The organisms that inhabit the World’s oceans can be classified in two ways. Firstly, and most obviously, by what type of organism they are; be they plant, fish or whale for example. Secondly they can be classified as to where they live in the ocean, for the oceans contain a range of habitats from the surface of the open ocean to the depths of the abyssal plain.
Beginning with the plant kingdom, true plants grow in shallow waters for they need sunlight for photosynthesis. Examples would be the sea grasses, such as eelgrass, which can form extensive ‘meadows on the ocean floor’. Seaweeds, which may resemble true plants, are macroscopic forms of algae. Most attach themselves to rocks but some are free floating, such as the genus Sargassum. The free floating microscopic organisms such as cyano-bacteria, various types of algae, and diatoms which also photosynthesize light and are the most abundant primary producers on Earth, are known as phytoplankton. Occupying the surface waters of the open ocean, the phytoplankton are the bottom of the food chain and the basis for all ocean life.
The microscopic and larger animal life that drifts in the open ocean is known as the zooplankton. Zooplankton contains organisms such as protazoa, but also the eggs and larvae of fish, crustaceans and mollusks. Zooplankton feed on the phytoplankton and upon each other. Some quite large animals, such as certain types of jellyfish are also included in the zooplankton as they drift with the tides and cannot propel themselves.
Next we come to the organisms classified as nekton. These are the free swimming organisms of the ocean. They may be small, such as the shrimp-like krill, or as massive as the blue whale. Invertebrates are represented by mollusks such as the octopus and squid, and crustaceans such as shrimps. There are no insects to be found living in the oceans. Vertebrates are represented primarily by fish, the bony fish such as cod and the cartilaginous fish such as sharks, but also by reptiles and mammals. Reptiles include various turtles and saltwater crocodiles. Mammals are represented by creatures such as whales and seals.
The organisms that occupy the ocean floor or the waters directly above it are known as benthos. Starfish, worms, various shellfish and snails are just some examples. The benthos can be further classified. The infauna live in the sediment of the ocean floor, whereas the epifauna live on the sediment, rocks or other substrates. The fish and other organisms that feed on the infauna and epifauna are known as demersal.
The main classifications of ocean life are therefore; true plants, seaweeds, phytoplankton, zooplankton, nekton and benthos. Various forms of fungi are also found in the ocean, playing a vital role in the decomposition process. All these groups can be further subdivided and many organisms occupy specialized ocean habitats, such as coral reefs or hydrothermal vents. The majority of the Earth’s oceans are yet to be explored by man and no doubt there are species of organism still to be discovered, still to be classified.