Turtles, despite their aquatic lifestyles, are air-breathing reptiles equipped with lungs for respiration. This fundamental aspect of their biology leads to curiosity about how they breathe, especially given their time spent in water. Like all other reptiles, they do not possess gills.
Turtles Possess Lungs
Turtles breathe by drawing air into their lungs, their primary respiratory organs. These lungs are located directly beneath their rigid upper shell, the carapace. Unlike mammals, turtles lack a diaphragm, the muscle responsible for inflating and deflating lungs. Their unique body structure, with ribs fused into the shell, necessitates a different approach to ventilation.
To overcome the constraint of their inflexible shell, turtles utilize specialized abdominal muscles for breathing. These muscles, including the transversus abdominis and obliquus abdominis, attach to the shell and internal organs. During inhalation, a muscle sling attached to the shell contracts, pulling the liver and other organs downwards, expanding the lung cavity and drawing air in. Exhalation occurs when other muscles contract, compressing the lungs and expelling air. Some turtles also use limb movements to alter lung pressure.
How Turtles Breathe Underwater
While turtles primarily breathe air using their lungs, many species have evolved supplementary methods to absorb oxygen while submerged. These adaptations extend time underwater, especially during inactivity or brumation, a state similar to hibernation. These alternative respiratory processes augment, rather than replace, lung breathing.
One notable adaptation is cloacal respiration, informally known as “butt breathing.” The cloaca is a multi-purpose opening at the base of a turtle’s tail. In some freshwater turtles, like the Fitzroy River and Mary River turtles, specialized sac-like structures called bursae are present within the cloaca. These bursae are lined with finger-like projections, or papillae, rich in blood vessels.
Turtles pump water into these bursae, allowing dissolved oxygen to diffuse across the thin membranes into their bloodstream. This method is less efficient than lung breathing but allows oxygen uptake when surfacing is not feasible.
Pharyngeal respiration is another aquatic breathing mechanism, involving gas exchange across the throat lining. Some turtles possess highly vascularized pharyngeal tissues that absorb oxygen directly from water. Additionally, cutaneous respiration, or breathing through the skin, contributes to underwater oxygen uptake in many species. Smaller turtles, with a higher surface area-to-volume ratio, are more efficient at this form of respiration. These accessory methods become significant in cold water, where a turtle’s metabolic rate slows, reducing oxygen demand.
Why Turtles Do Not Have Gills
The question of whether turtles have gills arises from their aquatic habitat, and the answer is definitively no. Gills are specialized respiratory organs found predominantly in aquatic animals like fish and crustaceans, designed to extract dissolved oxygen from water. Structurally, gills are external or internal laminar tissue outfoldings with a large surface area for gas exchange, constantly exposed to water flow. This contrasts fundamentally with lungs, which are internal, sac-like organs adapted for gas exchange with air.
The primary distinction lies in the medium from which oxygen is extracted: gills are for water, and lungs are for air. While some turtles use cloacal or pharyngeal surfaces that absorb dissolved oxygen, these are not true gills in anatomical or evolutionary terms. Turtles are reptiles, a group of vertebrates that evolved on land, and their respiratory system reflects this terrestrial ancestry. Their evolutionary path led to the development of lungs, highly efficient at processing atmospheric oxygen, rather than retaining or developing gills for aquatic respiration.
Therefore, even sea turtles, which spend almost their entire lives in the ocean, must periodically surface to breathe air into their lungs. Their ability to hold their breath for extended periods, sometimes hours when resting, is due to physiological adaptations like slowing their heart rate and tolerating low oxygen levels, not gills. The absence of gills highlights that despite their aquatic prowess, turtles remain fundamentally air-breathing animals.