Turtles do not breathe underwater in the sense of what we call breathing. That is, they do not take in oxygen through inhalation and exhalation. Rather, they absorb or take in oxygen through means other than just by their lungs.
Turtles have a set of lungs that it can breathe with. As its activity increases so does its demand for air. The turtle, depending on the species can hold its breath underwater for a long time. Most species can hold their breath anywhere from 30 minutes to 3 hours. How long the turtle can hold its breath is also in direct proportion to how active it is and if it is able to find pockets of air. These pockets of air can be found in small caves and/or under ice.
According to some studies that have been done, turtles were found to virtually not be breathing at all. Instead they would take in air by a different area of the body other than the means of the lungs. It was once thought that turtles did breathe underwater because movement of the water was seen around the turtle’s mouth. However later studies showed that the air was actually absorbed through gill like structures in the beck in some turtles and in through the skin at the throat on others. Water movement was also found near the turtle’s anal area where there was also absorption of air through two sacs. These sacs are similar to the throat in that they have tiny capillary blood vessels that are equipped to absorb the oxygen needed from the water.
Some turtles are able to hibernate for up to four months without breathing or eating while underwater. In addition to the absorption of air through both ends of its body, the turtle’s system slows down. Because these creatures are cold blooded, their body temperatures are comparable to the environment around it. When the temperature drops, the turtle cools down. The turtle’s heart can slow to as little as 5 to 10 beats per minute. The creature’s need for oxygen and/or food is diminished. Cold water not only lowers the turtle’s body temperature, it also retains more oxygen that the turtle will absorb. Turtles are no different from other living organisms in that they need oxygen to perform natural bodily functions. They are also able to go without air for short periods of time.
Turtles are still in need of oxygen when they are underwater hibernating. In fact, two things can still cause a turtle to die. One being the lack of oxygen due to a stagnant body of water. The second being the body of water freezing solid. Turtles often fare better in a pond or river that has a constant flow of water regardless of the temperature to bring in a fresh supply of oxygen. If the water freezes, the turtle will literally freeze to death.