The sunlit, shallow waters of the ocean Although the oceans make up by far the largest biome on Earth, only about 20% of Earth’s species live in the oceans, of which about 90% are bottom-living, shallow-water species. With the abundance of microscopic plants in sunlit, shallow waters, taken together with nutrients washed out to sea from the land or brought to the surface by upwelling currents, coastal marine life is by far the most abundant of all ocean habitats. In the rest of the ocean, especially the zone below a depth of 1000 metres (3300 feet) where no light penetrates at all, life is extremely sparse.
There are two main ocean habitats: the water itself, the pelagic habitat, and the ocean floor, the benthic habitat. Both are divided into several zones, according to the amount of sunlight that reaches down through the water. Most life is concentrated in the upper 200 metres (660 feet) in the relatively shallow waters of the continental shelf, where plankton, a rich source of food, is plentiful.
Plankton seen through a microscope
In surface waters, called the epipelagic zone, sunlight passes through the water, allowing plankton to grow. Plankton is the name given to the many different types of tiny plants and animals—some too small to see without a microscope—that drift in surface currents. For many animals, including even the very largest whales, plankton is all they eat. Others prey on small plankton-eaters such as jellyfish, squid, or small fish, while still others prey on them.
A sperm whale diving through the twilight zone
In the mesopelagic zone, or “twilight zone,” (200–1000 metres / 650–3300 feet) far less light can get through to these depths. The amount of food available is much less so there are far fewer kinds of animals. Some live off plant and animal remains that have fallen from above, while others travel to and from the surface to feed. All are prey for deep-living sharks or deep-diving whales.
Bioluminescent creatures of the bathypelagic zoneBelow the twilight zone the water is as black as night and icy cold: the bathypelagic zone (1000–4000 metres / 3300–13,000 feet). How do animals survive at this depth? A steady supply of food rains down from above in the form of decomposing parts of dead animals and plants. Scavengers living in deep waters feed on these remains and other animals feed on them. Many deep-sea animals can create light, a feature known as bioluminescence. The lowest level of the pelagic zone, below about 4000 metres (13,000 feet) but above the ocean bed itself, is known as the abyssopelagic zone.
Creatures of the abyssal plain
On the abyssal plain itself there is a greater diversity of species compared with the waters immediately above. Some of the animals that live here are attached to the ocean floor and look more like plants. A few creatures, like sea cucumbers and brittle stars, creep about searching for dead animal remains in the mud. Deathly white sea spiders pick their way across the mud, known as ooze. They feed by sucking out the juices of burrowing worms.
Consultant: Chris Jarvis