Anemone Fish

Due to the inherent visibility and charisma of our fellow land-dwelling animals, it is often easy to forget that vertebrate life, and therefore the vertebrate eye, originally evolved in the water. Indeed, even those organisms that migrated to dry land found ways to keep their eyes bathed in salt water. So, as the ocean is the birthplace of the eye, it is useful to examine the intricate adaptations needed to see in a watery world.

As anyone who has gone scuba diving or swam in a lake with eyes open knows — there is not as much light underwater. Not only does water absorb light incredibly faster than air, but also scatters it in the three dimensions. Thus, the remarkable vertebrate eye evolved in a rather darkened realm where pressures changes significantly as animals move up and down.

How do aquatic animals keep their eyes from collapsing or exploding? Eyes are kept firm by fluid pressure. The source of this pressure is osmosis, the single most important factor influencing the evolution of the vertebrate eye. In land-dwelling vertebrates (including humans) the eye fluid comes from the ciliary body, a combination of glandular tissue and muscle inside the eye at the edge of the lens. On the inner surface of the ciliary body is a series of projections that secrete the intra-ocular fluid, thus keeping the fluid pressure up inside the eye. The fluid is diffused into the blood or drained through a canal at the same rate it is produced, thus maintaining a constant ocular pressure.

Surgeon Fish

Fish do not have ciliary bodies, in part because fluid is all around them. To maintain water balance inside and outside the eye,freshwater fish absorb fluid into their eyes by osmosis directly through the cornea. Any excess salts from the eye are removed and excreted in their urine. Sharks maintain a high level of urea in their blood and eyes. This allows them to keep osmotic pressure inside their eyes higher than that of the surrounding salt water, thus keeping the eyes firm. Marine bony fishes have a lower salt concentration inside their eyes than the surrounding ocean. This means that the eye should theoretically shrink or collapse due to loss of water from inside the eye by osmosis. Somehow, fish manage to maintain fluid pressure inside their eyes despite this osmotic difference, but the mechanism is still a mystery.

Sculpin Fish

How are fish adapted to the optical properties of water? Water not only reduces the amount of light that penetrates a fish's world but also the kind of light. The extinction of sunlight in the water column happens very fast. On average, 90% of the light is gone by 10m depth, and 99% by 40m. Varying wavelengths of light are absorbed at different depths. UV light is eliminated in the first few millimeters of water; infrared in the first few centimeters. Of the wavelengths of visible light, the red wavelengths disappear in the first 5m or so, then the yellows. Eventually, at a depth of around 100m, all that remains is a narrow band of blue-green wavelengths (510nm-540nm).

There is complete darkness below about 200 meters. It makes good sense that fish eyes have maximum sensitivity for blue light (at about 520nm wavelength).

Fish visual pigments, called rhodopsins, are photosensitive protein pigments found in the rod cells of the retina. When light passes into the eye and reaches the retina, rhodopsin protein molecules become photo-excited. Biochemical changes in the structure of the molecule cause electrical impulses to run down the optic nerve sending information to the brain.

Rhodopsins, which are also found in all other vertebrate animals, are a dark reddish color. Like deep growing red algae, the rhodopsin pigment absorb the only wavelengths that penetrate the water column: greens and blues. Therefore, the rhodopsin, like the algae, reflects the light it does not absorb — which is generally the red wavelengths of light, causing its reddish color. Our own human rhodopsins are still red, a heirloom of our aquatic ancestry.

Why do fish have "fish-eyes" (bulging eyes)? The refractive index of a transparent substance is a measure of the relative speed of light in that medium. When light travels from one medium to another, such as passing from air into water, it slows down and changes direction. Everyone knows that objects in the water, when viewed from above the water, are not the same size or in the same place as they appear.

Bulging Eyes

Corneal tissue has the same refractive index as water. This means that the cornea of fish do not bend light as it enters the eye. In our eyes, light is slowed and bent as it moves through the cornea — the cornea acts like a lens to collect more light from a wider field of view. But the underwater cornea is "effectively absent", so light is first refracted at the lens of the fish eye. This means that the fish eye, already in a dark environment, will capture even less of the available light — less of the field of view. To compensate for this, the fish lens tends to bulge through the pupil allowing periscopic vision (180° field of view for each eye). Without bulging pupils or alternately a rotational neck, fish would have a very limited visual field.

Streamlining

How do fish streamline their eyes? It's not always beneficial for a fish to have a bulging periscopic eye. Friction from water moving over the eye has the potential to scratch the cornea and impair vision. Also, a bulging eye will experience high water pressure at its front edge and low pressure at the back surface when the fish is swimming rapidly. This could damage or deform the eye. Many fish have evolved a tradeoff between periscopy and a streamlined head that allows for faster swimming.

One adaptation involves altering the shape of the eye. An elliptical eye produces the least amount of friction and pressure. Sharks, among the fastest swimming fishes, have highly elliptical eyes. Another adaptation is vertical eyelids - thin transparent tissue that cover the cornea, thus reducing not only friction on the eye but also eddies that form around the eyes when the fish is swimming.