How Kayalvizhi (The Fish's Eye) Looks inside Salty Water Without Tears?
Answer:
Red.
Fishes have neither nictitating membrane nor tear glands to protect their eyes.
Fishes such as marlins and tunas roll their eyes downward
into their sockets in order to protect their delicate corneas, this behaviour helps protect these fishes' eyes.
white sharks, which, like marlins and tunas, lack nictitating membranes that would help protect their eyes. They roll their eyes for protection far down into their sockets, leaving them functionally blind when preparing to bite.
Even in clear water only relatively close objects can be sharply focussed. But this is the environment, the eyes of fishes must cope with, and their adaptation to the medium is extremely ingenious. Their primary need is to see movements and nearby shapes- and this they do to perfection with their eyes set on the side of their head, many of the fishes can practically register every thing that moves around them at any time. Needing no eyelids or tear ducts in their liquid medium, they have evolved ways of coping with varying amount of light.
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.
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