There are a wide variety of organisms living in the ocean. To date, over 250,000 species have been identified by marine biologist and new organisms are constantly being discovered. Marine organisms range in size from the largest living creature, the blue whale down the smallest algae and bacteria that can only be seen under a microscope.
The sunlit surface waters of the ocean are home to most marine creatures. Marine algae need the sunlight to photosynthesise and these are the basis of the ocean food chain as they provide food either directly or indirectly to most sea creatures. The majority of marine organisms live near the surface as that is where the algae on which they feed are found.
The classification of marine creatures is based on how they move and where they live. There are three main classifications – plankton that float, nekton that swim and benthos that live at the bottom of the sea. Most ocean creatures live near coastlines and where there are areas of upwelling such as coral reefs and atolls and near mid-ocean ridges.
The organism that are classified as plankton are those that drift in the ocean currents, these can be bacteria, animals or algae. Although plankton are classified as floaters even if they can swim as the ones that can swim can usually only move vertically or their movements are very weak.
Plankton are further classified into several sub groups. Phytoplankton are those that are autotrophic and can use photosynthesis to produce their own energy from the sun, the algae, and are mostly microscopic. They are also important as they take excess carbon dioxide out of the sea and add oxygen to the atmosphere. Phytoplankton come in many different types including coccolithophores, diatoms, dinoflagellates and picoplankton.
Zooplankton are the animal plankton and includes the larval stage of many marine creatures like crabs, lobsters, sea stars and fish. These are the heterotrophic plankton that eat the phytoplankton. Each type of zooplankton can only live in certain areas of the oceans as they have different salinity and temperature requirements.
There are also about 20,000 species bacteria throughout the oceans of the world and can be both planktonic and benthic. Billions of years ago, bacteria are thought to have been the start of life in the oceans. Now they help organic matter to decompose in the ocean and are important for the health of marine creatures and biochemical cycles.
Another abundant life form in the ocean are the viruses. This microplankton acts the same way as viruses on land and invade the cells of the living organisms. Some viruses are not destructive and may have an effect on nutrient cycling and are part of the biodiversity of the oceans.
These are the organisms that swim or use other ways of getting around under their own power rather than having to rely on the ocean currents. Nekton often migrate for long distances and know where they are in the ocean. Marine reptiles, marine mammals, squid and most adult fish are nektons. Most nektons have an internal skeleton, brain and spinal columns making them vertebrates.
Fish may seem to be everywhere in the oceans but they actually live in colder waters near islands and continents. Some fish don’t spend their whole lives in the oceans, salmon, for example, swim into freshwater rivers and go upstream to spawn whilst eels live in fresh water until they mature and then swim out of the streams into the depths of the ocean to breed.
Organisms in this group live on or in the bottom of the ocean. The coastal ocean floors are shallow and provide a variety of habitants and amount of nutrients. This is where most of the organisms in this group live. The shallow coastal areas are the only places with enough sunlight for large marine algae or seaweed to live.
In the deepest parts of the ocean floor no sunlight ever penetrates so it is not possible for plants to photosynthesise. So the animals that live here must eat each other or any nutrients that float down from the surface. Because there is only darkness, stillness and coldness at the bottom of the ocean, the pace of life is very slow and the distribution of creatures is very wide.