In order to be so efficient in the water, seals have to be able to see where they are going. When we go underwater and open our eyes, our vision is blurred, but perfectly clear while on land.
If seals saw like us, they wouldn’t be able to hunt and survive, so their eyes have some special adaptations like flattened, strengthened corneas and pupils that improve their sight while submerged.
Their eyes have to be able to focus in both air and water, this is why their eyes appear so large and round. You will notice on any pups that you see in the media that their eyes look so large on their face, to allow as much light in as possible. When at depth their pupils dilate (expand) into a large circle to let as much light in as possible. In bright light, the pupils constrict to a slit. Their vision underwater is better than humans, because their lenses are enlarged and round, adapted for focusing on any light that is refracted upon the water.
Seals eyes also contain a high number of photoreceptor cells that are rod cells. Rods are responsible for low light vision (scotopic vision). They do not coincide with colour vision and have a low spatial acuity.
These rods work in conjunction with a layer of reflecting plates behind the retina. These reflecting plates are used like a mirror and ensure light rebounds back through the retina again, increasing the light capturing ability of the rod cells. If you think of your cats’ eyes at night and how they shine, this relates back to reflecting plates in their eyes.
In the past, it was thought that seals were completely colour blind due to the high number of rod cells within their eyes. Recent studies have shown through the presentation of different colour stimuli, that seals cannot interpret the difference in isoluminant colours (same brightness). However, presenting seals with objects the same colour or different colours but with different brightnesses, the seals were able to distinguish between the two, meaning they have a level of colour vision, as long as the colours are not isoluminant. This supports the seals’ rod based vision.
When we look at dry seals on land, they have the appearance of tear stained eyes. Unlike most mammals, seals lack a duct for draining eye fluids into the nasal passages. To protect their eyes a mucus coating continually washes over them, this mucus surrounds the eye when out of the water, which gives that tear-stained appearance. A protective third eyelid is used to produce this tear film, remove sand to protect the eyes
Next time you’re out diving, check how well you can see without your mask.