The Living Sea    

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Model showing the relative depths of the ocean zones

From the seashore to the deepest depths, oceans are home to some of the most diverse life on Earth. There are big animals and little ones; long and short ones, multicolored and drab ones, and those that just sit while others that never stop swimming. There are even some organisms that light up.

Oceanographers divide the ocean into five broad zones according to how far down sunlight penetrates:

  • the epipelagic, or sunlit, zone: the top layer of the ocean where enough sunlight penetrates for plants to carry on photosynthesis.
  • the mesopelagic, or twilight, zone: a dim zone where some light penetrates, but not enough for plants to grow.
  • the bathypelagic, or midnight, zone: the deep ocean layer where no light penetrates.
  • the abyssal zone: the pitch-black bottom layer of the ocean; the water here is almost freezing and its pressure is immense.
  • the hadal zone: the waters found in the ocean's deepest trenches.

Plants are found only in the sunlit zone where there is enough light for photosynthesis, however, animals are found at all depths of the oceans though their numbers are greater near the surface where food is plentiful. Still, over 90 percent of all species dwell on the ocean bottom where a single rock can be home to over ten major groups such as corals, mollusks and sponges.

To the left is a scale model of the ocean zones, from the warm sunlit waters of the surface to the cold dark depths of the Mariana Trench, the deepest point in the oceans at 36,200 feet (11,033 m). Notice how shallow the sunlit zone is compared to the midnight or abyssal zones.

Dive deeper into the living sea by continuing your explorations below.

Life Near the Surface

Most of the living things in the ocean are in a very small portion near the surface. Nearly all marine life depends directly or indirectly on microscopic algae found only at the ocean surface. Therefore, most of the animals in the ocean live in the sunlit zone or migrate to it in search of food.

Predators and Prey

Some animals eat only plants; they are called herbivores. Animals that eat meat are called carnivores. Omnivores are animals that eat both plants and animals. No matter what animals eat, all their food can be traced back to the ability of plants to produce organic material from the energy of the sun.

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