BioGalleries Eye To Eye
How many eyes can one animal have?
Article and Photographs by David Denning

It may come as a surprise, but not all animals have two eyes. True enough, most familiar animals ARE "binocular", but you don't have to go very far to find out that TWO does not always RULE.

Pineal Eye
Pineal Eye

Four Legs and A Third Eye?

It's pretty common knowledge that vertebrate animals have two eyes, but this was not always the case. Our earliest fish-like ancestors, evolved with a third hole in their skulls and an eye that connected directly to the brain called the pineal eye. It appears that this adaptation became converted over time to a small organ associated with the control of hormone production — the pineal body or gland. But in tadpoles, other amphibians, and some of the lizards, among them the Tuatara, a living relic lizard of the Sphenodont group, a third centrally located eye has remained although the function is not fully understood. It is sensitive to changes in light and dark, but it does not form images, having only a rudimentary retina and lens. It is visible as an opalescent gray spot on the top of the animal's head. Also, some snakes have an "extra" pair of "eyes" (more correctly, sensory organs) located on the forehead that can detect infrared radiation. They can "see" the heat of a mouse from a meter away, even in conditions where our eyes would see only pitch dark.

Jumping Eyes:

The visual ability of Jumping Spiders far exceeds that of most of its relatives or any other creature of equal size. Jumping spiders are active hunters, not the passive wait-until-it-comes-to-you predators like web-weaving spiders. When they see a prey from a distance, they can jump over 20 times their body length to pounce on an insect or another spider.

Such an active lifestyle requires vision that excels. Little wonder then that natural selection favoured jumping spider ancestors with better visual systems — in this case with multiple sets of eyes. Jumping spiders use their 8 eyes to gauge distance and detect moving prey. Four long tubular eyes on the face form very sharp images but have a narrow field of view. In order to focus on objects at different distances and directions, the jumping spider has small muscles that move the retina (where the image forms) and not the lens which is fixed in place. Jumping spiders also have four eyes on the top of the head — 2 toward the back and 2 toward the front. These eyes effectively broaden the spider's field of view, acting mostly as motion detectors and do not form sharp images.

Some types of spiders use auxiliary eyes to detect polarized light. This helps in navigation and guides them back to their nest after a day of hunting.

Jumping Spider
Jumping Spider

SEM image by Rachel Robson
SEM image by Rachel Robson
Jellyfish
Jellyfish

Jelly Eyes:

A jellyfish or medusa is one of the simplest animals. It is equipped with a loose network of nerve cells called a nerve net, and it lacks a major concentration of nerve cells that could be called a brain. Nevertheless, a medusa, such as this Polyorchis, can use it's nervous system to respond to environmental conditions and adjust its behaviour to benefit food-getting or predator-avoiding. To observe the light in its environment it has simple eyespots at the base of each tentacle, each shielded by a patch of bright red pigment. The shield casts a shadow on the eyespot which helps the animal to determine its orientation and the surrounding light conditions.

An Army of Eyes:

Poke around any rocky seashore and you are likely to see a sea star, a member of the Phylum Echinodermata, the spiny-skinned animals. But will the sea star see you? Maybe, but probably not too clearly. In addition to spines on its skin and a wonderful adornment of other structures (see our gallery - Beauty is Skin Deep) sea stars have a small eyespot at the end of each arm. It's not a sophisticated eye, and probably capable only of detecting light direction and large shadows, but it is useful in helping the sea star to orient itself. While most sea stars have 5 arms, and thus 5 eyespots, one common North Pacific species, Pycnapodia helianthoides, regularly has 21 arms, and species in other parts of the world may have 40 or more eyed arms.

Sea Star Eyes
Sea Star Eyes
Scallops
Scallops

Brimming With Eyes:

When we think of eyes and vision, clams are probably one of the last creatures that come to mind. But for some types of clams that live on the surface of the ocean floor, rather than buried beneath it, eyes can have a vital function for protecting against predators. In these animals, eyes have evolved that are effective motion detectors, and since the bivalve is a flattened circular creature, it is beneficial to have eyes all around the margin in order to see problems coming in from any direction.

Scallop Eyes
Scallop Eyes

Scallops, the bivalve with the pretty ridged shell and delicious muscle, take the prize as one the animal kingdom's most "eyed" creatures. A single scallop can possess over a hundred eyes! The mantle is lined with small, blue eyes. Each eye has a lens and a retina which is attached to a branch of the optic nerve. Behind the retina is a reflective layer called a tapetum that bounces light back to the retina, enhancing the signal. The hundred or so eyes work together to alert the animal to changes in light and motion. Changes in light and motion and very rough images are registered on the retinas of the mollusc.

If you scuba dive in the coastal waters of the Pacific Northwest, you may experience the effectiveness of scallop eyes for predator avoidance. Should you cast a shadow over a collection of scallops attached lightly to the bottom, the scallops will often pull up their anchors and swim away awkwardly by flapping their two shells together. It's not only awkward — the scallops only swim a few meters at most — it's also hilarious, as the scallops look for all the world like sets of false teeth jouncing around the intruding divers.

Perhaps the champion of all in the "eyes have it" category is the giant clam of the South Pacific, Tridachna gigas. These bivalves can have thousands of small eyes dotting the edge of the mantle. However, the eyes are relatively simple photoreceptors, not the sophisticated lens-bearing eyes of the scallop.

 
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