| Vision in Underwater Beings |
By Fabio Cupul -
April, 2005 - Leer Español
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The octopus, squid and crab have the most developed eyes of all invertebrates and of some of the biggest animals in the world. The eyes of a giant squid, which is 18 m long, have the size of an American football.
The structure of the eyes of these animals is similar to that of human beings, even though there are some differences. The human eye changes the form of the crystalline lens to achieve a precise focus whereas the eyes of an octopus or squid move forward or backwards, which is the same way how the focusing mechanism of a slide projector works. Just like the rest of the sharks, the ctopus and squid uses his senses in order to locate its prey. But vision is the most important of the senses when it is about determining precisely the location of the victim on the surface.
The same thing happens to a shark than to a person who turns blind when passing from a dark place to a room full of light. Both of them take a few seconds in order to adjust themselves to the change of intensity. A shark that comes up to the surface from the dark marine depths has to face a quick increase of light. He cannot wait for his eyes to adjust since the prey could escape in a second.
The secret of this skill lies in the tunic, a layer of membranes located in the back part of the eyeball. In cats the reason why their eyes become bigger when they are blinded by lights lies in the nictitate membrane (this is the layer that produces the shine in the eyes when light is projected in them and it helps them to absorb a bigger quantity of light during darkness). By reflecting the light in the retina, a shark doubles the amount of light that his eyes can endure. This is essential in order that night hunters, like the cat, can see with clarity, especially on nights without moonlight. During the day an excess of light could represent a real problem. Cats manage by reducing the size of the pupil with the help of movements of the iris.
The tunic of the white shark improves vision during darkness. This animal does not have an iris that protects the eyes when it swims towards the surface. In order to compensate the deficiency it has developed a curtain of cells containing pigments. Upon approaching a source of intense light, the cells expand themselves over the tunic and they contract themselves upon returning to the depths.
An eye fit to function in the air is almost useless underwater. In these conditions the cornea, which usually is curved and sends the image to the retina, finds itself flattened out by the pressure of the water. As a result of this distortion the cornea does not function as it should. That is why the majority of terrestrial animals cannot see underwater. However some birds indeed can see.
The tern and the pelican cannot see with clarity underwater, but they are able to make out fish from the air in order to let themselves get down and catch them with their beak. Nevertheless nature has provided the penguins and cormorants with a better solution of the problem. The cornea of the penguin is flatter than that of other birds and can function adequately in the air as well as in the water.
The cormorant for its part has soft crystalline lenses that can extend and contract without any difficulty through the action of the ocular muscles. The crystalline lens can deform itself to such a degree that it comes out of the disk of the iris and reaches the pupil. This modification increases the curvature of the crystalline lens in order to allow the registration of the image on the retina, even underwater. The oil engineers have observed some cormorants in the Northern Sea that swim up to a depth of almost 45 meters .
When the cormorant is above the surface, the crystalline lens is flattened by the ocular muscles so that the eye functions in the air. This permits it to hunt above water and underwater.
Nonetheless more than a pair of eyes is needed for an underwater vision. The scallop, a kind of clam, has a multitude of eyes, each one provided with two retinas sensible to the increase and decrease of the intensity of light.
Maybe the reasons for this complexity are related to the mobility of the organism, since these animals - unlike some of their relatives like the mussel and the oyster - can move with jet propulsion just by closing their valves and thus being powered by the water course that is produced.
Even though its eyes may detect different intensities of light, but not details of its surroundings (it does not see images), the scallop has enough capacity of perception in order to notice danger on time and close its valves.
Prof. Fabio German Cupul Magaña
Email: fcupul@pv.udg.mx
The publication of this article is possible thanks to the authorization of Prof. Fabio German Cupul Magaña, author of the books "Tales of natural history in Vallarta - University of Guadalajara" (Relatos de la historia natural vallartense - Universidad de Guadalajara) and "Natural environment: Selection of essays of scientific disclosure - Coastal University Center of the University of Guadalajara". (Ambiente natural: Selección de ensayos de divulgación científica - Centro Universitario de La Costa Universidad de Guadalajara.
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