Major Marine Habitats

Geographic zones and biomes that support marine life

Earth is nicknamed "the blue planet" because it looks blue from space. That's because about 70% of its surface is covered with water, 96% of which is ocean. The oceans are home to a number of marine environments ranging from the lightless, frigid deep seas to tropical coral reefs. Each of these habitats presents a unique set of challenges for the plants and creatures that inhabit them.

Mangroves

Pean Krasop Mangrove Sancturay. Cambodia.
Eitan Simanor / Photodisc / Getty Images

The term “mangrove” refers to a habitat comprised of a number of halophytic (salt-tolerant) plant species, of which there are more than 12 families and 50 species worldwide. Mangroves grow in intertidal areas or in swampy coastal estuaries, which are semi-enclosed bodies of brackish water (water containing more saline than freshwater but less than saltwater) fed by one or more freshwater sources that eventually flow out to the sea.

The roots of mangrove plants are adapted to filter saline, and their leaves can excrete salt, allowing them to survive where other land plants cannot. The mangroves' tangled root systems are often visibly exposed above the waterline, leading to the nickname “walking trees.”

Mangroves are an important habitat, providing food, shelter, and nursery areas for fish, birds, crustaceans, and other forms of marine life.

Seagrasses

A dugong and cleaner fish graze on seagrass off the coast of Egypt.
A dugong and cleaner fish graze on seagrass off the coast of Egypt. David Peart / Getty Images

Seagrass is an angiosperm (flowering plant) that lives in a marine or brackish environment. There are about 50 species of true seagrasses worldwide. Seagrasses are found in protected coastal waters such as bays, lagoons, and estuaries and in both temperate and tropical regions.

Seagrasses attach to the ocean bottom by thick roots and rhizomes, horizontal stems with shoots pointing upward and roots pointing downward. Their roots help stabilize the ocean floor.

Seagrasses provide important habitat to a number of organisms. Larger animals such as manatees and sea turtles feed on organisms that live in seagrass beds. Some species use seagrass beds as nursery areas, while others shelter amongst them for their entire whole lives.

Intertidal Zone

A tide pool along the coast of southern California is home to starfish, mussels, sea anemones, and much more.
magnetcreative / E+ / Getty Images

The intertidal zone is found on the shoreline where land and sea meet. This zone is covered with water at high tide and exposed to air at low tide. The land in this zone can be rocky, sandy, or covered in mudflats. There are several distinct intertidal zones, starting near dry land with the splash zone, an area that's usually dry, moving down toward the sea to the littoral zone, which is usually underwater. Tide pools, the puddles left in rock indentations as tidewater recedes, are characteristic of the intertidal zone.

The intertidal is home to a wide variety of organisms that have had to adapt to survive in this challenging, ever-changing environment. Species found in the intertidal zone include barnacles, limpets, hermit crabs, shore crabs, mussels, anemones, chitons, sea stars, a variety of kelp and seaweed species, clams, mud shrimp, sand dollars, and numerous species of worms.

Reefs

Bleached corals on the Great Barrier Reef
Sirachai Arunrugstichai / Getty Images

There are two types of corals: stony (hard) corals and soft corals. While there are hundreds of coral species found in the world’s oceans, only hard corals build reefs. It's estimated that 800 unique hard coral species are involved in building tropical reefs.

The majority of coral reefs are found in tropical and sub-tropical water within the latitudes of 30 degrees north and 30 degrees south, however, there are also deep water corals in colder regions. The largest and most well-known example of a tropical reef is the Great Barrier Reef in Australia.

Coral reefs are complex ecosystems that support a wide array of marine species and birds. According to the Coral Reef Alliance, "Coral reefs are believed by many to have the highest biodiversity of any ecosystem on the planet—even more than a tropical rainforest. Occupying less than 1% of the ocean floor, coral reefs are home to more than 25% of marine life."

The Open Ocean (Pelagic Zone)

Green turtle (Chelonia mydas) feeding on jellyfish. Juvenile mackerel still hides beside the jellyfish about to lose its home
Jurgen Freund / Nature Picture Library / Getty Images

The open ocean, or pelagic zone, is the area of the ocean outside of coastal areas. It's separated into several subzones depending on water depth, and each provides habitat for a variety of marine life including everything from larger cetacean species including whales and dolphins, to leatherback turtles, sharks, sailfish, and tuna to myriad forms of minuscule creatures including zooplankton and sea fleas, to otherworldly siphonophores that look like something straight out of a science fiction movie.

The Deep Sea

Ocean exploration off the coast of Cocos Island, Costa Rica
Jeff Rotman / Photolibrary / Getty Images

Eighty percent of the ocean consists of waters greater than 1,000 meters in depth known as the deep sea. Some deep-sea environments may also be considered part of the pelagic zone, but the areas in the deepest reaches of the ocean have their own special characteristics. While extremely cold, dark, and inhospitable, a surprising number of species thrive in this environment, including numerous varieties of jellyfish, the frilled shark, giant spider crab, fangtooth fish, six-gill shark, vampire squid, angler fish, and Pacific viperfish.

Hydrothermal Vents

An active hydrothermal vent chimney spewing out hydrothermal fluids. Ring of Fire 2006 Expedition. East Diamante Volcano, April 29, 2006.
Image courtesy of Submarine Ring of Fire 2006 Exploration / NOAA Vents Program

Hydrothermal vents, located in the deep sea, are found at an average depth of about 7,000 feet. They were unknown until 1977 when they were discovered by geologists aboard of the Alvin, a U.S. Navy manned research submersible that operates out of the Woods Hole Oceanographic Institution in Woods Hole, Massachusetts who had set out to study the phenomenon of undersea volcanoes.

Hydrothermal vents are essentially underwater geysers created by shifting tectonic plates. When these huge plates in the Earth’s crust moved, they created cracks in the ocean floor. Ocean water pours into these cracks, gets heated up by the Earth’s magma, and is then released through the hydrothermal vents, along with minerals such as hydrogen sulfide. Water exiting thermal vents can reach incredible temperatures of up to 750° F, but as improbable as it sounds, despite the extreme heat and toxic substances, hundreds of marine species can be found in this habitat.

The answer to the conundrum lies at the bottom of the hydrothermal vent food chain, where microbes convert chemicals into energy in a process called chemosynthesis and subsequently become the foodstuff for larger species. Marine invertebrates Riftia pachyptila, a.k.a. giant tube worms and the deepwater mussel Bathymodiolus childressi, a bivalve mollusk species in the family Mytilidae, both thrive in this environment.

Gulf of Mexico

An American flag lays in a slick of oil that washed ashore from the Deepwater Horizon oil spill in the Gulf of Mexico on July 4, 2010 in Gulf Shores, Alabama.
Joe Raedle / Getty Images

The Gulf of Mexico covers about 600,000 square miles off the coast of the southeastern United States and a portion of Mexico. The Gulf is home to several types of marine habitats, from deep canyons to shallow intertidal areas. It is also a haven for a wide variety of marine life, from huge whales to tiny invertebrates.

The Gulf of Mexico's importance to marine life has been highlighted in recent years in the wake of a major oil spill in 2010, and the discovery of the presence of Dead Zones, which the U.S. National Oceanic and Atmospheric Administration (NOAA) describes as hypoxic (low-oxygen) areas in oceans and large lakes, that have resulted from "excessive nutrient pollution from human activities coupled with other factors that deplete the oxygen required to support most marine life in bottom and near-bottom water."

Gulf of Maine

A whale breaching in the Gulf of Maine.
RodKaye / Getty Images

The Gulf of Maine is a semi-enclosed sea next to the Atlantic Ocean that covers over 30,000 square miles just off the U.S. states of Massachusetts, New Hampshire, and Maine, and the Canadian Provinces of New Brunswick and Nova Scotia. The cold, nutrient-rich waters of the Gulf of Maine provide a rich feeding ground for a variety of marine life, particularly in the months from the spring through late fall.

The Gulf of Maine encompasses a number of habitats including sandy banks, rocky ledges, deep channels, deep basins, and a variety of coastal areas featuring rock, sand, and gravel bottoms. It's home to more than 3,000 species of marine life including about 20 species of whales and dolphins; fish including Atlantic codbluefin tunaocean sunfishbasking sharksthresher sharksmako sharks, haddock, and flounder; marine invertebrates such as lobsters, crabs, sea starsbrittle starsscallops, oysters, and mussels; marine algae, such as kelp, sea lettuce, wrack, and Irish moss; and the plankton that larger species rely on as a food source.