Animal Vision: Gazing into the abyss

Dr Ilse Daly continues her series with a look at sight in the deepest reaches of the sea

Given that over 70% of our planet is covered in water, it is hardly a surprise that the oceans are the least explored corners of our world. Each excursion we make into the depths reveals strange new animals whose lives are almost unimaginably different from our own. In this article, I will highlight some of their weird and wonderful visual systems; nevermind outer space, aliens, it turns out, live right here on Earth.

The ocean is broadly divided into four zones: the sunlight zone (0-200m), the twilight zone (200-1,000m), the midnight zone (1,000-6,000m), and the sinisterly named abyssal zone (below 6,000m). As the names of these zones suggest, the deeper the water, the darker it gets. That is because light does not travel as well through water as it does through air.

Sunlight can penetrate pretty well through the first 200m of water, which is why this uppermost zone is referred to as the sunlight zone. Most of the fish and sea creatures that we are familiar with live in this zone. There is enough light for animals to be able to see their surroundings and for photosynthesis to occur. However, descend below 200m and there is almost no sunlight, certainly not enough to support photosynthesis. This is the twilight zone. Dropping even deeper still, past 1,000m and there is complete darkness. The sunlight bathing the surface of the ocean will never reach these depths. This is the midnight zone and it is completely devoid of sunlight. So, how do these light-environments affect the vision of the animals that live in them?

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