Marine Biology Vocabulary in English

Phytoplankton are microscopic photosynthetic organisms — essentially tiny plants and plant-like algae — that float near the surface of the ocean and use sunlight, water, and carbon dioxide to produce energy. They form the foundation of almost all marine food chains and are responsible for producing roughly half of all the oxygen in Earth’s atmosphere. Zooplankton are microscopic animals that drift in ocean water and feed on phytoplankton (and on each other). They are the vital link between the primary producers (phytoplankton) and larger animals such as fish, seabirds, and whales. Both groups are collectively called plankton, from the Greek word for “wandering.”

Coral reefs are built by tiny colonial animals (coral polyps) that live in a mutualistic symbiosis with microscopic algae called zooxanthellae. These algae live inside the coral’s tissues, providing it with food through photosynthesis and giving it its colour. When corals are stressed — most commonly by water that is too warm, but also by pollution, disease, or changes in salinity — they expel the zooxanthellae, turning white. This is bleaching. If the stress is short-lived, the algae can return and the coral recovers. But if the stress continues for too long, the coral starves and dies. Mass bleaching events driven by climate change have killed large sections of reefs worldwide.

The ocean absorbs about 30% of the carbon dioxide (CO&sub2;) released into the atmosphere by human activities. When CO&sub2; dissolves in seawater it forms carbonic acid, which lowers the pH of the water — a process called ocean acidification. Since the industrial revolution, ocean pH has fallen from about 8.2 to about 8.1, which represents a 26% increase in acidity. This threatens organisms with shells or skeletons made of calcium carbonate — including corals, oysters, mussels, and many types of plankton — because carbonic acid dissolves calcium carbonate. If acidification continues, it could disrupt entire marine food webs and fishing industries.

Bioluminescence is the ability of living organisms to produce and emit light through chemical reactions. It is remarkably widespread in the ocean, particularly in the deep sea where sunlight never reaches. Many deep-sea fish, squid, jellyfish, and bacteria are bioluminescent. The light is produced by a chemical reaction involving a molecule called luciferin and an enzyme called luciferase. Marine animals use bioluminescence for a range of purposes: to attract prey (like the anglerfish’s glowing lure), to confuse predators (by producing a blinding flash), to communicate with potential mates, or to provide camouflage by matching the dim light from above.

A keystone species is one that has a much larger effect on its ecosystem than its abundance alone would suggest. Remove it, and the ecosystem changes dramatically, often collapsing or being transformed. The term comes from the keystone at the top of an arch — remove it and the whole structure falls. A classic marine example is the sea otter. Sea otters eat sea urchins; sea urchins eat kelp. Without otters, urchin populations explode and devour the kelp forests, destroying the habitat of hundreds of other species. The concept of keystone species has been important for conservation because it shows that protecting certain species can save entire ecosystems.

The pelagic zone is the open ocean — the water column away from the coast and the ocean floor. Pelagic organisms such as tuna, sharks, and jellyfish live and feed in this open water. The pelagic zone is divided into depth-based sub-zones from the sunlit surface (epipelagic) down through twilight zones to the pitch-black hadal trenches. The benthic zone is the seafloor and the layer of water just above it. Benthic organisms such as sea urchins, starfish, crabs, worms, and many types of coral live on or in the sediment. Both zones are interconnected — organic matter from the pelagic zone sinks to the benthos as “marine snow,” feeding benthic communities.

Upwelling is an oceanographic phenomenon in which winds push surface water away from the coast, causing cold, nutrient-rich water from the deep to rise and replace it. Cold deep water is rich in nutrients such as nitrate and phosphate that have sunk from the surface in dead organic matter. When this water rises to the sunlit surface, it fertilises massive blooms of phytoplankton, which in turn support abundant populations of fish, seabirds, and marine mammals. The Humboldt Current upwelling system off the coast of Peru and Chile is one of the world’s most productive fishing regions, accounting for a disproportionate share of the global fish catch despite covering a small area.

A dead zone (technically called a hypoxic zone) is an area of the ocean where dissolved oxygen levels are so low that most marine life cannot survive. They are usually caused by eutrophication: excess nutrients — typically nitrogen and phosphorus from agricultural fertilisers or sewage — flow into the ocean, causing explosive blooms of algae. When the algae die and decompose, bacteria consume huge amounts of oxygen in the process, leaving the water severely depleted of oxygen. Dead zones have been growing in number and size worldwide; the largest one forms seasonally in the Gulf of Mexico, driven by nutrient runoff from the Mississippi River basin.

A marine protected area (MPA) is a defined region of the ocean where human activities are restricted or managed to protect marine life, habitats, and ecosystems. MPAs vary widely in the level of protection they offer, from “no-take” zones where all fishing and extraction are banned to areas with limited sustainable fishing allowed. Research shows that well-designed and properly enforced MPAs can significantly increase fish biomass, restore habitat, and enhance biodiversity within their boundaries — and often benefit fishing in adjacent areas by allowing populations to recover and spill over. The international target agreed at the 2022 COP15 biodiversity summit is to protect 30% of the world’s oceans by 2030.

Symbiosis describes a close, long-term relationship between two different species. In the ocean, there are many striking examples. Mutualistic symbiosis benefits both partners: the clownfish and sea anemone are a famous example — the clownfish gains protection from the anemone’s stinging tentacles and in return drives away the anemone’s predators. Coral and zooxanthellae algae are another: the algae photosynthesise and provide the coral with energy; the coral provides the algae with shelter and nutrients. Parasitic symbiosis benefits one species at the expense of the other, such as sea lice infecting salmon. Understanding symbiosis is fundamental to understanding how marine ecosystems function.