How Many Species Live in the Sea?

This guest blog was written by Nancy Knowlton, Sant Chair of Marine Science at the Smithsonian Institution’s National Museum of Natural History and Matthieu Leray, Post-doctoral fellow, Marine Science Network, Smithsonian’s National Museum of Natural History.

You would think that would be an easy question to answer, but it’s not. The most recent estimates range from about 0.7-2.2 million species (excluding the bacteria and viruses – they are almost too diverse to contemplate). But many scientists suspect there could be many more than that. In fact it took quite a lot of work just to figure out how many ocean organisms even have names (answer? About 226,000, listed in the aptly named World Registry of Marine Species, or WoRMS).

Why is getting such a seemingly simple number so hard? The ocean represents a lot of territory – about 1.3 billion cubic kilometers, or about 95% of the habitable real estate on the planet. Since we’re obviously never going to catch and catalogue every species, one method is to take samples, count the number of species in the samples, and then calculate how many are likely to be in the entire ocean.

So how do you count the number of species in a sample? Even small samples can have large numbers of species, and most of them (maybe as many as 90%) haven’t been studied by scientists so they don’t even have names. Fortunately, DNA “barcodes” allow us to recognize a species as distinct, even if it doesn’t have a name.

What about the samples then? The key is STANDARDIZATION. If you take the samples in the same way from many places, they can be directly compared without worrying about who took them or exactly how they were taken.

The Smithsonian has pioneered this approach with Autonomous Reef Monitoring Structures, or ARMS. These underwater condominiums attract a large number of species that either settle onto the floors and ceilings of the ARMS, or nestle in the spaces between. In some places, like coral reefs, there can be more than 1000 species on a single ARMS.

This approach is about to get a big boost from the establishment of the Tennenbaum Marine Observatories Network at the Smithsonian. Designed to use these and other standardized methods across space and over time, we should one day have a much better idea of who lives in the ocean and where. Will it be one million or

closer to ten? Stay tuned!

This entry reflects the author’s personal judgments and does not represent the views of the United States Government or the Department of State.

3 thoughts on “How Many Species Live in the Sea?

  1. i guess mapping the entire species would be far-fetched through such (simple) sampling methods. Would it be correct to say we know – despite our intelligence – one percent of the under-water species and be modest about it rather than 10%?
    When I look at the plant life in the Himalayas, I realize how little we have mapped and know.

    Best wishes.

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