While most species of turtles have shells which appear to be similar, subtle differences in design make them anything but generic. Each species has customized its shell for its own peculiar life style.

   Probably the most obvious variation in shell design is the color. Colors and patterns are usually for concealment. Lines, streaks and spots break up the outline of the shell and thereby make the turtle less conspicuous. In our spotted and bog turtles the shells are essentially all black. The dark color allows the small turtles to soak up solar radiation, raising body temperatures quickly to allow the turtles to be active early in the spring and late into the fall. Higher body temperatures allow the "cold blooded" reptiles to be active longer and to maintain internal temperatures necessary for digestion. It is not unusual to see spotted turtles sunning on cool winter days, and I once saw an individual sunning on a snowy February afternoon.

   Sometimes the coloration for the young and adults differ. One of the more interesting aspects of this are the ventral spots and other markings which are prevalent on young pond dwelling turtles, but are absent on the adults. The patterns actually serve as underwater advertising to large predatory fishes. Bass and other predators quickly learn to recognize and avoid eating young turtles. Little turtles are difficult to swallow whole, and live ones, trying to escape from a fish's mouth will claw and damage the gills.

   The shell's camouflage is not limited to coloration. Often the carapace will have ridges or bumps which add in breaking up the outline, and provide surfaces for attachment of live algae. The alligator snapper of southeastern North America is one the best examples of this type of camouflage.

   Since the shell's main function is protection, it is not surprising that some of the most interesting modifications are ones which enhance protection. The box turtle with its hinged plastron is an excellent example of this. Because of its terrestrial nature it does not have the luxury of underwater escapes. When confronted with a dangerous situation, the box turtle simply seals itself up in its shell. Mud turtles, which are semi-terrestrial also have hinged plastrons, but the seal is not complete and parts of the legs can be seen when the turtle is tucked tightly in its shell.

   American alligators have developed wide strong jaws, in part because of their turtle eating habits. Because of this several species of sliders have developed high domed shells. The dome, like the Roman arch, provided additional structural support, and at the same time makes such a mouthful that the gators have trouble clamping down. The high dome creates extra resistance for swimming turtles and reduces swimming speed. Therefore, the same species of turtles, where they occur outside the range of alligators, have shell which are less highly arched. Swimming species of turtles which are primary inhabitants of rivers have additional drag on their shells caused by currents. These species typically have flattened streamlined carpaces, and while alligators may still be a concern, the gators are not very effective hunters in swift currents.

   The main problems with shells is that they limit movement. Custom design is everything. Our common mud turtle perhaps has the simplest of designs. A smooth rounded shell that does not catch on anything allows it to crawl through grass, or burrow in mud with little resistance. Mud turtles live mostly in shallow water and it is mostly of a bottom crawler. Species for which swimming is a major way of movement have modified openings in the shells to accommodate the swimming motion of the legs. The cooters and other pond turtles have leg openings which allow for strong lateral swimming movements. On land these same turtles seem to scuttle, because the positioning of the leg openings does not allow them to get their legs under the shell. Terrestrial species, like box turtles and tortoises, have leg openings positioned so that their legs can lift their shells off the ground and they can walk more freely.

   Snapping turtles have a variety of leg movements, which include swimming and walking on the bottom. The plastron of snapping turtles is extremely reduced and the large leg openings allow for much freedom of movement but limited protection. The snapper compensates for its vulnerability with its long flexible neck, strong jaws and short temper. Aquatic turtles which confine most of their foraging to the bottoms of swift moving streams have tent shaped shells which direct the water currents in fashion that actually helps hold them to the bottom. By turning out of the current they lose the effects of the redirected water pressure and can easily swim to the surface.

   In the interest of speed and power the entire shape of sea turtle shells has been redesigned to accommodate the huge muscle mass necessary for their modified, paddle like front limbs. The hind legs are used primarily for steering and need far less muscle. Because of this the back third of the shell sharply tapers and the overall result is minimized resistance for the swimming turtle.

   Protection can be sacrificed for speed. By having lighter weight shells, aquatic turtles can move about more easily. Soft-shelled turtles and leatherback sea turtles are both active swimmers. They have lightened their shells by the loss of the external scale-like scutes and reduction in the amount and thickness of the underlying bony plates. While their rubbery and leathery shells do not afford as much protection as traditional turtle shells, the increased speed cuts down on their vulnerability to predators and increases their ability be effective hunters.

    Ogden Nash wrote: 
      "The turtle lives 'twixt' plated decks
       Which practically conceal its sex
       I think it cleaver of the turtle
       In such a fix to be so fertile."