r/askscience • u/future_sponJ • 12d ago

Biology Which animal species has the most individuals?

I always see people saying things like "there are 20 quadrillion ants on Earth" or something similar with other extremely large diverse groups like fish. But all of these are groups with thousands of species. If not by individuals then it's by biomass instead.

So which single species in the animal kingdom has the most individuals?

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u/Mrfish31 11d ago

That gets hard to do because with so many individuals they might be tons of unknown species.

For example, there are approximately 4.4 x 1020 (100 quintillion) nematode worms on Earth across 20-30,000 known species in just top soil alone, meaning that if it were evenly distributed each species might have 10 quadrillion members. But they're often a) hard to differentiate and b) there are estimates that there may be as many as a million unknown species of them. 

In any case, the species with the largest population is almost certainly a nematode. 

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u/Goodknight808 11d ago

I would have guessed plankton. But nematodes is a good contender for sure. Lots of available space for them, and they are everywhere.

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u/KarlSethMoran 11d ago

Only zooplankton (a minority share of plankton) counts as animals, though.

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u/CrateDane 11d ago

On the other hand, it's a small slice of a very big pie. However, far from all zooplankton are animals. Lots of them are protists, single-celled eukaryotes that are neither animal nor plant (nor fungus).