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Zooplankton
Zooplankton are considered drifting animals that depend upon the ocean's current for movement. Because these animals have the inability to direct their own mobility, they are entirely dependent upon wind drifts and ocean currents for transportation about the ocean. Zooplankton can be of any size; some examples of animals in this category are jellyfish and the Portuguese man-of-war, which is a floating colony of four different types of polyps that can be seen floating on the ocean's surface.
Nekton
Nekton make up the second and largest animal population in the ocean. These animals are free swimmers and exist in three subcategories: chordates, molluscan and arthropods. Chordates are boney fish such as sharks, clown fish, tuna and whales. The molluscan group includes animals such as octopus and squid. An example of an arthropod is the shrimp family that can move freely throughout the ocean.
Benthos
Benthos animal life is the third and last category. Benthos consists of animals that spend their entire life on the ocean floor. Some examples of the animals in this category are lobsters, starfish, oysters and snails. The three subcategories of benthos are macrobenthos, meiobenthos and micorbenthos. Macrobenthos are larger than one millimete,r such as starfish, crabs and coral. Meiobenthos include animals that are between one tenth of a millimeter and one millimeter, such as sea worms. The last group, microbenthos, includes animals less than one tenth of a millimeter, such as bacteria.
Interesting Facts
Some ocean animal life, such as fish eggs and larvae, are temporary members of the zooplankton category, but when they mature they cross over to the nekton category. Additionally, each animal category is dependent upon the other for survival. Free swimmers and zooplankton both consume animals from the benthos category. And when animals of all categories die, their carcasses are consumed by animals in the benthos category as part of the natural process of decomposition.