The question of how many animals live in the ocean is one of the most complex mysteries in science. Earth’s oceans cover over 70% of the planet’s surface, representing the largest continuous habitat on the planet. This sheer volume and depth mean that the vast majority of the marine environment, particularly the deep sea, remains unexplored.
Scientists cannot provide a single, fixed number for the total amount of life. Instead, they rely on a combination of baseline counts and broad projections to grasp the scale of marine biodiversity, distinguishing between formally identified species and those only estimated to exist.
The Counted: Currently Described Marine Species
The most concrete figure available is the number of species formally cataloged and named by scientists through taxonomy. According to the World Register of Marine Species (WoRMS), approximately 242,000 valid marine species have been formally described.
These species represent the baseline of our knowledge, where a physical specimen has been collected, studied, and given a unique scientific name. This total includes all forms of marine life, from whales and fish to small invertebrates and simple organisms. The majority of these known species are invertebrates, such as mollusks, crustaceans, and worms, which dominate the current count.
The Hidden Majority: Estimating Undiscovered Life
Moving beyond the known catalog, scientists use sophisticated models to project the total number of species likely inhabiting the ocean. These models often rely on extrapolation techniques, such as analyzing the rate at which new species are discovered over time, known as species accumulation curves.
By observing how quickly the number of described species is growing, researchers can project where the curve will eventually level off, suggesting the total number of species. Due to varying assumptions and different datasets, estimates for total marine species range widely, from about 700,000 to over 2.2 million species. This wide range reflects the scientific uncertainty inherent in counting a population that is largely invisible. Estimates tend to be highest for smaller, less-studied groups like nematodes, copepods, and other tiny invertebrates, which hold the greatest reservoir of undiscovered diversity.
Factors That Obscure the True Number
Several physical and logistical challenges contribute to the difficulty in obtaining an accurate count of marine life. The most significant challenge is the inaccessibility of the deep sea, which constitutes the largest unexplored habitat on Earth. Sampling efforts are often biased toward coastal areas and surface waters, leaving vast, deep-ocean zones largely unsampled and their species undocumented.
The nature of some marine animals also complicates the counting process, particularly the existence of cryptic species. These are distinct species that are morphologically identical to a known species but are genetically different, a distinction that often requires complex DNA analysis. Furthermore, the immense number of microscopic marine animals, such as meiofauna and zooplankton, are extremely difficult to sample and classify because of their minute size.
Measuring Life Beyond Species Counts
Measuring life in the ocean can also be approached by focusing on the sheer quantity of organisms rather than just the number of species. Total marine biomass, the collective mass of all living material, provides a different perspective on the magnitude of ocean life.
The total biomass of the ocean is estimated to be approximately six gigatons of carbon, which is significantly less than the total living mass on land. Despite this, the ocean environment harbors the majority of the world’s animal biomass, estimated at 78% of the global total. Within this animal mass, invertebrates like arthropods (e.g., copepods) and fish dominate in terms of total weight.
Microbial life, while not classified as animals, contributes overwhelmingly to the total living mass in the ocean. Bacteria and archaea form a colossal living layer, and understanding their abundance is necessary to grasp the overall scale of life in the marine environment.