Introduction to chelonian (turtle) medicine (Proceedings)
Turtles are found throughout the world on all continents and in all oceans except Antarctica. There are over 300 species of turtles (far fewer than snakes or lizards) that belong to about 90 genera in 13 families (http://www.reptile-database.org/db-info/SpeciesStat.html; accessed 7/09). Turtles appeared in the fossil record over 200 million years ago and were on earth long before mammals and other forms of present day reptiles. They occur in terrestrial, freshwater aquatic, semiaquatic, and marine environments. They range in size from 11 cm to 185 cm and one species can weigh close to a ton, making it (the leatherback sea turtle) the world's largest turtle and one of the largest living reptiles!
Anatomy and physiology:
Witherington & Wyneken (2003) have published a nice color anatomy guide for the turtle (see Further Reading).1. Turtles can live a long time and tortoises generally live longer than aquatic species. Reports of Aldabra and Galapagos tortoises living up to 260 years exist. Some species can be aged by growth rings on the scutes, but as the animal ages accuracy suffers. This does not hold true for many aquatic species that periodically shed their scutes.
2. Both the pelvic and pectoral girdles are contained entirely within the rib cage that is fused to the protective shell. The shell is a vascular bony structure that should be included when calculating drug dosages from the animal's weight.
3. Sexual dimorphism exists in many species. Male tortoises have a concave plastron and male aquatic turtles usually have very long toenails on their front feet. The tail is relatively larger in males than in females but this does not always hold true.
4. Turtles lack teeth but most possess a sharp beak called a rhamphotheca or tomium. They also have tongues that aid in prehending food but cannot be extended beyond the mouth (as is the case with many snakes and lizards).
5. The gastrointestinal tract is standard in that it includes a simple S-shaped stomach, liver, gall bladder, pancreas, spleen, small and large intestine. A cecum may be present but fermentation, when applicable, occurs primarily in the large intestine. Nearly all aquatic turtles must eat in the water.
6. Like most other reptiles, the heart has three chambers (one ventricle and two atria).
7. Turtles lack a diaphragm and since they are housed in a shell most have little or no abdominal breathing component. Most pressure changes allowing for lung expansion are accomplished by muscles in the pockets surrounding the fore and hind limbs. Aquatic species can also respire through their skin and the mucus membranes of the throat and cloaca.
8. Turtles have paired kidneys and a cloacal opening for the urogenital and gastrointestinal tracts. The ureters open into the cloaca and urine then passes from the cloaca to the more cranial urinary bladder.
9. The chelonian cloaca is divided into three zones: the cranial coprodaeum where the rectum attaches; the medial urodaeum where the ureters and reproductive tracts attach; the proctodaeum where urinary and fecal waste are stored.
10. All turtles have internal fertilization. Courtship is usually a part of most chelonian copulation. Some freshwater species (sliders, cooters) have an elaborate ritual in which the male waves its long forelimb nails in front of the female's face. The single phallus is employed during copulation. Unlike snakes and lizards, the chelonian penis has erectile tissue. The semen is conducted through a seminal groove that terminates between distinct folds (known as plicae) of the glans. Some female turtles have a clitoris located in the ventral cloaca that anatomically resembles the penis.
11. All turtles lay eggs and most bury them in the earth. Some species may lay several clutches per year and females of certain species can store sperm for several years.
12. Sea turtles possess special salt glands in their head behind each eye that allow them to drink seawater.
-More details can be found by reading: Boyer TH and Boyer DM. 2006. Turtles, Tortoises, and Terrapins. In: Mader D.R. Reptile Medicine and Surgery, Second Edition. Elsevier/Saunders Co., Phila, 78-99.