If you would like to listen to an audio recording of this module, please click here.

Introduction

A more comprehensive discussion of chelonian anatomy and physiology can be found in the book chapter cited in the Additional Resources at the end of this module. The focus of the information below is to present you with the key concepts of turtle anatomy and physiology that are essential for the treatment of wild turtles in your practice.

Ventral view of the internal anatomy of a turtle with the plastron removed. Source: Boyner and Innis 2019.

Ventral view of internal anatomy of a turtle with the plastron removed, liver and bladder have been reflected to reveal gallbladder and intestines, respectively. Source: Boyner and Innis 2019.

Ventral view of internal anatomy of a turtle. The intestinal tract and liver have been removed for a view of the lungs and urogenital system. Source: Boyner and Innis 2019.

Integumentary System

• The boney shell is covered with keratinous structures called scutes
• Turtles continuously grow new scutes underneath their previous ones. Aquatic turtles can and do shed old scutes.
  • Scutes and the underlying bone are able to regenerate following debridement
  • Scutes are named according to their position on the body.

Names of the various scutes on a chelonian shell. Source: Boyner and Innis 2019.

• The skin in most turtles seen in NC is scaled – injections must be given between scales.
  • Turtles also periodically shed their skin, though not as complete as in snakes

A box turtle with the carapace bones exposed from under the scutes.

Musculoskeletal System

• The shell is made up of the carapace (dorsal portion of shell), plastron (ventral portion of shell), and the bridge, which connects the plastron and carapace.
• The ribs, vertebrae, and clavicles are fused with the boney shell
• Box turtles contain a connective tissue hinge on their plastron that allows them to withdraw completely into their shell.
• The pectoral and pelvic girdles are within the ribcage rather than outside of it as in other animals.
• Turtles have more musculature along the limbs than on the trunk.
• Bones of the limbs are similar to those in other animals.

Ventral view of the chelonian skeleton. Source: Boyer and Innis 2019.

Lateral view of the chelonian skeleton. Source: Boyner and Innis 2019.

Cardiovascular System

• The heart consists of one sinus venosus, two atria, and one ventricle with three subchambers
• Major vessels: Right and left cranial vena cavas, caudal vena cava, hepatic vein, right and left aortas, pulmonary artery, brachiocephalic trunk.
  • During diving, a R-L shunt occurs to allow turtles to tolerate apnea, decreasing blood flow by 50-80%
• Due to the presence of a renal portal system, drugs are given in the front half of a turtle whenever possible.

Illustration of the intracardiac right-to-left shunt in reptiles. PA: Pulmonary artery. LAo: Left aortic arch. RAo: Right aortic arch. RAt: Right atrium. LAt: Left atrium. Source: Hicks and Wang 2012.

Respiratory System

• Turtles are obligate nasal breathers, taking in air through the external nares and into the internal nares to the trachea
  • Open-mouth breathing is always considered abnormal
• The glottis is located at the base of the tongue and leads to the trachea
• The trachea is short and bifurcates cranially
• The lungs are thin and adhered to the dorsal carapace
  • Turtles lack a diaphragm and so do not have a thorax that can be expanded and contracted – they move their limbs and necks in order to breathe.
• Turtles’ lungs have a large volume but low surface area relative to mammals
  • Positive pressure in anesthesia should not exceed 10 cm H2O
• Aquatic turtle species use gular respiration, while terrestrial turtle species use gular breathing mostly for olfaction
  • Some aquatic turtles can also respire through their cloaca while submerged
• Turtles are able to tolerate apnea for long periods of time, especially aquatic turtles
  • This can make use of inhalant anesthetics difficult
• Pneumonia is difficult to manage in turtles and can be life-threatening.

Nervous System

• The brain consists of the olfactory bulb, cerebral cortex, thalamus, hypothalamus, pituitary, optic lobes, cerebellum, and medulla
• Turtles have two meningeal layers – the inner leptomeninx and outer dura mater. The space between these is used for intrathecal injections and CSF collection.
  • There is no subarachnoid space.
  • The epidural space does not contain CSF
• Turtles have the same 12 cranial nerves as in other animals.

Gastrointestinal System

• Turtles lack teeth and instead use their beak to break up food.
• Aquatic turtles require water to eat due to adaptations of the salivary glands.
• Much of the rest of the digestive tract functions similarly to that of other animals, though transit time can be longer.
  • Transit time is impacted by temperature, fiber and water content, and food frequency
• Feces are passed through the cloaca.

Urogenital System

• The kidneys are located retrocoelomically.
• Reptiles lack a loop of Henle and so cannot concentrate urine more than plasma
• Aquatic turtles tend to excrete more ammonia and urea while terrestrial turtles excrete more uric acid and urates; this is due to the amount of water needed to excrete the different compounds.
  • Measurement of BUN is not specific to renal disease in turtles
• Urine is excreted through the cloaca.
• Gravid ovaries in female turtles can be easily palpated in the inguinal region.
• Engorged phalluses extending from the cloaca can be mistaken for prolapsed organs – it is also possible for the phallus itself to become prolapsed.

Radiographs showing eggs in a female turtle.

Relevant Videos

Guided turtle dissection video (YouTube)

Additional Resources

Boyer TH and Innis CJ. Chelonian Taxonomy, Anatomy, and Physiology. Chapter 7 in Divers S and Stahl S (eds.), Mader’s Reptile and Amphibian Medicine and Surgery, 3rd ed., Elsevier, Amsterdam, 2019. (PDF)