Turtle Tweets: Chelonian Ophthalmology

Introduction

A simple retweet of a turtle eye examination at the National Aquarium inspired a day of terrapin-friendly tweets by LafeberVet.com.

 

Adnexa

 

Turtle Tweet 2: Turtles and tortoises also possess a well-developed nictitating membrane.

 

 

Turtle Tweet 4: Blepharedema is a common finding in aquatic turtles and is often associated with hypovitaminosis A.

Most Chelonia have a Harderian gland in the medial region and a lacrimal gland laterally.

 

Lacrimal system

The nasolacrimal duct is absent in all Chelonia that have been studied therefore tears naturally spill over the eyelids and down the sides of the face to eventually evaporate. Any fluorescein placed in the eye does the same.

 

Shown above, two Julia butterflies (Dryas julia) drinking the tears of turtles in Ecuador.

 

Globe

The globe of most reptiles is almost spherical, and this holds true for turtles and tortoises. The globe is supported by a series of scleral ossicles or small bones that overlap to form a cup-like structure. Chelonia also possess a scleral cartilage that measures up to 1 cm thick. The scleral cartilage is a continuous band of cartilage that circles the posterior segment of the globe.

 

Intraocular pressure (IOP) has been shown to vary with eye position relative to the body. Loggerhead sea turtles (Caretta caretta) in dorsoventral and ventrodorsal positions have lower IOP than when the head is in a down position (Chittick 2001).

Looking for additional information on chelonian ophthalmology? Here are some additional facts:

  • As in birds, the reptile iris contains skeletal muscle and is therefore under voluntary control.
  • Turtles and tortoises possess an avascular retina.
  • The conus papillaris is a vascular structure, similar to the pecten in birds, that projects into the vitreous in some chelonian species. A conus papillaris was not observed in thirty, adult red-eared slider turtles (Trachemys scripta elegans) (Somma 2014).
  • Chelonia possess cones and rods, allowing significant color vision.
  • Finally, some Chelonia have been shown to possess ultraviolet vision.

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References