The question of how many creatures inhabit the ocean is complex because the answer depends entirely on what is counted as a “creature.” The ocean’s vastness and the incredible diversity of its life forms mean that the number shifts dramatically from a few hundred thousand documented species to a number so large it is measured by the individual cells of microscopic organisms. Our understanding of marine life moves from the known, cataloged species to the theoretical estimates of the undiscovered. Exploring the ocean’s biodiversity requires distinguishing between the count of different species and the count of individual organisms.
Documenting Known Ocean Species
The most concrete answer to the question of marine life comes from formal scientific records of species that have been collected, analyzed, and officially named. This process, known as taxonomy, is the foundation of our current knowledge of marine biodiversity.
The World Register of Marine Species (WoRMS) serves as the authoritative catalogue for named marine species. This database, which is constantly updated by a global network of taxonomists, provides the most reliable count of formally described life in the ocean. As of recent tallies, the number of known and accepted marine species is approximately 242,000.
On average, scientists add around 2,000 new marine species to the global record each year. However, this recorded number is just the floor of ocean biodiversity, representing only the life forms that have been formally classified.
Predicting the Total Number of Species
While the number of named species is firm, scientists believe the true diversity of the ocean is vastly greater than what has been formally recorded. The immense size of the oceans, especially the unexplored deep sea, suggests that most species still await discovery. Estimates for the total number of marine species—including the known and the unknown—range widely, from about 500,000 to over two million.
To arrive at these estimates, researchers employ predictive methods that extrapolate from the rate of discovery and the diversity of life in sampled habitats. One approach models diversity based on factors like habitat complexity and geographic distribution.
These theoretical estimates suggest that less than 10% of all marine species have been formally described. The majority of the unknown species are likely to be small invertebrates, such as mollusks and crustaceans, or specialized organisms living in remote areas like hydrothermal vents.
Accounting for Microbial Organisms
The numbers shift completely when the question moves from counting distinct species to counting individual organisms. The total count of individual life in the ocean is overwhelmingly dominated by the microscopic world, which includes bacteria, archaea, protists, and viruses.
The total number of prokaryotic cells—bacteria and archaea—in the world’s oceans is estimated to be around 10^29, or one hundred nonillion. A single liter of seawater can contain approximately one billion bacteria. This microscopic abundance means that if you weighed all the living organisms in the ocean, about 90% of that total biomass would be from microbes.
Viruses, which are often considered microbes even though their status as “living” is debated, outnumber the prokaryotic cells they infect. There are an estimated 10^30 viruses in the ocean, a number that is ten times greater than the count of bacteria and archaea. The sheer scale of this microbial population means that the answer to “how many creatures are in the ocean” primarily reflects the count of individual cells.