Sea snakes spend much of their lives living in the world’s oceans, occasionally popping up to the surface to breathe. With their paddle-like tail to aid their aquatic locomotion, they are certainly suited to their mostly marine environment. However, very little is known both about their vision and their ability to sense their prey and surroundings in water, which is almost always murky to some extent.
Now, a new study published in the journal Open Biology has shed some light on this sensory mystery. Researchers have discovered that small, elevated structures on the scales on their heads – their so-called scale sensilla – may be far more sensitive to movement than their terrestrial counterparts. Whereas lizards and land snakes use their scale sensilla to detect objects using direct touch, sea snakes may use theirs, which are far more dome-shaped and protrusive, to pick up on movement at a distance.
“We believe sea snakes use these organs to sense objects at a distance by 'feeling' movements in the water,” lead author Jenna Crowe-Riddell, a Ph.D. student at the University of Adelaide, said in a statement. “This hydrodynamic sense is not an option for land animals. In water, a new way of sensing the environment becomes possible.”
After closely examining sea snakes that exclusively inhabit marine environments, the team found that their head scales contained a far higher proportion of scale sensilla compared to sea snakes that can temporarily inhabit the land. This suggests that the most hydrophilic snake species have the most finely-tuned vibrational detection sense.
The head of a beaked sea snake, featuring a zoomed-in image of a single scale on its head, which contains small raised structures called scale sensilla. Jenna Crowe-Riddell
Sea snakes are evolutionary descendants of land snakes, splitting off from their terrestrial ancestors between 9 and 20 million years ago. They vary from being partly land-based – like the sea kraits, which feed off prey and lay eggs on the land before retreating into the water – to being fully aquatic – like the yellow-bellied sea snake, which spends its entire life in tropical oceanic waters.
Very little is known about their evolution and, in particular, about their sensory mechanisms. Land snakes have remarkable vision, chemoreception (the ability to detect chemical changes in the environment) and hearing, but all these senses are highly distorted in water. Still, it’s clear that sea snakes of all kinds are frighteningly efficient hunters of fish, so zoologists naturally have been wondering how they manage it.
For this study, the international team of researchers looked at 19 species of fully-aquatic, partly-aquatic, and terrestrial snake, measuring and assessing the quantity and form of their scale sensilla with the aid of powerful microscopes. It became rapidly clear that those sensilla on the more aquatic snakes were higher in number and differently shaped to their land-based equivalents.
Considering that land snakes use their sensilla to directly feel the world around them, they suggested that the sea snakes' sensilla, which branch out into the water around them, are used to detect tiny vibrations in the aquatic environment. This could be how they hunt their prey, by tracking precise movements based on the internal turbulence of the water.
The adaptations of sea snakes. BBC via YouTube
Admittedly, the team have yet to find any direct evidence of this scale sensilla sense in sea snakes, but the fact that they’ve suggested it is a start.
“What we now need to do is to investigate the physiology of these scale sensilla and demonstrate exactly what they can sense,” Kate Sanders, an ARC Future Fellow at the University of Adelaide and co-author of the study, added in the statement. “If they are hydrodynamic tactile sense organs, as we suspect, then… we can start to understand how evolution has changed these organs from direct-touch sensors to distance vibration-sensors that work underwater.”