Tuesday, September 23, 2008

Which Animal Phylum Has the Most Species?

I bet almost all of you think it's Arthropoda since there are so many insects and especially beetles, which God was very fond of.1

However, recent advances in taxonomy have suggested that there are one million species of nematode. This is about the same number of species as arthropods so the primacy of insects has been challenged.

1. This is a reference to a famous quip by J.B.S. Haldane. When asked to name the most important thing he has learned about God from studying biology he reportedy said, "I'm not sure, but He seems to be inordinately fond of beetles."

[Photo Credit: Iowa State University Plant Disease]


  1. It's kind of a silly question, but also hard to answer without knowing the minimal criteria for declaring two genes different. And how are you defining “gene”? (Are you using a “common man” definition that would count different alleles as different genes?)

    Clearly my myosin LC2 and my alpha actinin genes count as two (or is it 4, because I have two copies of each), but do we have 2 more if we add in those of the mouse? And how about the chimpanzee, if we discover that (unlike the mouse) their genes/alleles are absolutely identical to mine? Do we have 2 more when we add in the alleles of the mayor of my town (which, unbeknowst to her have synonomous substitions that make the protein-encoding portion of her DNA, but not the protein itself, different from mine)? Or how about those of the guy next door, who has a neutral mutation in an intron region? Or my third cousin, who has a neutral AA substition in his proteins?

    Or do we take a functional approach: a gene is counted if it differs functionally in anything more than a non-trivial way? (that would be tough to discern!)

    Finally, what does the winning phylum win? (Lunch with you? :)

  2. Sorry, it was a typo in the title of the posting. I'm referring to the phylum with the most species.

  3. A gentleman at the University of Utah once quipped that if we destroyed every ounce of soil and rock, air and ocean, you could still see the earth from outerspace - the outline made purely from nematodes.

    And the best part of such `facts` is that they're entirely unverifiable. ;)

  4. Haldane's quip was rehearsed. He said it in many different contexts, like any originator of a good quip should. He also said the "two brothers, eight cousins" quip many times.

  5. I thought the "two brothers or eight cousins" quote was Fisher. Is there an on-line repository of great quotes from scientists? I can only find collections of quotes from all sorts of people.


    A large fraction of the described species diversity of nematodes is in the form of parasites, either of other animals or of plants. I think there are a few described species that are obligate parasites of fungi, too. Assuming every multicellular organism is host to at least one species of specialist parasite, then I find it quite likely that nematode species may outnumber insects or even arthropods.

    However, estimates of undescribed species richness are always a little suspicious to me - the mathematical models used to generate these estimates always seem pretty simplistic, like just multiplying the species found per square metre of sea-floor by the area of the Earth's oceans.

  6. im guessing from my best judgment that it would be more likely that nematodes, since there are so many different types and species of them........ but then again many say that arthropods are the most numerous on earth, i also have a question have nematodes when did the nematodes go through cephalization?- around what time was it before the annelids when 2 system digestive tracts came about or some other time........?

  7. oh sorry about the type *have nematodes* should have been deleted