Scales let sea snakes feel movement in water

As sea snakes moved from land to their new underwater habitat around 10 million years ago, they developed a scaly organ on their heads which lets them “see” underwater, new research suggests.

A team of researchers from Australia and South Africa used microscope images and genomic testing to trace sea snakes’ evolution. Their findings, published in Open Biology, suggest the organ may have developed from a touch-sensor on land-dwelling snakes to a subaquatic vibration sensor when snakes ventured into the sea.

The sensors, known as scale sensilla, are sensitive organs that protrude from scales on a snake’s head. In fully aquatic snakes, which diverged from land-dwelling snakes between nine and 20 million years ago, these head-organs facilitate awareness of water movements, but the extent of their awareness isn’t well understood.

It turns out that fully aquatic snakes have larger sensilla than their land-dwelling and semi-aquatic cousins. And snakes that hang out in water have more dome-shaped sensilla, “potentially making them more likely to be able to sense vibrations from all directions”, explains lead author Jenna Crowe-Riddell from the University of Adelaide in Australia.

“We also found that scale sensilla on some of the fully aquatic snakes covered a much higher proportion of the scales’ surface.”

Crowe-Riddell and her colleagues combined DNA sequencing with microscopic images of snakes. To identify the shape and size of the sensory organs, the team used software specifically designed to detect the sensilla from silicone casts of snakes’ heads.

Underwater, even the slightest movement creates a vibration, which led the researchers to believe these sensilla contributed significantly to the snakes’ awareness of their surroundings.

Next, says co-author Kate Sanders, also from the Unversity of Adelaide, the team must investigate exactly what they can sense.

“If they are hydrodynamic tactile sense organs, as we suspect, then by comparing them to the scale sensilla of closely related land-snakes we can start to understand how evolution has changed these organs from direct-touch sensors to distance vibration sensors that work underwater.”

This knowledge could help illustrate how sea snakes exist in their underwater environment, and contribute to a broader understanding of reptile biology.