Thursday, June 9, 2016

[Herpetology • 2016] The Evolution of Scale Sensilla in the Transition from Land to Sea in Elapid Snakes

Figure 2. High-depth-of-field photographs of the heads of six elapid species:
(a) Hydrophis schistosus, (b) Hydrophis platurus, (c) Aipysurus duboisii, (d) Emydocephalus annulatus, (e) Hydrelaps darwiniensis and (f) Pseudonaja textilis. Species are representative of (a–d) fully aquatic, (e) semi-aquatic and (f) terrestrial ecologies. Insets show sensilla within the postocular scale(s).
 Scale bar, 3 mm.  DOI: 10.1098/rsob.160054


Scale sensilla are small tactile mechanosensory organs located on the head scales of many squamate reptiles (lizards and snakes). In sea snakes and sea kraits (Elapidae: Hydrophiinae), these scale organs are presumptive scale sensilla that purportedly function as both tactile mechanoreceptors and potentially as hydrodynamic receptors capable of sensing the displacement of water. We combined scanning electron microscopy, silicone casting of the skin and quadrate sampling with a phylogenetic analysis to assess morphological variation in sensilla on the postocular head scale(s) across four terrestrial, 13 fully aquatic and two semi-aquatic species of elapids. Substantial variation exists in the overall coverage of sensilla (0.8–6.5%) among the species sampled and is broadly overlapping in aquatic and terrestrial lineages. However, two observations suggest a divergent, possibly hydrodynamic sensory role of sensilla in sea snake and sea krait species. First, scale sensilla are more protruding (dome-shaped) in aquatic species than in their terrestrial counterparts. Second, exceptionally high overall coverage of sensilla is found only in the fully aquatic sea snakes, and this attribute appears to have evolved multiple times within this group. Our quantification of coverage as a proxy for relative ‘sensitivity’ represents the first analysis of the evolution of sensilla in the transition from terrestrial to marine habitats. However, evidence from physiological and behavioural studies is needed to confirm the functional role of scale sensilla in sea snakes and sea kraits.

KEYWORDS: sea snake; sensilla; mechanoreceptor; hydrodynamic; sensory; elapid


Our study devised a novel approach to quantify the traits of scale sensilla, which enabled meaningful comparison across a broad sample of elapid snakes. In particular, our estimates of overall coverage of sensilla provided a proxy for putative mechanoreceptor sensitivity and allowed the first analysis of sensilla evolution in the transition from terrestrial to marine habits in snakes. Our results indicate multiple increases in overall coverage of sensilla within the fully aquatic sea snakes, in addition to a more dome-shaped sensillum ultrastructure in fully aquatic and semi-aquatic lineages compared with terrestrial lineages. These findings are consistent with a derived, possibly hydrodynamic, sensory role for scale sensilla in sea snakes and sea kraits, but rigorous testing of this hypothesis will ultimately require behavioural and physiological studies. The novel methodological approach presented here is easily transferable to other reptilian lineages that have undergone adaptive shifts.

Jenna M. Crowe-Riddell, Edward P. Snelling, Amy P. Watson, Anton Kyuseop Suh, Julian C. Partridge and Kate L. Sanders. 2016. The Evolution of Scale Sensilla in the Transition from Land to Sea in Elapid Snakes. [Royal Society] Open Biology. DOI: 10.1098/rsob.160054

Sea snakes can sense objects at a distance by feeling movements in the water via @physorg_com