Lack of oxygen curtails vision in red-eared sliders

Abstract

For many animals, winter is a time to go into hibernation and sleep until the weather gets warmer and food is more plentiful. But for some animals, hibernation requires a lot more than just sleeping through the frigid winter months. Some turtles, such as the red-eared slider (Trachemys scripta elegans), are condemned to overwinter in ponds that freeze over, meaning that they can't surface for air for weeks or months in temperatures hovering around 3°C. Even worse, the water these turtles settle down in often has very little oxygen. Because oxygen is usually required by animals to make energy, these turtles slow down their metabolism by 90 to 95%, saving the energy they do make to keep the systems going that are absolutely necessary. But even these systems aren't functioning as normal. The turtles slow their heart rates to 1 or 2 beats every 10 min, stop moving around and even turn off parts of their brain to save energy. But what happens to their senses? Are they still able to hear and see while in this state of suspended animation? These are the questions that Michael Ariel, Shivika Ahuja and Daniel Warren of Saint Louis University, USA are attempting to answer.Ariel and colleagues first tested whether the turtles’ hearing was affected when they didn't have any oxygen. They vibrated a rod near the turtle's eardrum and measured the response from where hearing is processed the brainstem. The shelled reptiles’ hearing was no different whether they had normal amounts of oxygen or very little. This suggested that the turtles can still hear during their hibernation because hearing isn't as reliant on the amount of oxygen they have available. But what about their vision? Do the turtles even need to see in the murky waters of their winter homes?This time, the team shined flashes of light on the sliders’ eyes and measured responses of one area of the brainstem that processes vision. Although very short 0.05 ms flashes of LED light showed no differences in the response, the responses of the brainstem decreased by 50% when the turtles didn't have much oxygen if the light flashes were longer than 0.2 ms. This wasn't surprising to Ariel and colleagues as the visual system is takes a lot of energy to run. ‘The retina in general, and the light sensing cells in particular, use the highest amount of metabolic energy of all vertebrate sensory systems’, states Ariel.With that in mind, the researchers measured the response of the same flashes of LEDs directly from the retina. When oxygen levels were very low, the response to the lights by the retina was lower, but not as low as the response by the brainstem. So, the turtles are shutting off their energetically expensive visual system all the way from the eyes back to the brain, where their vision is processed. Also, it took the turtles’ brainstems longer than normal to respond to flashes of light, whereas the brainstems responded normally to vibrations near the eardrum, suggesting that the cascade of events that follow sensing a flash of light is slower when very little oxygen is present. So, much like high-altitude mountain climbers who report problems such as hallucinations and dimming of their vision, in these turtles, it seems like eyesight is the first thing to go when the oxygen levels drop.