Most vertebrates have a flexible rib cage that allows the lungs to expand and contract during breathing. Not so the turtles, who long ago traded away flexible ribs in favor of a fixed, protective shell. Various species of turtles have evolved different means of drawing air into their lungs. Turtles have also developed indirect ways of obtaining oxygen during times when they are sealed away from contact with the air, as when hibernating or remaining underwater.
In turtles, the lungs lie just beneath the carapace and above the other internal organs. The upper surface of the lungs attaches to the carapace itself, while the lower portion is joined to the viscera (heart, liver, stomach, and intestinal tract) by a skin of connective tissue known as diaphragmaticus. The viscera themselves are also contained within a membrane that attaches to the diaphragmaticus. Groups of muscles rhythmically change the volume of the abdominal cavity. One set of muscles moves the viscera upward, pushing air out of the lungs. Then other muscles contract, pulling the viscera away from the lungs, which lets the lungs expand and draw in air.
When turtles walk about, the motions of their forelimbs promote the suction and compression actions that ventilate the lungs. A turtle can change its lung volume simply by drawing its limbs inward, then extending them outward again: Turtles floating on top of the water often can be seen moving their legs in and out, which helps them breathe. A turtle pulled back inside its shell has no room in its lungs for air. At these and other times, turtles use different strategies to obtain oxygen.
One aid to respiration is the hyoid apparatus, a system of bony and cartilaginous rods located at the base of the tongue. Raising and lowering the hyoid apparatus causes a turtle’s throat to rise and fall, pulling in air. (In addition to promoting ventilation, this air movement allows a turtle to better use its sense of smell.) In the highly aquatic soft-shell turtles, the throat is lined with fingerlike projections of skin called villi, which are richly supplied with blood. The villi work like gills, expelling carbon dioxide and taking in oxygen from the water. To process oxygen rich water, a soft shell uses its hyoid apparatus to repeatedly fill and empty its throat in a process known as buccopharyngeal breathing. When underwater, a soft shell typically pumps water in and out about sixteen times per minute. Turtles that hibernate underwater also exchange gases through the throat lining, cycling the water inside the throat cavity several times each minute. Many turtles practice this method of breathing, and some turtles even take in oxygen through the cloaca.
Many of the details of turtles breathing remain unknown. What is clear however is that different kinds of turtles have evolved different methods of fulfilling their oxygen needs. Through evolution, they have gotten very good at obtaining this essential gas. As Ronald Orenstein notes in Turtles, Tortoises, and Terrapins: Survivors in Armor, turtles seem able to breathe “with the least amount of effort no matter what their circumstances.”