Life exists throughout the ocean from the surface to the deep sea and from the polar regions to the tropics. Generally the species differ from region to region because the physical and chemical conditions differ. Thus there are distinct life zones.

Scientists have devised a wide range of instruments for studying these organisms. The most basic device for studying plankton is the simple plankton net. Variations of this basic design e.g. an opening-closing net have allowed scientists to distinguish plankton from various depths. Bongo nets allow scientists to collect replicate samples. And finally multiple net systems such as the MOCNESS allow scientists to collect samples from various depths on one deployment while at the same time collecting measurements of parameters such as temperature, salinity, depth, oxygen etc.

Very small plankton were also underestimated for many years because they slipped through the finest plankton nets. These small organisms must be collected in water bottles. The bottles may be arranged individually on a wire or grouped as part of a rosette system.

All of these devices are blindly and remotely operated and typically useful in the upper 1000 or so meters of the ocean. Sampling of the deep ocean has been difficult. Now manned submersibles such as the French vessel Nautile and remotely operated vehicles with camera systems enable researchers to see marine organisms in situ and observe the environment in which they live.

The oceans harbor a tremendous diversity of organisms ranging in size from microscopic bacteria to the enormous blue whale. Phytoplankton are responsible for most of the photosynthesis that takes place in the oceans. Some of the more important groups in this regard are the diatoms and dinoflagellates. These two groups are considered part of the Kingdom Protista. The key characteristics of protists are that they are unicellular and have a nucleus.

Much of the phytoplankton is consumed by copepods. These small crustaceans, often referred to as the 'insects of the sea' are are extremely abundant in the oceans both in terms of numbers and biomass. Copepods are important components of the food web. Ctenophores or "comb jellies" are also zooplankton. They are soft-bodied carnivores that capture their prey with the aid of tentacles or sticky cells. The common term 'jellyfish' refers to a different taxonomic group of organisms that resemble ctenophores in being jelly-like. These are the scyphozoans of the phylum Cnidaria. These animals also capture their prey with the aid of tentacles. However, they also possess stinging cells called nematocysts that disable the prey with a 'barbed hook' and poison.