4.20.2013

Marine Reptile Anatomy

Reptiles are not usually associated with marine environments. In fact, of the 6,000 known species of reptiles, only about 1 percent inhabits the sea. Members of this select group include lizards, crocodiles, turtles, and snakes. Each of these organisms shares many of the same anatomical structures that are found in all reptiles: They are cold-blooded, air-breathing, scaled animals that reproduce by internal fertilization. Yet, to live in salt water, this subgroup has evolved some special adaptations not seen in terrestrial reptiles.

Marine Reptile Anatomy

In turtles, the shell is the most unique feature. The lightweight, streamline shape of the shell forms a protective enclosure for the vital organs. The ribs and backbone of the turtle are securely attached to the inside of the shell. The upper part of the shell, the carapace, is covered with horny plates that connect to the shell’s bottom, the plastron. Extending out from the protective shell are the marine turtle’s legs, which have been modified into paddle-like flippers capable of propelling it at speeds of up to 35 miles per hour (56 kph) through the water. These same legs are cumbersome on land, making the animals slow and their movements awkward.

Most air-breathing vertebrates cannot drink salty water because it causes dehydration and kidney damage. Seawater contains sodium chloride and other salts in concentrations three times greater than blood and body fluids. Many marine reptiles drink seawater, so their bodies rely on special saltsecreting glands to handle the excess salt.

To reduce the load of salt in body fluids, these glands produce and excrete fluid that is twice as salty as seawater. The glands work very quickly, processing and getting rid of salt about 10 times faster than kidneys.
Salt glands are located on the head, often near the eyes.

There are more than 50 species of sea snakes that thrive in marine environments. Sea snakes possess adaptations such as nasal valves and close-fitting scales around the mouth that keep water out during diving. Flattened tails that look like small paddles easily propel these reptiles through the water. The lungs in sea snakes are elongated, muscular air sacs that are able to store oxygen. In addition, sea snakes can take in oxygen through the skin. Their adaptations to the marine environment enable sea snakes to stay submerged from 30 minutes up to two hours; however, this ability comes at a cost. Because marine snakes routinely swim to the surface to breathe, they use more energy and have higher metabolic rates than land snakes. To balance their high energy consumption, they require more food than their terrestrial counterparts.

Finally, crocodiles usually occupy freshwater, but there are some species that live in brackish water (in between salt water and freshwater) and salt water. These animals have salivary glands that have been modified to excrete salt. Their tails are flattened for side-to-side swimming and their toes possess well-developed webs. Saltwater crocodiles are equipped with valves at the back of the throat that enable them to open their mouths and feed underwater without flooding their lungs.