size) and phylogeny (shared descent), respectively. We found that there is substantial variation in the overall coverage of sensilla (0.8 to 6.5%) among aquatic and terrestrial lineages of elapid snakes. However, only the aquatic taxa sampled had protruding, dome-shaped sensilla and exceptionally higher overall coverage was found in only a few of the fully-aquatic species of sea snakes. These results suggest that changes in sensilla traits do not coincide with the transition to aquatic habitats in sea kraits or sea snakes, but rather that sensilla traits might be correlated with specific ecologies of a few species of sea snakes, and indeed with the new ‘co-opted’ ability to sense water motion.
Even though we found that these organs are more protruding and (in some cases) larger in sea snakes, it was still uncertain whether these scale organs are even sensory! So we described the ultrastructure of scale sensilla in two species of sea snakes and found that they are indeed homologous with the scale sensilla on land snakes. This shows that these are clearly mechanoreceptors and perhaps the larger size/higher coverage of these organs in some sea snakes confer greater sensitivity in the marine environment.
Crowe-Riddell, JM, Snelling, EP, Watson, A, Kyuseop Suh, A, Partridge, JC, Sanders, KL (2016) ‘The evolution of scale sensilla in the transition from land to sea in elapid snakes’, Royal Society Open Biology, 6, 160054. doi: 10.1098/rsob.160054.
Crowe-Riddell, JM, Williams, R, Chapuis, L, Sanders, KL. (2019) ‘Ultrastructural evidence of a mechanosensory function of scale organs (sensilla) in sea snakes (Hydrophiinae)’, Royal Society Open Science, 6. doi: 10.1098/rsos.182022.