Turtles (L. testudo = tortoise) have adapted to a remarkable variety of environments, but the greatest number of species occurs in southeastern North America and South Asia. In both areas, most species are aquatic, living in bodies of water from small ponds, bogs and swamps to large lakes, and rivers.
A few types are strictly terrestrial, others divide their time between land and water, and some spend their entire lives in water. Although turtles as a group are broadly distributed, each species has a preferred habitat and is seldom found elsewhere.
The Turtle Body Plan
Like other reptiles, turtles are ectothermic (cold-blooded), breath air only through powerful lungs (although there is limited skin breathing in some aquatic types), and lay their eggs on land.
The approximately 300 species of turtles are characterized by a bony or leathery box-like shell, a keratinized beak rather than teeth, and unique among vertebrates, limbs and limb girdles located inside the ribs.
The shell, which is the key to their success, has also limited the group’s diversity. Obviously, flying or gliding turtles have never existed, and even aboreality (life in the trees) is only slightly developed.
A turtle’s shell is essentially a box of bone consisting of a domed upper shell or carapace and a flat lower shell or plastron. The carapace is formed from a fusion of vertebrae, expanded ribs, and bones in the dermis of the skin and consists of a number of interlocking bony plates.
The plastron is formed from bones of the pectoral girdle and dermal bone. It also consists of interlocking bony plates which are larger but fewer in number than those of the carapace. The morphology (structure) of the shell reflects the ecology of the turtle species.
The four limbs of freshwater aquatic turtles and tortoises (land turtles) are stubby, stout and equipped with claws. Sea turtles have paddle-like limbs with no claws.
Four different categories of turtles are recognized: freshwater aquatic turtles, tortoises, terrapins, sea or oceanic turtles
Freshwater Aquatic Turtles
Freshwater turtles have broad paddle-like legs with webbed toes and a low carapace that offers little resistance to movement through water. Because they have a fixed tongue, aquatic turtles must eat their food in water. Most familiar of the freshwater aquatic types are pond turtles, including the painted turtles and the red-eared slider which is a very popular pet around the world.
The snapping turtles and the mud and musk turtles prowl along the bottoms of ponds and slow rivers and are not particularly streamlined. They have a reduced plastron which makes these species more agile than most turtles. Musk turtles may even climb several feet into trees, probably to bask.
Tortoises
Tortoises (commonly called box turtles) are completely terrestrial (land-living). They have elephant-like feet and their toes are not webbed. The bottom shell of the tortoise has flexible areas, or hinges, that allow the anterior and posterior sections of the plastron to be pulled up tightly against the upper shell. This allows the box turtle to pull the head, limbs, and tail in and seal them off from attack by potential predators.
Smaller species of tortoises may show adaptations for burrowing. The gopher tortoises of North America are an example – their front legs are flattened into scoops, and the dome of the carapace is reduced. The Bolson tortoise of northern Mexico constructs a burrow a meter (6-7 feet) or more deep and several meters (12-14 feet) outward in the hard desert soil.
Endemic to nine islands of the Galapagos archipelago, the Galapagos tortoise (Geochelone nigra) is the largest tortoise. Adults can weigh over 300 kg (660 pounds), reach lengths of 1.2 meters (4 feet) long, and live to be 200 years old.
Terrapins
Terrapins have a smaller, lighter shell than do terrestrial types, and like freshwater types, their legs are paddle-like with webbed toes. They inhabit tidal marshes, estuaries, and lagoons where the water contains some salt, but is not as salty as the ocean (brackish water).
Sea (Oceanic) Turtles
Sea turtles are totally aquatic and found in all the world’s oceans except the Arctic Ocean. Their shells are generally reduced and flattened, and their limbs are flat flippers that lack toes entirely.
The leatherback (Dermochelys coriaca) does not have a hard shell, instead it carries a mosaic of bony plates embedded in connective tissue beneath its leathery skin. The leatherback is the largest marine turtle reaching shell lengths of more than 2 m (6-7 feet) and weights in excess of 660 kg (1,323 pounds).
“The turtle lives twixt plated decks which practically conceal its sex. I think it clever of the turtle in such a fix to be so fertile.” (Ogden Nash)
We should all be thankful that turtles, in spite of their cumbersome shell, have managed to survive because the world would be much less interesting place without these magnificent creatures.
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