Marine organisms can be classified taking into account the area of the ocean they occupy.
Species associated with the seabed are called benthic species or benthos. Theones that live on the sediment are parto f the epifauna, but if they live in the sediment they are part of the infauna.
The species that occupy the pelagic domain are divided into two big groups – nekton and plankton. Nekton includes all active swimmers such as fishes (e.g. hammerhead shark Sphyrna zygaena), reptiles (e.g. loggerhead turtle Caretta caretta) and cetaceans (e.g. northern bottlenose whale Hyperoodon ampullatus).
Plankton encompasses living beings with little or no motility, although some organisms manage to swim vertically. Photosynthetic organisms are called phytoplankton, while non-photosynthetic organisms are called zooplankton.
There are several species that vary in their life cycle between pelagic and benthic stages. Na example is the duskin grouper (Epinephelus marginatus) – presents a pelagic larval stage that lasts about 30 days.
Adults of the species spend most of the time associated with the seabed.