**Three-Eyed Animal: The Anableps**

The Anableps, commonly known as the "four-eyed fish," is an intriguing aquatic creature that possesses an unusual feature: it appears to have three eyes. Despite its name, the four-eyed fish actually has only two functional eyes, but their unique arrangement creates the illusion of a third eye. Let's explore the characteristics and visual abilities of the Anableps in detail.

**Anatomy and Eye Structure**

1. **Head and Body**: The Anableps is a small fish native to freshwater environments in Central and South America. It typically grows to a length of about 10-12 inches (25-30 cm) and has a slender, elongated body.
2. **Eyes**: Unlike most fish, the Anableps has eyes that are positioned on the top of its head. These eyes are divided into two sections, each having a separate lens and retina, effectively creating two distinct visual fields.
3. **Eye Arrangement**: The eyes of the Anableps are positioned in such a way that they can simultaneously see above and below the water's surface. The upper half of each eye protrudes above the water, while the lower half remains submerged.

**The "Third Eye" Illusion**

1. **Pupil and Cornea**: The Anableps has a unique horizontal pupil, which allows it to keep an eye on both the underwater and above-water environments simultaneously. The cornea of each eye is curved, enabling it to refract light more effectively.
2. **Surface Vision**: The upper half of each eye is adapted for vision in air. This part of the eye has a more developed lens and retina, allowing the Anableps to focus on objects and detect movements above the water surface.
3. **Underwater Vision**: The lower half of each eye is specialized for underwater vision. It has a flatter lens and retina, enabling the fish to see clearly and detect prey or predators beneath the water.

**Advantages of the Eye Arrangement**

1. **Predator Awareness**: The Anableps' unique eye arrangement provides it with a distinct survival advantage. It can constantly scan both the air and water for potential threats, such as aerial predators or underwater predators.
2. **Prey Detection**: With its specialized eyes, the Anableps can detect prey items swimming beneath the water's surface, such as insects or small aquatic organisms, while keeping an eye out for potential aerial predators.
3. **Adaptability**: The Anableps' visual system allows it to adapt to different environments. It can easily transition from hunting underwater to grazing on floating insects at the water's surface without the need to reposition its eyes.

In conclusion, the Anableps, commonly known as the four-eyed fish, possesses a unique eye arrangement that creates the illusion of having three eyes. This adaptation enables the Anableps to simultaneously monitor both the underwater and above-water environments, providing it with increased predator awareness and prey detection capabilities.

The lizard-like reptile tuatara has a "well-developed parietal eye, with small lens and retina".[2][3] Parietal eyes are also found in lizards, frogs and lampreys, as well as some species of fish, such as tuna and pelagic sharks, where it is visible as a light-sensitive spot on top of their head.