Turtle Eye Muscle Adapts to Deal With Obstructed Vision

Sep. 19, 2013 — In a recent study published in The Journal of Comparative Neurology, Saint Louis University professor of pharmacological and physiological science Michael Ariel, Ph.D., reported surprising findings about the eye movements of pond turtles who can retract their head deep into their shell. While researchers expected that the pond turtle's eyes would operate like other animals with eyes on the side of their heads, this particular species of turtle appears to have characteristics of both front and side-eyed animals, affecting a specific eye muscle's direction of pull and the turtle's eye position when its peripheral vision is blocked by its shell.

Humans, and many mammals like cats and monkeys, have their eyes viewing forward. In contrast, most lower vertebrates, including turtles, have eyes that are lateral -- on the side of their heads. Of the six muscles that move each eye, the muscles that move lateral eyes differ from the muscles of animals that move eyes viewing forward. In an earlier study, Ariel and his research team made an unexpected observation that a nerve that moves one of the pond turtle's eye muscles, the superior oblique muscle, was active when that turtle moved its head from side to side, much like that of animals whose eyes view forward .

In the current study, Ariel and the research team tested his theory that the pond turtle had characteristics of a front-eyed animal in three ways: physiologically, looking at the eye movement response to nerve stimulation; anatomically, examining how muscles were attached to the eyes and head; and behaviorally, examining eye positions.

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