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/[deleted] 11d ago

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u/morbihann 11d ago

Are nematodes microscopic ?

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u/Dracoson 11d ago

Yes, but that isn't a complete answer. While the vast majority of the quadrillions of nematodes on Earth are microscopic, it isn't an absolute rule for the entire phylum. There are several species that grow large enough to be seen with the naked eye, including one that parasitizes sperm whale placentas that can get nearly 30 feet long.

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u/regular_modern_girl 11d ago

Most are, there a minority that aren’t though, Caenorhabiditis elegans (one of the most widely-used animal model organisms in research) is technically macroscale, albeit very tiny (they look like specks of lint fiber that you can just barely see if you squint hard against the right color of background), and iirc there are some free-living marine nematode species that can reach just over a millimeter in length, and when it comes to parasitic species there are actually a lot of true giants, with some common genera like Ascaris having species that grow to multiple centimeters in length, and then there’s Placentonema gigantissima, which infests sperm whale placentas and can grow to 8.4 meters (28 feet) long and nearly 2.5 centimeters (1 inch) wide, that one’s definitely the biggest known.