The concept of a biological “third eye” is often associated with myth, but a specialized organ on the top of the head of some animals serves as a genuine, light-sensing structure. This median eye, known scientifically as the parietal eye, is a vestige of an ancient sensory system. While this unique feature is present in a few modern species, one reptile possesses the most complex and developed example, giving it a unique sensory advantage.
The Tuatara: The True Three-Eyed Animal
The tuatara, a reptile endemic to New Zealand, holds the distinction of having the most prominent third eye among living vertebrates. Despite its lizard-like appearance, the tuatara is not a lizard; it is the sole surviving member of the distinct reptilian order Rhynchocephalia. This ancient lineage, dating back approximately 240 million years, flourished during the Mesozoic Era and gives the tuatara its status as a “living fossil.” The tuatara retains many primitive anatomical features lost in modern lizards and snakes, including its remarkable parietal eye, which is positioned centrally on the head, just beneath the skin.
Anatomy of the Parietal Eye
The tuatara’s parietal eye is a sophisticated structure emerging from the pineal complex in the brain. This median organ possesses components analogous to a fully formed visual eye, though its function is not image-forming. The structure includes a rudimentary lens that focuses incoming light and a parietal plug that acts similarly to a cornea.
Beneath the lens lies a retina-like structure containing photoreceptor cells sensitive to light, but lacking the complexity for detailed sight. A nerve pathway connects this photoreceptive layer directly to the brain, integrating light information with the central nervous system.
In newly hatched tuatara, the third eye is clearly visible through a translucent patch of skin on the head. As the animal matures, the parietal eye becomes covered by opaque scales and pigment, obscuring it from external view in adults. The retention of a lens and a nerve connection makes the tuatara’s parietal eye the most highly developed of any living tetrapod.
Primary Functions of the Third Eye
The primary role of the tuatara’s third eye is to function as a specialized light-intensity meter, not to form visual images. Its photoreceptors detect changes in light levels, distinguishing between day and night. This light perception is directly linked to the tuatara’s internal clock and hormonal regulation.
The parietal eye regulates the circadian rhythm by signaling light exposure to the pineal gland. This gland controls the production and release of melatonin, a hormone that governs sleep and seasonal cycles. Detecting shifts in daylight hours also helps the tuatara time critical behaviors, such as reproduction and seasonal activity.
Thermoregulation
For this cold-blooded reptile, the parietal eye aids in thermoregulation, the control of body temperature. By sensing the intensity of sunlight, the tuatara determines how long to bask to reach its optimal body temperature without overheating. This ability to gauge sun exposure allows it to maintain a stable internal environment.
Similar Photosensitive Structures in Other Animals
While the tuatara has the most developed example, a median, light-sensitive organ is not unique in the animal kingdom. Many species of lizards, such as iguanas and some skinks, also possess a less complex parietal eye. These structures typically lack the distinct lens and retina found in the tuatara, functioning only as simpler light sensors.
The pineal complex, the evolutionary origin of the parietal eye, is present across a wide range of vertebrates. Certain groups, including fish, amphibians, and lampreys, possess a photosensitive organ derived from this complex, often referred to as a pineal or parapineal organ. In most of these animals, the organ is less organized, typically buried beneath the skull or skin, and acts only as a simple photoreceptive spot.
In contrast, mammals and birds have lost the external parietal eye. The pineal complex has been wholly internalized as the pineal gland, situated deep within the brain. Though no longer a direct light sensor, this gland retains its function as a regulator of biological rhythms, controlling hormonal cycles in response to light signals received indirectly through the lateral eyes.