Most turtles that spend most of their lives on land have their eyes looking down at objects in front of them. Some aquatic turtles, such as snapping turtles and soft-shelled turtles, have eyes closer to the top of the head. These species of turtle can hide from predators in shallow water, where they lie entirely submerged except for their eyes and nostrils. Near their eyes, sea turtles possess glands that produce salty tears that rid their body of excess salt taken in from the water they drink.
Turtles have rigid beaks and use their jaws to cut and chew food. Instead of having teeth, which they appear to have lost about 150-200 million years ago, the upper and lower jaws of the turtle are covered by horny ridges. Carnivorous turtles usually have knife-sharp ridges for slicing through their prey. Herbivorous turtles have serrated-edged ridges that help them cut through tough plants. They use their tongues to swallow food, but unlike most reptiles, they cannot stick out their tongues to catch food.
The upper shell of the turtle is called the carapace. The lower shell that encases the belly is called the plastron. The carapace and plastron are joined together on the turtle’s sides by bony structures called bridges. The inner layer of a turtle’s shell is made up of about 60 bones that include portions of the backbone and the ribs, meaning the turtle cannot crawl out of its shell. In most turtles, the outer layer of the shell is covered by horny scales called scutes that are part of its outer skin, or epidermis. Scutes are made up of the fibrous proteinkeratin that also makes up the scales of other reptiles. These scutes overlap the seams between the shell bones and add strength to the shell. Some turtles do not have horny scutes; for example, the leatherback sea turtle and the soft-shelled turtles have shells covered with leathery skin instead.
The rigid shell means that turtles cannot breathe as other reptiles do, by changing the volume of their chest cavities via expansion and contraction of the ribs. Instead, they breathe in two ways. First, they employ limb pumping, sucking air into their lungs and pushing it out by moving the limbs in and out relative to the shell. Secondly, when the abdominal muscles that cover the posterior opening of the shell contract, the pressure inside the shell and lungs decreases, drawing air into the lungs, allowing these muscles to function in much the same way as the mammalian diaphragm. A second set of abdominal muscles face the opposite way and when they contract they expel air under positive pressure. The shape of the shell gives helpful clues about how a turtle lives. Most tortoises have a large, dome-shaped shell that makes it difficult for predators to crush the shell between their jaws. One of the few exceptions is the African pancake tortoise, which has a flat, flexible shell that allows it to hide in rock crevices. Most aquatic turtles have flat, streamlined shells, which aid in swimming and diving. American snapping turtles and musk turtles have small, cross-shaped plastrons that give them more efficient leg movement for walking along the bottom of ponds and streams. Another exception is the Belawan Turtle (Cirebon, West Java), which has sunken-back soft-shell.
The color of a turtle’s shell may vary. Shells are commonly colored brown, black, or olive green. In some species, shells may have red, orange, yellow, or grey markings, often spots, lines, or irregular blotches. One of the most colorful turtles is the eastern painted turtle, which includes a yellow plastron and a black or olive shell with red markings around the rim.
Tortoises, being land-based, have rather heavy shells. In contrast, aquatic and soft-shelled turtles have lighter shells that help them avoid sinking in water and swim faster with more agility. These lighter shells have large spaces called fontanelles between the shell bones. The shells of leatherback sea turtles are extremely light because they lack scutes and contain many fontanelles.
It has been suggested by Jackson (2002) that the turtle shell can function as pH buffer. To endure through anoxic conditions, such as winter periods trapped beneath ice or within anoxic mud at the bottom of ponds, turtles utilize two general physiological mechanisms. In the case of prolonged periods of anoxia, it has been shown that the turtle shell both releases carbonate buffers and uptakes lactic acid.
Skin and molting
As mentioned above, the outer layer of the shell is part of the skin; each scute (or plate) on the shell corresponds to a single modified scale. The remainder of the skin has much smaller scales, similar to the skin of other reptiles. Turtles do not molt their skins all at once as snakes do, but continuously in small pieces. When turtles are kept in aquaria, small sheets of dead skin can be seen in the water (often appearing to be a thin piece of plastic) having been sloughed off when the animals deliberately rub themselves against a piece of wood or stone. Tortoises also shed skin, but dead skin is allowed to accumulate into thick knobs and plates that provide protection to parts of the body outside the shell.
By counting the rings formed by the stack of smaller, older scutes on top of the larger, newer ones, it is possible to estimate the age of a turtle, if one knows how many scutes are produced in a year. This method is not very accurate, partly because growth rate is not constant, but also because some of the scutes eventually fall away from the shell.