Ocean Life: Species, Biodiversity, Oldest, Most Numerous, Threats | Sea Life, Islands and Oceania

OCEAN LIFE

Marine worms painting made in late 19th century

In “The Extreme Life of the Sea”, Stephen and Anthony Palumbi wrote: There are about as many bacteria in a liter of water of seawater as there are people in India — over a billion. In April 2010, scientists at Woods Hole involved with 10-year Census of Marine Life, estimated there was a mind-mind-boggling nonillion, or 1,000,000,000,000,000,000,000,000,000,000 (1 followed by 30 zero) individual microbial cells in the oceans, weighing as much as 240 billion African elephants and making up 50 percent to 90 percent of the oceans’ biomass. Among their findings were a bacteria colony — comprised of whitish mats — the size of Greece on the floor of the Pacific Ocean off Chile and Peru.

Out of the millions of insects species only five can live in the open ocean — five species of water strider. The only reptiles that can do so are salt water crocodiles, sea turtles, sea snakes, Galapagos marine iguanas and some monitor lizards that occasionally take to saltwater. There are lots of mammals — whales, dolphins, seals, walruses and dugongs — that live in the sea..

Oceans cover 71 percent of Earth and 94 percent of life on Earth is aquatic, but only two-thirds of all marine life remains unidentified according to Live Science as of 2016. Censuses of marine species are taken in coastal waters and reefs by scuba divers who measures out a an area on the ocean floor and count the numbers and estimate the size of different species they observe and record the numbers with a pencil on waterproof paper. In the past scientists have traditionally relied catch numbers from fishermen to determines the health and population size of marine species.

Taxonomic Category or Rank: 1) Kingdom; 2) Division: 3) Class; 4) Order: 5) Family; 6) Genus; 7) Species

Websites: Animal Diversity Web (ADW) animaldiversity.org; NOAA site fisheries.noaa.gov; Fish Base fishbase.se; World Register of Fish Species marinespecies.org; Smithsonian Ocean ocean.si.edu;

How Many Species Live in the Ocean?

According to one estimate around 250,000 of the world’s 2 million known species are found in the sea. Some scientists estimate there could be as many as 10 million species out there and that number could reach 1 billion of all the existing microbes are factored in.

Given the vast size of the ocean, it is impossible to know the exact number of species that live there. Research suggests, however, that the number of species in the ocean is decreasing. The continued decline in the health of many ecosystems, coupled with rising extinction rates, are likely outpacing species’ ability to evolve to tolerate the conditions of our rapidly changing planet. It is not all bad news, though. Researchers around the world continue to study marine life and habitats to help develop new strategies to preserve vital ocean ecosystems. [Source: NOAA]

Scientists estimate that 91 percent of ocean species have yet to be classified, and that more than eighty percent of our ocean is unmapped, unobserved, and unexplored.. While these statistics may sound daunting, they have not stopped the global scientific community from striving to amass as much knowledge as possible about ocean life.

Ocean Biodiversity

20120516-Nudibranch_-_Glossodoris_symmetricus_1.1.jpg
The Nudibranch Glossodoris symmetricus

According to the Census of Marine Life, completed in October 2011, crabs, lobsters and other crustaceans are now believed to be the most common species in the seas of Australia and Japan, whose waters are thought of as the most varied .

The worlds seas are full of food and it is no surprise that so many forms of life are found there. A typical drops of sea water is teaming with life: bug-like copepods; long, glassy arrowworms; coiled filaments of cyanobacteria; rectangular-shaped diatoms; fish eggs and bug-eyed larva; and microscopic crabs.

Biodivesity in the sea is as high or higher than in the rain forest, with many areas, especially at depths of over 1,000 meters completely unexplored. One sampling of an area the size of a bathroom (5.4 square meters) deep in the South Atlantic Ocean turned up 700 species of copepod, a type of crustacean, 99 percent of them unknown to science.

Reefs are among the worlds most diverse habitats. They are home to about 35,000 to 60,000 species (of the world's 274,000 known marine species) and a third of the 12,000 kinds of marine fishes. They are also incredible rich in phyla, with creatures as diverse as sea lilies, sponges, crabs, eels and sea anemones all living there. Some patches of reef boast over a 100 species of fish and more than a thousand other kinds of plants and animals.

A single reef wall often contains a wider variety of life forms’species from more phyla, or major groups — than an entire continent. The rich biodiversity of the reefs is due to abundance of light warmth provided by the tropical sun and an abundance of oxygen saturated into the water by crashing waves.

In March 2025, scientists announced that they had discovered 866 new marine species have been identified over two years as part of global research by the Ocean Census, a partnership involving scientists, governments, museums, and over 400 institutions, backed by the Nippon Foundation and Nekton,. The discoveries came from 10 expeditions and 8 workshops, using divers, submersibles, and deep-sea robots reaching depths of 5,000 meters. The new species include sharks, sea butterflies, mud dragons, bamboo corals, octocorals, shrimp, and water bears. The Ocean Census aims to speed up species identification, which currently takes an average of 13.5 years—long enough for some species to go extinct before being documented. Interesting discoveries include 1) a new species of guitar shark — part shark, part ray; from one of the most threatened vertebrate groups — identified off Mozambique and Tanzania at a depth of 200 meters; 2) Turridrupa gastropod, a deep-sea snail from New Caledonia and Vanuatu that hunts with venomous harpoon-like teeth; and 3) Tylaster sea star, only the second known species in its genus, found in the Arctic at depths of 2,770–3,575 meters. [Source: Cara Tabachnick, CBS News, March 13, 2025]

Biodiversity in Different Seas

The waters of Japan and Australia are home to around 33,000 different life forms that have been elevated to the category of "species." The waters of China, the Mediterranean Sea and the Gulf of Mexico are also in the top five most biologically diverse marine regions, according to a preliminary census in the public opening of data in the Public Library of Science (PLoS ONE).

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reef sponge
The relatively isolated waters of Australia, New Zealand, Antarctica and South Africa have the largest number of endemic species, or which are unique to that area. By contrast, the Mediterranean has more "exotic" creatures: with more than 600 species, many of which came from other places.

Pedro Martinez Arbizu, from the German Centre for Marine Biodiversity Research, was astonished to find that a tiny creature — Ceratonotus steiningeri — also was within the cataloging he had made earlier 13,000 km away in the central Pacific. There was that critter again. Same shrimpy creature, different ocean. “We were really very, very surprised about that,” Arbizu said. “We think this species has a very broad distribution area.”

The census also describes a species of strange large squid that was only recently found in waters more than 900 meters deep. The 7-meter-long squid has large fins with arms and tentacles that have elbow-like bends. Scientists had seen it in larvae form before, but not in its full-blown glory until it was filmed at depth.

5,000-Year-Old Coral — the World’s Oldest Living Animal?

Studies show that some corals can live for up to 5,000 years, making them the longest living animals on Earth. Scientific studies of elkhorn coral (Acropora palmata) in the Caribbean and off the coast of Florida show that coral genotypes can survive longer than expected. Genotype refers to the genetic makeup of an organism. [Source: NOAA]

Scientists are now using a genetic approach to estimate the ages of corals. The method determines when a coral egg and sperm originally met to form the genome of a coral colony. The researchers then track the number of mutations that have accumulated in the genome since that time. Because mutations tend to arise at a relatively constant rate, researchers can estimate the approximate age of the coral genomes in their study.

These studies can help clarify how corals will respond to current and future environmental changes. Though corals can be resilient and live for thousands of years, elkhorn corals are listed as threatened under the U.S. Endangered Species Act. The corals have suffered recent population declines, indicating that there are limits to how much change even these resilient creatures can handle.

Intelligent, Feeling, Conscious Sea Creatures?

Evan Bush of NBC News wrote: The cleaner wrasse fish appears to recognize its own visage in an underwater mirror. Octopuses seem to react to anesthetic drugs and will avoid settings where they likely experienced past pain. These discoveries serve as indications that the more scientists test animals, the more they find that many species may have inner lives and be sentient (able to perceive or feel things). A surprising range of creatures have shown evidence of conscious thought or experience, including insects, fish and some crustaceans. [Source: Evan Bush, NBC News, April 20, 2024]

In other studies, researchers found that zebrafish showed signs of curiosity when new objects were introduced into their tanks and that cuttlefish could remember things they saw or smelled. One experiment created stress for crayfish by electrically shocking them, then gave them anti-anxiety drugs used in humans. The drugs appeared to restore their usual behavior.

State laws vary widely. A decade ago, Oregon passed a law recognizing animals as sentient and capable of feeling pain, stress and fear, which Reddy said has formed the bedrock of progressive judicial opinions in the state. Meanwhile, Washington and California are among several states where lawmakers this year have considered bans on octopus farming, a species for which scientists have found strong evidence of sentience.

British law was recently amended to consider octopuses sentient beings — along with crabs and lobsters. “Once you recognize animals as sentient, the concept of humane slaughter starts to matter, and you need to make sure that the sort of methods you’re using on them are humane,” Birch said. “In the case of crabs and lobsters, there are pretty inhumane methods, like dropping them into pans of boiling water, that are very commonly used.”

Marine Life Facing Mass Extinction as a Result of Global Warming

In June 2011, Thair Shaikh, reported in CNN, “Marine life is under severe threat from global warming, pollution and habitat loss, with a high risk of "major extinctions" according to a distinguished group of marine scientists who met at Oxford University, England, in April to discuss the impact of human activity on the world's oceans. The meeting, led by the International Programme on the State of the Ocean (IPSO), examined the combined effects of pollution, acidification, ocean warming, over-fishing and depleting levels of oxygen in the water. [Source: Thair Shaikh, CNN, June 21, 2011, Marlowe Hood, AFP, June 20, 2011]

Dying coral reefs, biodiversity ravaged by invasive species, expanding open-water "dead zones," toxic algae blooms, the massive depletion of big fish stocks are all are accelerating, the panel said. These symptoms, moreover, could be the harbinger of wider disruptions in the interlocking web of biological and chemical interactions that scientists now call the Earth system. The panel found that oceanic conditions are similar to those of "previous major extinctions of species in Earth's history," and that we face losing marine species and entire marine ecosystems, such as coral reefs, within a single generation. The interim report, produced in partnership with the International Union for Conservation of Nature (IUCN), was presented to the U.N.

Three main drivers are sickening the global marine environment, and all are a direct consequence of humans activity: global warming, acidification and a dwindling level oxygen, a condition known as hypoxia. The study also said that the speed of decline of marine ecosystems is faster than predicted. Alex Rogers, IPSO's scientific director, said: "The oceans are a common heritage of mankind. The extinction threat we believe is real." Rogers, professor of Conservation Biology at the Department Of Zoology, University of Oxford, told CNN: "The rate of change we are seeing in the quantities of carbon dioxide going into the atmosphere and then being absorbed into the oceans is so great that it is difficult to compare what is happening now with what has happened in the past but we do know that past disturbances in the carbon cycle have been a feature of mass extinction events."

According to the panel — which consisted of 27 marine experts from 18 organizations — most if not all the five "global mass extinctions" in Earth's history were probably caused by the "deadly trio" of global warming, ocean acidification and lack of water oxygen or hypoxia. It states that these three factors are present in the ocean today and gives examples of marine ecosystems suffering severe disturbance, such as the mass "coral bleaching" in 1998 that killed 16 percent of all the world's tropical coral reefs.

"The results are shocking," said Alex Rogers, an Oxford professor who heads IPSO and co-authored the report. "We are looking at consequences for humankind that will impact in our lifetime." "The rate at which carbon is being absorbed is already far greater now than during the last globally significant extinction of marine species 55 million years ago," when some 50 percent of deep-sea life was wiped out, the report said. That event, called the Paleocene-Eocene Thermal Maximum, or PETM, may be an ancient dress rehearsal for future climate change that could be even more abrupt and more damaging, some scientists fear.

"We now face losing marine species and entire marine ecosystems, such as coral reefs, within a single generation," said Daniel Laffoley, head of the International Union for Conservation of Nature's (IUCN) World Commission on Protected Areas, and co-author of the report.

Difference Between a Threatened and Endangered Species

Under the Endangered Species Act (ESA), a species may be listed as either threatened or endangered depending on their risk for extinction. An endangered species is defined as "any species which is in danger of extinction throughout all or a significant portion of its range." Endangered species are automatically protected by prohibitions of several types of "take," including harming, harassing, collecting, or killing, under Section 9 of the ESA. There are some limited exceptions to these rules listed in Section 10 of the ESA. The Kemp's ridley turtle, considered the smallest marine turtle in the world, is listed as an endangered species throughout its range of the Gulf of Mexico and entire U.S. Atlantic seaboard. [Source: NOAA]

The ESA defines a threatened species as "any species which is likely to become an endangered species within the foreseeable future throughout all or a significant portion of its range." Threatened species receive protections through separate regulations issued under Section 4(d) of the ESA. These regulations occur separately from the listing and detail what take prohibitions are in effect. Also called 4(d) rules, they can include the same prohibitions under Section 9. Elkhorn coral — a large, branching coral with thick and sturdy antler-like branches — is listed as a threatened species throughout its range.

NOAA scientists use the best scientific and commercial information available as the basis for their listing decisions. Scientists may not consider the economic impact of listing a particular species. A species must be listed if it is threatened or endangered due to any of the following five factors: 1) Present or threatened destruction, modification, or curtailment of its habitat or range; 2) Overutilization for commercial, recreational, scientific, or educational purposes; 3) Disease or predation; 4) Inadequacy of existing regulatory mechanisms; and 5) Other natural or human-made factors affecting its continued existence.

There are approximately 2,150 total species listed under the ESA. NOAA's National Marine Fisheries Service and the U.S. Fish and Wildlife Service share responsibility for implementing the ESA.

Invasive Species

Invasive species are animals or plants from another region of the world that don’t belong in their new environment. They often cause ecological or economic harm in a new environment where it is not native. [Source: NOAA]

Invasive species can harm both the natural resources in an ecosystem as well as threaten human use of these resources. An invasive species can be introduced to a new area via the ballast water of oceangoing ships, intentional and accidental releases of aquaculture species, aquarium specimens or bait, and other means.

Invasive species are capable of causing extinctions of native plants and animals, reducing biodiversity, competing with native organisms for limited resources, and altering habitats. This can result in huge economic impacts and fundamental disruptions of coastal and Great Lakes ecosystems. Invasive species can lead to the extinction of native plants and animals, destroy biodiversity, and permanently alter habitats.

Image Sources: Wikimedia Commons; YouTube, Animal Diversity Web, NOAA

Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 uv.es/hegigui/Kasper ; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated December 2025