The global ocean, covering over 70% of the planet’s surface, is a vast biome hosting an immense diversity of life. Marine animals inhabit every depth and temperature, from surface waters to the deepest trenches. Creatures span an incredible range of sizes, from microscopic organisms forming the base of the food web to the largest animals on Earth. Ocean inhabitants are often classified by whether they possess a backbone or based on their ecological role and location.
Familiar Residents: Marine Vertebrates
Marine vertebrates are the animals most commonly recognized, all sharing the defining characteristic of having a backbone. This group includes four distinct classes that have successfully adapted to life in a saltwater environment. Marine mammals include fully aquatic species like whales and dolphins (cetaceans), and semi-aquatic pinnipeds such as seals and sea lions. These mammals are warm-blooded, breathe air through lungs, and possess adaptations like blubber for insulation.
The ocean is dominated by fish, which are separated into two main categories based on their skeletal structure. Bony fish (Osteichthyes) represent the majority of species, including examples such as tuna, cod, and salmon. These fish possess a rigid, bony skeleton, a protective gill cover called an operculum, and a swim bladder that regulates buoyancy.
Cartilaginous fish (Chondrichthyes) feature a skeleton made of cartilage. This class includes all species of sharks, skates, and rays, which rely on a large, oil-filled liver for buoyancy control instead of a swim bladder. Marine reptiles are air-breathing and cold-blooded, requiring them to live in tropical and subtropical waters. This group encompasses the seven species of sea turtles, numerous sea snakes, and the marine iguana of the Galápagos Islands.
The Majority: Diverse Marine Invertebrates
The majority of ocean animals are invertebrates, lacking a spinal column. This group encompasses over 95% of all described animal life, featuring an astonishing array of body plans and lifestyles. The phylum Mollusca contains soft-bodied animals that often secrete a hard shell for protection.
Mollusks include Bivalves, such as clams, oysters, and mussels, which are filter feeders characterized by two hinged shells. Cephalopods, like octopuses, squid, and cuttlefish, are among the most intelligent invertebrates, using jet propulsion and possessing advanced camouflage abilities. The phylum Arthropoda is characterized by a segmented body, jointed appendages, and a tough, external exoskeleton. Marine arthropods are predominantly crustaceans, a diverse group that includes crabs, lobsters, shrimp, and krill.
The phylum Cnidaria is defined by animals that exhibit radial symmetry and possess specialized stinging cells called cnidocytes. This group includes free-swimming jellyfish and sessile forms like sea anemones and corals. Corals secrete calcium carbonate structures, building the complex habitats of coral reefs. These reefs are maintained through a symbiotic relationship with photosynthetic algae, which provide essential nutrients.
The exclusively marine phylum Echinodermata includes animals like sea stars, sea urchins, and sea cucumbers. These organisms are recognized by their five-sided radial symmetry and their unique water vascular system. This system uses hydraulic pressure to operate tube feet for locomotion and feeding.
Life by Location: Plankton, Nekton, and Benthos
Beyond taxonomic classification, marine life can be grouped by its ecological role and position within the water column. Plankton are organisms defined by their inability to move against ocean currents. This category includes phytoplankton, which are microscopic plant-like organisms that use photosynthesis and are responsible for producing a large portion of the Earth’s oxygen. Zooplankton are the animal-like components, consisting of tiny crustaceans and the larval stages of many larger animals, feeding on phytoplankton and forming the foundational consumer level of the food web.
Nekton consists of organisms that are active swimmers, able to propel themselves independently of water movement. This group includes most adult fish, such as sharks and tuna, as well as all marine mammals and cephalopods like squid. Nektonic species can be found throughout the water column, often undertaking extensive migrations.
The third major ecological grouping is the Benthos, which are bottom-dwellers living on, in, or near the ocean floor. Benthic organisms are divided into infauna, which burrow into the sediment like clams and worms, and epifauna, which live on the surface, such as sea stars and crabs. This diverse community thrives in habitats ranging from shallow coastal seagrass beds to the deep-sea abyssal plains. Unique benthic communities also exist around hydrothermal vents, where life is supported not by sunlight but by chemical energy in a process called chemosynthesis.