The parietal eye, often referred to as a “third eye,” is a unique light-sensing organ found in certain animal species. It provides insight into how animals interact with their environment. This organ highlights a rich evolutionary history, showing ancient sensory pathways that persist in various forms across different lineages, retained or specialized in some animals.
What is a Parietal Eye? Anatomy and Location
The parietal eye is a photoreceptive organ located on the dorsal, or top, surface of an animal’s head, typically positioned between the two conventional eyes.
Structurally, the parietal eye is simpler than a conventional eye. It often includes a rudimentary lens and a retina-like structure containing photoreceptors. These photoreceptors connect directly to ganglion cells, whose axons form a nerve that projects to the brain. The parietal eye is part of the epithalamus and connects to the pineal gland, an endocrine gland. In some species, it may even protrude through a small opening in the skull, known as the pineal or parietal foramen.
How the Parietal Eye Works and Its Purpose
The primary function of the parietal eye is to detect changes in light intensity and variations in day-night cycles, rather than forming detailed images. This light information is then relayed to the brain, influencing various physiological processes.
One significant function is its role in thermoregulation, particularly in ectothermic animals like lizards. By sensing changes in light, the parietal eye can influence basking behavior, helping the animal regulate its body temperature and determine appropriate sun exposure.
The parietal eye also plays a part in regulating circadian rhythms, which are the daily activity cycles and sleep-wake patterns of an animal. The detection of light and darkness by the parietal eye helps synchronize the animal’s internal biological clock with the external environment. This sensory input is also linked to the pineal gland, which produces hormones such as melatonin. Melatonin production, regulated by light exposure, further influences sleep patterns and other physiological processes tied to circadian rhythms.
Animals With a Parietal Eye
A functional parietal eye is observed in a variety of animal groups, often linked to ancient evolutionary lineages or specific ecological adaptations. Examples include:
The tuatara, a reptile native to New Zealand.
Many lizard species, such as iguanas, anoles, bearded dragons, blue-tongue skinks, and water dragons, also possess this organ, visible as an opalescent gray spot on the top of their heads.
Certain amphibians, including some frogs and toads, sometimes referred to as a “frontal organ.”
Some fish species, such as lampreys and sharks, which exhibit a median eye.
The presence of this structure in these diverse groups suggests its long evolutionary history, with fossil evidence indicating that many ancient vertebrates, including ostracoderms and placoderms, had sockets for functional third eyes.
Parietal Eye vs. Conventional Eyes
The parietal eye differs significantly from the paired, image-forming eyes found in most vertebrates. Conventional eyes, also known as lateral or camera-type eyes, are complex structures designed for detailed vision, including image formation, depth perception, and color discrimination. They achieve this through a sophisticated lens system, a highly developed retina with rods and cones, and extensive neural pathways to the brain.
In contrast, the parietal eye has a much simpler structure. It possesses a rudimentary lens and a retina-like layer, but it lacks the intricate components necessary for image formation or discerning fine details and colors. Its function is limited to detecting changes in light intensity and the presence or absence of light. While conventional eyes provide comprehensive visual information, the parietal eye serves a more specialized role, primarily sensing ambient light levels to influence physiological processes.