What Is Marine Life? An Overview of Ocean Ecosystems

Marine life encompasses the variety of organisms that inhabit the planet’s saltwater environments. This biodiversity ranges from microscopic, single-celled organisms to the largest animal ever to have lived, the blue whale. Oceans provide approximately 90% of the Earth’s living space by volume, serving as a cradle for the earliest known life forms. The health of these underwater realms is connected to the stability of life on land, influencing global weather patterns and the air we breathe.

Classifications of Marine Organisms

The diversity of life in the oceans is organized by scientists into groups based on shared biological traits. This classification reveals a spectrum of adaptations for survival in saltwater environments and helps in understanding the evolutionary paths that allowed life to flourish.

Marine mammals are warm-blooded, air-breathing animals adapted to an aquatic existence. This category includes cetaceans, such as whales and dolphins, which spend their entire lives in water and use communication methods like echolocation. Pinnipeds, including seals and sea lions, are recognized by their flippers and ability to live both in water and on land. Other mammals, like sea otters and manatees, also inhabit marine environments.

Fish represent the most diverse group of vertebrates in the ocean and are divided into two main categories. Bony fish, like tuna and clownfish, possess skeletons made of bone and are varied in their shape and function. Cartilaginous fish, including sharks and rays, have skeletons made of flexible yet strong cartilage.

Marine reptiles are another group, comprising species tied to both aquatic and terrestrial environments. Sea turtles, for example, roam vast ocean basins but return to specific beaches to lay their eggs. Sea snakes, marine iguanas, and saltwater crocodiles are other reptiles adapted to a life in or near the sea, with unique traits for swimming and feeding in saltwater.

Over 90% of animal species in the ocean are invertebrates, meaning they lack a backbone. This category includes a wide array of organisms that are foundational to many ecosystems. Examples of marine invertebrates include:

• Corals, which build complex reef structures
• Jellyfish and mollusks, such as the octopus and shelled clams
• Crustaceans, such as crabs and lobsters
• Echinoderms, like starfish and sea urchins

At the foundation of marine ecosystems are microorganisms, including plankton and bacteria. Plankton are organisms that drift with ocean currents and are divided into phytoplankton (plant-like) and zooplankton (animal-like). Phytoplankton produce a significant portion of the Earth’s oxygen through photosynthesis, while marine bacteria break down organic matter and recycle nutrients.

Marine Habitats and Ocean Zones

The ocean is a complex mosaic of different habitats, each with unique physical and chemical characteristics that shape the life within them. These habitats are broadly divided into coastal areas, where land meets the sea, and the open ocean. The marine environment is also categorized vertically into zones based on depth and sunlight penetration.

The uppermost layer, from the surface to about 200 meters, is the Sunlight Zone (Epipelagic), where enough light is present for photosynthesis. This zone is the warmest and supports the highest concentration of marine life, including phytoplankton, fish, and marine mammals. Below this is the Twilight Zone (Mesopelagic), extending from 200 to 1,000 meters, where sunlight is faint. Life here is adapted to low light, and many species produce their own light through bioluminescence.

The Midnight Zone (Bathypelagic), from 1,000 to 4,000 meters, is a region of complete darkness, high pressure, and near-freezing temperatures. Organisms in this zone have slow metabolisms and rely on “marine snow”—a shower of organic detritus from the upper layers—for food. Below this lie the Abyssopelagic Zone (4,000 to 6,000 meters) and the Hadopelagic Zone (below 6,000 meters) in deep-sea trenches. Life in these depths is adapted to withstand immense pressure and the absence of light.

Coastal and shallow water habitats are among the most productive marine environments. Coral reefs, found in warm, clear waters, are diverse ecosystems built by tiny coral polyps. Kelp forests thrive in cool, nutrient-rich waters, creating underwater canopies that provide food and shelter. Estuaries form where rivers meet the sea, creating brackish water nurseries for many fish and invertebrates, while mangrove forests stabilize coastlines.

The open ocean and deep sea contain their own unique habitats. The pelagic zone is the vast expanse of water away from the coast and seafloor. The abyssal plains are flat, deep regions of the ocean floor that support life adapted to high pressure and cold temperatures. Deep-sea hydrothermal vents form around geothermally heated water, where life is sustained by chemical energy in a process called chemosynthesis, not sunlight.

Ecological Roles Within Marine Food Webs

The connections between different marine organisms are understood through the food web, which illustrates the flow of energy and nutrients. Each organism occupies a specific position, or trophic level, based on its feeding habits. These roles create a balanced system where energy is transferred from one life form to another.

At the base of most marine food webs are producers, primarily phytoplankton, algae, and some bacteria. Through photosynthesis, these organisms convert sunlight into organic matter, forming the primary source of energy for the entire marine community.

The energy from producers is transferred to consumers, which are categorized into levels. Primary consumers are herbivores that feed on producers, such as zooplankton consuming phytoplankton. Secondary consumers are carnivores that prey on primary consumers. Higher up the web are tertiary consumers, which are larger predators, and at the top are apex predators, such as orcas or great white sharks, which have few natural predators.

Keystone species have a disproportionately large effect on their environment relative to their abundance. The presence or absence of a keystone species can alter the structure of an ecosystem. For instance, sea otters in kelp forests prey on sea urchins; without otters, urchin populations can explode and decimate the kelp, leading to the collapse of the forest habitat.

Marine ecosystems are also characterized by symbiotic relationships, where different species live in close association. In mutualism, both species benefit, such as the relationship between clownfish and sea anemones. Commensalism is a relationship where one species benefits and the other is unaffected. In parasitism, one organism, the parasite, benefits at the expense of the host.

Human Impact on Marine Ecosystems

Human activities have a growing influence on marine life and habitats. These impacts range from direct exploitation of marine resources to widespread changes in the chemical and physical properties of the oceans. The consequences disrupt food webs and threaten biodiversity across all marine ecosystems.

Pollution is a stressor on marine environments. Plastic pollution is a pervasive problem, with animals becoming entangled in debris or ingesting smaller plastic particles. Chemical pollutants, including pesticides and industrial waste, enter the ocean from land-based sources. Nutrient runoff from agriculture can lead to eutrophication, where excess nutrients cause algal blooms that deplete oxygen, creating “dead zones” where most marine life cannot survive.

Overfishing and destructive fishing practices have led to the decline of many fish populations. When fish are removed from the ocean faster than they can reproduce, their populations can collapse, which has cascading effects on the food web. Some fishing methods, such as bottom trawling, are damaging as they involve dragging heavy nets across the seafloor, destroying habitats like coral reefs.

The consequences of climate change are affecting the oceans. As the ocean absorbs excess heat from the atmosphere, its temperature rises, leading to coral bleaching, where corals expel the symbiotic algae living in their tissues and turn white. The ocean also absorbs a significant amount of carbon dioxide, which causes ocean acidification. This change in water chemistry makes it more difficult for organisms like corals and clams to build their shells.