Marine life, which encompasses all organisms inhabiting saltwater environments, is categorized into three major ecological groups based primarily on their location and method of movement within the ocean. These classifications allow scientists to understand the distribution, roles, and interconnectedness of species across the marine biome, from the sunlit surface waters to the deepest trenches. The three classifications that define all marine organisms are Plankton, Nekton, and Benthos. These groups describe the drifting life, the powerful free-swimmers, and the organisms associated with the sea floor, respectively.
Plankton: The Drifting Base of the Food Web
Plankton are organisms unable to swim effectively against ocean currents; their movement is primarily dependent on the flow of the water. Although often microscopic, this category includes larger species like jellyfish, which drift passively. Plankton form the base of the marine food web, supporting nearly all other life in the ocean.
Plankton are split into two groups based on how they obtain energy. The plant-like group, known as Phytoplankton, are autotrophs that perform photosynthesis, converting sunlight into energy. They include organisms such as diatoms and dinoflagellates, and their photosynthetic activity is responsible for generating a large portion of the oxygen in the Earth’s atmosphere. Because they require light, Phytoplankton are restricted to the photic zone, the upper layer of the ocean where sunlight penetrates.
The second group, Zooplankton, consists of animal-like consumers that feed on phytoplankton and other smaller zooplankton. This group includes tiny crustaceans like copepods, as well as the larval stages of many larger organisms, such as fish and sea stars. Zooplankton often undergo a daily vertical migration, moving to deeper waters during the day to avoid predators and rising to the surface at night to feed. Their consumption establishes the flow of energy through the marine ecosystem.
Nekton: Powerful Swimmers of the Water Column
Nekton are marine organisms that engage in strong, sustained locomotion, allowing them to actively swim against ocean currents and control their position. This independent movement separates them from the drifting plankton. Their bodies are adapted for mobility, often featuring streamlined shapes, powerful tail fins, and strong musculature.
These organisms primarily inhabit the pelagic zone, the open water region, and can be found at all depths. The Nekton category encompasses a diverse range of animals across multiple phyla. Examples include most fish, such as pelagic species like tuna and mackerel, and cartilaginous fish like sharks.
Marine mammals, including whales, dolphins, and seals, are classified as Nekton due to their powerful swimming capabilities. Invertebrates such as cephalopods, like squid and cuttlefish, are also strong swimmers that use specialized funnels for jet propulsion. Their ability to migrate across vast distances and pursue prey makes them important predators and consumers within the marine food web.
Benthos: Organisms of the Sea Floor
Benthos refers to the community of organisms that live on, in, or are closely associated with the sea floor, known as the benthic zone. This classification applies to species found from the shallowest intertidal zones down to the abyssal depths. Their physical relationship with the substrate—whether sand, mud, or rock—is the common link among all benthic life.
Benthic organisms are categorized into two lifestyle types based on their movement. The first type, Sessile Benthos (Epifauna), includes organisms permanently attached to the sea floor surface or those that move very slowly across it. Examples include corals, sponges, and sea anemones.
The second type, Motile Benthos, is comprised of organisms that live buried within the sediments of the ocean floor (Infauna) or crawl along the surface (Epifauna). Infauna examples include clams, marine worms, and burrowing crustaceans, which create tunnels and mix sediment layers. Other Motile Benthos, such as crabs, lobsters, and sea stars, are surface crawlers. Benthos play a role in nutrient cycling by processing organic matter that sinks from the water column, often called marine snow.