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Wikipedia says Nematodes represent no less than 80% of the animals on earth.

I don't know anything about biology, but I've had this thought: isn't their habitat much smaller (in terms of volume) than the one of other species (for example: fishes) ?

Indeed, the volume of Nematodes' habitat is about $10m × S$ where $S$ denotes the surface of the earth, whereas the volume of fishes habitat is about $3 700m × S$. (considering $3700 << \sqrt S$).

So, how come Nematodes be the most numerous species on earth ?

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  • $\begingroup$ If only the universal language was my mother tongue...! $\endgroup$
    – niobium
    Mar 12, 2023 at 22:27
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    $\begingroup$ Where are you getting your numbers for habitat volume for each? Seems likely there are some unfounded assumptions in there. Also fish are really big. $\endgroup$
    – Bryan Krause
    Mar 13, 2023 at 1:57
  • $\begingroup$ I took the average depth of oceans (I Googled it but don't remember the link) and for Nematodes I've just guessed the number. However, I just asked chatGPT (the artificial intelligence) the depth at which Nematodes can live: it answered me $15$ to $20cm$. $\endgroup$
    – niobium
    Mar 13, 2023 at 9:55
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    $\begingroup$ Never ever ever take information from chatGPT like that. ChatGPT does not "know" anything. It cannot check or understand information it gives. It has no concept of "correct" or any concept of concepts at all. $\endgroup$
    – Bryan Krause
    Mar 13, 2023 at 13:25
  • $\begingroup$ Fish don't live at all depths of ocean, certainly not with uniform abundance. Nematodes live in aquatic environments as well as soil. The Wikipedia page you started your question with talks about nematodes living very deep. Did you read the page that you yourself linked here? $\endgroup$
    – Bryan Krause
    Mar 13, 2023 at 13:27

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For future reference, the exact Wikipedia quote is: "Their numerical dominance, often exceeding a million individuals per square meter and accounting for about 80% of all individual animals on earth, their diversity of lifecycles, and their presence at various trophic levels point to an important role in many ecosystems."

I could not look at a free online copy of either source, but I have a guess where the 80% figure comes from. Imagine giving every living individual animal a number and then holding a lottery where every number you gave out has an equal chance of being picked. What I believe this quote is saying is that there is an 80% chance that the winner of the lottery will be a nematode.

The reason so many individual can live is so small a space is because most species are extremely small. This site offers data on the average weight of individuals in each nematode families and the largest was steinernematidae at less than 115 micrograms. Wolfram Alpha says that is less than half the mass of a grain of sand.

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  • $\begingroup$ Half the mass of a grain of sand seems to me a bit small. Actually this picture from Wikimedia let me think they have a length of a few ( more than 5) centimeters. It's also possible that I'm not reading correctly the ruler next to the Nematode's picture $\endgroup$
    – niobium
    Mar 12, 2023 at 21:00
  • $\begingroup$ @niobium indeed that is one of the larger species - Females of this species are up to 35 cm I think. $\endgroup$
    – bob1
    Mar 12, 2023 at 22:50
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    $\begingroup$ Nematodes can actually get quite a bit larger. The largest species (according to my first link) can be up to 13 meters long and is a parasite of sperm whales. However, these species are the exceptions, not the general rule. It's like ostriches are birds, but that does not mean the most birds weight over 200 lbs. Those nematodes species are a few centimeters big, but the vast majority are microscopic. $\endgroup$
    – E Tam
    Mar 13, 2023 at 0:20

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