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Book Review

Skin that Sees: Evolution and Mechanism of Phototaxis in Sea Snake Tails

Researchers have discovered light sensing abilities in the tails of sea snakes. This unique adaptation in one genus of snakes may allow them to sense and respond to impending danger.

Crowe-Riddell, J. M., Simões, B. F., Partridge, J. C., Hunt, D. M., Delean, S., Schwerdt, J. G., … Sanders, K. L. (2019). Phototactic tails: Evolution and molecular basis of a novel sensory trait in sea snakes. Molecular Ecology. doi:10.1111/mec.15022

 

Imagine having eyes on your feet. This is essentially the body plan of some sea snakes. With a paddle-shaped tail for locomotion, the olive sea snake (Figure 1) goes to extremes to protect it. The skin in this snake’s tail region is embedded with unique light sensors, allowing it to respond to impending danger. Crowe-Riddel et al. have discovered other sea snakes that have also evolved this phototaxis, or response to light, and begin to understand the evolution of and underlying mechanisms of the trait.

Figure 1. Olive sea snake, Source: Wikimedia Commons

In a marine environment, we find many invertebrates (organisms without a backbone) that use non-visual light sensors. Octopuses, for example, use chromatophores to sense light and respond by changing color, or camouflaging. There are not many marine vertebrates (animals with a backbone), however, that have the ability to sense light in this way. In 1990, Zimmerman and Heatwole reported that the olive sea snake senses light with its skin. It is believed that this ability protects the vulnerable tail from predators during the snake’s resting period. Crowe-Riddel’s team wanted to know whether there were other sea snakes that shared this light sensing ability.

 

Where did Phototactic Tails Originate?

If other sea snakes could sense light with their tails, scientists would be able to figure out where the trait arose, evolutionarily. Crowe-Riddel et al. therefore began testing various sea snakes for this phototactic tail. The team collected 17 live snakes from 8 species and began testing for phototaxis. Equipped with colored flashlights, the scientists shined light on the paddle-like tails of the specimens, recording the responses.

The sea snakes’ reactions to the light varied by species. Some snakes responded to certain colors of light by retracting their tails, as if escaping a predator. Others exhibited no response to the light stimulation. Because Crowe-Riddel et al. noticed that only sea snakes of the Aipysurus genus had phototactic tails, they suggested that the trait evolved from a common ancestor of Aipysurus. Since only 10% of known sea snakes are from genus Aipysurus, this photosensitive tail may not be a very common trait among the serpents (Figure 2).

 

Figure 2. Aipysurus sea snake taxonomy, Source: Constance Sartor

 

Skin that “Sees”

Although we know that the tails of sea snakes respond to light, we do not understand how this occurs. Since snakes do not have eyes or visible photoreceptors, their light-sensing ability most likely lies within their dermis, or skin.

To take a closer look at the skin of sea snakes, Crowe-Riddel et al. collected skin tissue samples from the tails of the specimens that responded to light. Using a genetic method called transcriptomics, the team was able to take a “snapshot” of the genes that were expressed in the snakes’ tails during a response to light.

After genetic analysis, the team found two genes that were likely associated with the sea snakes’ phototactic tails. The first was xenopus-like melanopsin. The other was neuropsin, a gene found in tadpoles.

 

Injured Tails

Most organisms cannot sense light without eyes. So, what is the advantage of a light-sensing tail? Perhaps the trait protects them from predators that could injure their tails. Crowe-Riddel decided to take a look at 111 museum specimens to see if the species of snakes with tail injuries was correlated with the species of snakes who could not sense light. Essentially, they wanted to see if the phototaxis served as a defense mechanism from predators who injured their tails.

After comparing light sensing to tail injuries of the specimens, Crowe-Riddel et al. found no correlation between the two. Perhaps the light sensing tails had not evolved to keep the snakes free from harm.

With only 10% of sea snakes known to possess phototactic tails, there is still much to learn about this unique sense. Crowe-Riddel’s team unraveled some of the evolution and genetic basis of the trait, but we still do not understand what the Aipysurus sea snakes use their light sensing tails for. With this new information on the genes involved in phototaxis, we can begin to test other individuals to see if they too can sense light with their skin. Perhaps there are more marine vertebrates with “skin that sees”.

 

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