Whales – Up Close

Our almost spherical eye, which is why it is also called an eyeball, has a diameter of approx. 2.5 cm and consists of three layers of tissue, as well as the lens and the vitreous body, which is filled with a gelatinous mass. The eyeball, surrounded by a lubricating fatty body, lies well protected in the eye sockets formed by the skull bones. The three layers of tissue form the wall of the eyeball. The first outer layer with the cornea and the sclera, the second with the choroid and iris and on the back part the third layer with the retina.

The outer dermis is whitish in color and is therefore also called “white eye skin”. If we look in the mirror, this white sclera can be clearly seen as the “white” in our eyes. It extends from the posterior entry point of the optic nerve, almost completely encloses the eyeball, merges into the transparent and avascular cornea at the front and has the task of keeping the eye in shape and protecting it. Between the dermis and the retina, on the second middle layer of tissue, lies the choroid, which, with its many blood vessels, supplies the adjacent layers with nutrients and oxygen. When we look in the mirror we can sometimes see the small red choroidal vessels shining through the “white” of our eyes.

A striking feature of the front section of the choroid is the iris, which is responsible for the color of our eyes. This is influenced by the pigment melanin. A high concentration of melanin results in brown eyes, while a lower pigment content makes the iris appear green, blue or gray, depending on the dosage.

In the middle of the iris is a round hole called the pupil. With the help of muscles, the iris can enlarge or shrink the pupil, thereby controlling how much light passes through the pupil onto the lens and the retina behind it.

When relaxed, our lens has the shape of an ellipse, is more flat and thin and in this shape regulates the sharpness of distance. Muscles that are connected to the lens and the eyeball enable us to change the shape of the lens through an interaction of tension and relaxation and thus create different degrees of curvature. When these muscles contract and the lens takes on a more rounded shape, we can see objects well at close range.

After being “focused”, the light rays bundled by the lens are passed on through the vitreous body to the inner tissue layer of the eyeball, to the retina, where an inverted, reduced image is initially created.

The retina then continues to work like an image editing program on the computer, because before the image is forwarded, the light receptors and the retinal nerve cells are responsible for displaying the movements in the image more clearly, improving the image contrast and making the colors brighter.

The retinal ganglion cells are also located in the retina, nerve cells whose projections act as “optic nerves” and carry the electrically encoded signals into the brain.

The light world ends at the retina.

Light therefore plays a very important role in our “seeing” so that we can orientate ourselves in our environment.

But despite their poor vision, the eyes of marine mammals are masterpieces of evolution. They have adapted excellently to the marine living conditions and we can discover amazing things in the eyes of the whales, because we can find cornea, sclera, lens, retina and much more, even tear fluid in their eyes.

Their protection is also adequately ensured, because the white outer skin of the eye, the dermis, is particularly thick in whales, almost rock hard, which is why it is also called the sclera, which is derived from the Greek “scleros = hard”, and the eye in front It is intended to protect against deformations, while the oily fluid from the tear glands protects the cornea.

Human eyes are almost spherical, although whale eyes are more like a hemisphere because they are noticeably flattened in the front area. 

But not only the sperm whale, but also we humans do not have this ability of a built-in negative contact lens, because we have all had the experience that with our eyes open underwater everything appears blurred and blurry to us.

The reason for this lies in the different optical densities of water and air. In order to create a sharp image for us humans, the light is refracted on the surface of the cornea, which then passes it on to the lens. This in turn directs it to the retina, to the yellow spot, the point of sharp vision. The distance between the object, the lens of the eye and the retina must be in a certain relationship to one another.

By curving the lens, we can change the focal length and thus focus the distance between the object and the lens. However, due to the fixed lens size of the eye, the distance between the eye lens and the retina can never be changed.

Due to the higher density of water, light is refracted and scattered when air enters water. It hits the cornea more differently than in the air. Our lens now has to adapt much more to focus on the distance between the object and the lens. However, it only succeeds to a limited extent and as a result the image that it transmits to the retina is no longer transported at the correct distance. The resulting image ends up behind the retina and we see it blurry.

The whales have solved this problem with the so-called negative built-in contact lens. If we want to see clearly under water, we would need glasses with 40 diopters or we could simply put on diving goggles. This creates the normal density of air in front of the cornea and our usual refraction of light. We see clearly again.

However, marine mammals are not only very nearsighted, they are even color blind compared to us humans and other mammals.

Around 127 million cells in the retina are responsible for our color and light vision. 120 million of these cells are so-called rods that are very sensitive to light and can only distinguish between light and dark. The other 7 million are called cones and are divided into three types. They respond to light in the blue, green and red wavelength range, which is why we also speak of trichomatic color vision. We are able to distinguish almost ten million shades from mixtures of these three basic colors.

Most mammals only have two types of cones, namely the blue and green cones. Some only have blue and red cones. This bichromatic color vision is, so to speak, the basic equipment of mammals. But two large groups of marine mammals, whales and seals, fall completely outside of this pattern.

Leo Peichl from the Max Plank Institute for Brain Research, Günther Behrmann from the Alfred Wegener Institute in Bremerhaven and Ronald Kröger from Sweden discovered an astonishing deficit in joint examinations of 14 different species of toothed whales, seals and sea lions. Whales and seals even lack blue cones. They only have one type of cone, namely the green cone. They are therefore cone monochromats and practically color blind.

For the sperm whales, the loss of the blue cones should be irrelevant anyway, because they spend most of their time diving and in the depths to which they penetrate, colors and light no longer play a major role.

However, in order to use the remaining light more efficiently even at shallower depths, the whales have developed a reflective mirror layer, the tapetum lucidum, which lies behind the retina. Undoubtedly, we are all familiar with the bright green eyes of a cat or dog when they are illuminated in the evening by the rays of a car lamp or the flash of a camera. This light-amplifying layer serves to capture the incident light and reflect it back, whereby the light passes through the retina again.

This back and forth improves light sensitivity because the sensory cells on the retina have a second chance to react to the incoming light quantum and thus utilize it better and more. Many nocturnal land mammals have this tapetum lucidum; the only difference in marine mammals is that theirs is a little grayer instead of green.