| 1. |
Harbor seals have large eyes. Their vision under water is better than a human's, but inferior on land. Lenses are enlarged and almost round, adapted for focusing on light that is refracted upon entering water. The lenses are not as well-adapted for sight in air.
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Harbor seals have large, round eyes that are adapted for sight in dark and murky water.
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| 2. |
Harbor seals' eyes are adapted for sight in dark and murky water.
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Like the eyes of other pinnipeds, harbor seals' eyes contain high numbers of rod cells - photoreceptor cells that are sensitive to low light levels. |
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Harbor seals have a well-developed tapetum lucidum, a layer of reflecting plates behind the retina. These plates act as mirrors to reflect light back through the retina a second time, increasing the light-gathering ability of the rod cells. (The tapetum lucidum is the same structure that makes a cat's eyes appear to "glow" when reflecting light at night.) |
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Under water, the pupils dilate (expand) into a wide circle to let in as much light as possible. In bright light, the pupils constrict to a slit. |
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| 3. |
Mucus continually washes over the eyes to protect them. Unlike most land mammals, pinnipeds lack a duct for draining eye fluids into the nasal passages. When a harbor seal is out of the water, mucus surrounding the eyes gives them a wet, "tear-rimmed" look. |
| 4. |
Good vision does not seem to be essential to harbor seal survival; scientists have found blind but otherwise healthy individuals, including mothers with pups, at sea. |
| 5. |
Harbor seals probably do not have color vision. |
