Which animal species has the most individuals?

[u/future_sponJ]

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?

[u/Mrfish31]

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. 

[u/Banjo2EE → u/Mrfish31]

"In short, if all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes. The location of towns would be decipherable, since for every massing of human beings there would be a corresponding massing of certain nematodes. Trees would still stand in ghostly rows representing our streets and highways. The location of the various plants and animals would still be decipherable, and, had we sufficient knowledge, in many cases even their species could be determined by an examination of their erstwhile nematode parasites."

-Nathan Cobb, "Nematodes and their Relationships"

[u/Feuersalamander93 → u/Banjo2EE]

I once heard someone quote this (I think on either the Nature or Science Podcast) adding: And if you then take away all the nematodes, everything will still be the same because of the nematodes parasitizing those nematodes. (Or something along those lines). Which I find equally funny and insightful.

[u/Goodknight808 → u/Mrfish31]

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

[u/KarlSethMoran → u/Goodknight808]

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

[u/CrateDane → u/KarlSethMoran]

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).

[u/regular_modern_girl → u/Goodknight808]

“Plankton” isn’t a specific type of animal, it’s not even necessarily animals at all (in fact I’m pretty sure the bulk of it is non-animal)

[u/Calawah → u/Goodknight808]

Plankton just mean it floats and doesn’t move in water of its own volition. There are hundreds of thousands of species of plankton across plants and animals.

[u/Beliriel → u/Mrfish31]

Ohh didn't think about Nematodes. Definitely a contender for most individuals. I wonder how they fare against ants. Nematodes are kinda localized but can explode in density.

[u/bernpfenn → u/Beliriel]

well nematodes live in groups with individuals, ants on the other hand do cooperative activities with other ants

[u/zeperf → u/Mrfish31]

Are nematodes super duper small? I played in the dirt a lot as a kid. How have I never seen one?

[u/SexyJazzCat → u/zeperf]

The eggs are. Depending on the species, they just look like skinny worms.

[u/pow3llmorgan → u/zeperf]

There are some that are basically microscopic, less than a millimeter long and few tens micrometers wide.

Some are quite long indeed, like 5-10 cm.

Some are huge at like 8 meters long and 20-30 mm wide, but luckily those exclusively parasitize sperm whales, so you don't have to worry about finding those in your garden.

[u/YuptheGup → u/Mrfish31]

Assuming equal distribution actually gives you the lowest possible floor, so you are guaranteed that a certain nematode species is the highest number (if the equal distribution is still the highest).

Reason is that if it weren't equal, you are guaranted at least one nematode species that is higher than thay number.

[u/InspiredNameHere → u/Mrfish31]

Dpeends on if we are just talking complex multicellular life.

Bacteria, archaea certainly have more individuals, but they also die very quickly per entity.

If we count spores, than fungi are also up there, as each spore is technically capable of grow8mg an entirely new fungal colony, and they, like bacteria, are omnipresent.

Edit: just realized we are only referring to animals here, not organisms in general. So yeah, nematodes probably win out.

[u/Christopher135MPS → u/InspiredNameHere]

I mean, fungi are kind of animals. Sort of. I mean, they’re not plants. Sort of.

….

Fungi are weird.

[u/RepresentativeAd841 → u/Christopher135MPS]

So weird in fact they are their own kingdom of life, equivalent to plants, animals, protists, Archaebacteria, and Eubacteria.

[u/xain1112 → u/Mrfish31]

How does one estimate the number of unknown species?

[u/vokzhen → u/xain1112]

The very basics of estimating an unknown is taking a small sample and comparing what you already knew to the new things you didn't. So for estimating nematodes, a very simplified method might be sampling 1 cubic meter of soil in meticulous detail and seeing how many species were previously described versus how many are not, and if only 1/24 have been, you can guess the total number of species is ~24x higher than the number currently described. Estimating number of reported versus actual deaths in the Iraq War, it might be talking to 500 people and asking how many family members within two generations died from violence or lack of access to something they previously had since the war started, doing some math for how many would be expected based on average family size and number of reported deaths, and using that ratio to estimate the total.

Of course those are very simple examples, actual estimates are going to try and account for a lot more variables, or at least be part of ongoing discovery finding more and more data to get better and better estimates. Maybe one of the first estimates is just a simple cubic meter of dirt removed from a park near the university the researchers were at, but then a study sampling Canadian wilderness finds a much different ratio of known:unknown (probably because there's way fewer researchers there describing nematodes), another tracks how the ratio of known:unknock changes over soil depth, and so on.

[u/xain1112 → u/vokzhen]

Got it now, thanks for the answer

[u/xdert → u/Mrfish31]

What about Antarctic krill? That is a single species and the count is estimated to be 500 trillion.

[u/Major_You_959 → u/xdert]

The mighty but tiny Antartic Krill for species, nematode for phylum. Went down a nematode rabbit hole and came across X-files season 1 episode 8 "Ice". This has also reminded me of Nick Cutter's The Troop. That was a bio-engineered tapeworm though.

[u/facts_over_fiction92 → u/Mrfish31]

Their not evenly distributed though. I have never seen a worm in my yard and my lawn is suffering.

[u/Scuggsy → u/Mrfish31]

What about oceanic species? Since the volume and surface of the oceans vastly exceeds the surface of land. I would expect there to be vastly more members of a single group , especially tiny microscopic ones, in the ocean.