#elapidae

Important background: Most animals run on a day-to-day, or 'diel' schedule called a 'circadian rhythm', which dictates when they're asleep and when they're awake. The most common triggers for this rhythm are light and darkness, and temperature.

The yellow-bellied sea snake Hydrophis platurus spend most of their active time diving, but will also come up to the surface for important activities like breathing, drinking, and resting. Most research suggests that this species is active during the day, but there is some evidence that a subspecies, Hydrophis platurus xanthos, which is smaller and completely yellow in color, is more active at night.

Question/hypothesis: Is H. p. xanthos truly nocturnal?

Methods: In March and April of 2021, the researchers took a boat and searched the waters of inner basin of Golfo Dulce, Costa Rica, where H. p xanthos occurs. They kept to a set straight-line path, called a transect line, which stretched for 6 km (3.7 mi); there were five of these paths in total, which were searched for 24 hours each. Every time a snake was sighting, the researchers noted the time of day and what the snake was doing. They also took GPs coordinates of where the snake was.

In addition to the sightings collecting during this survey, the researchers also analysed data collected from other studies of H. p. xanthos collected between 2010 and 2021.

The researchers then took the data and tested whether sightings were mostly in the day, mostly in the night, or if they occurred randomly without regard for the time; this last assumption is called the 'null hypothesis'. They tested the significance of their results by comparing their results by creating a randomly generated dataset of 1000 sea-snake sightings, and determining the day/night distribution of that to determine the p-value. The p-value is a number that indicates the likelihood that the results of a test or model are random; the lower the number, the more 'significant' the results, aka the less likely they were due to random chance.

Results: Over the course of their observations, the researchers logged 358 sea snake sightings. Only 16% of those sightings occurred during the day. The p-value for that number was p < .001, which means there's a very good chance that this number was not due to random chance.

This figure shows all the different behaviors recorded, and the time the observation was made:

Each number corresponds to a behavior: A (diving or surfacing), FS (feeding sinusoidal), K (knotting), N (swimming), RO (resting open), and RS (resting serpentine). The percentages of total recorded behaviors by period are shown inside moons (night) and suns (day), and scaled for quick reference (larger icons for higher percentages).

The type of circadian rhythm H. p. xanthos exhibits is called a multimodal crepuscular circadian rhythm; they are active at dawn and at dusk. During this time, they are more likely to be hunting feeding.

So what: This study shows that this subspecies of yellow-bellied sea snake is primarily crepuscular. However, it remains unclear why. One reason may be due to the temperature. This study occurred at the beginning of the rainy season, and temperatures were roughly the same during the day and the night. As a reptile, sea snakes are reliant on their environment to maintain a constant internal body temperature (aka homeostasis). Other studies have suggested that H. p. xanthos became smaller and all-yellow to cool itself off more quickly; being crepuscular may also help them to maintain homeostasis. Another reason why H. p. xanthos might have become crespucular is to protect their venom from ultraviolet radiation from the sun, or to avoid predators.

Most other species of sea snake are active primarily during the day or are active randomly without regard to day or night. It's therefore important to study why H. p. xanthos has diverged from other species, and what implications that may have for its future evolution. It's also important to examine the behavior of sea snakes as many species are either vastly understudied or endangered; one potential threat may be the rising temperature of their marine habitat, which could be causing changes in their diel behavior.