I was reading about Copepods on Christopher Taylor's blog Catalogue of Organisms [Taxon of the Week: Some Copepods for your Reading Pleasure].It got me thinking about whether Crustaceans should be elevated to phylum status as proposed by Lynn Margulis and Karlene Schwartz in Five Kingdoms 3rd ed.. A quick search of the web revealed that scientists seem to be split on the issue. Several competing classifications are listed on Answers.com. The NCBI site lists Crustacea as a subclass of Phylum Arthrododa but the taxonomy on that site is notorious for being very conservative.
Are there any taxonomists out there? Is there a consensus?
[Photo Credit: South Australian Research and Development Institute (Sardi)]
10 comments :
I have enough problems with "class aves" and "class reptilia". But I would still be interested in hearing more on this topic of Crustacea. I don't know much about invertebrate taxonomy.
~ Nick
crustaceans belong to the phylum arthropoda along with stuff like the chelicerates (spiders & scorpions, horseshoe crab, pycnogonids, etc). I guess it's fair to call crustacea a class.
What's interesting is that insects turned out to e crustaceans.
I don't really see why this should be such a serious question right now. Arthropode systematics and phylogeny are so messed up, especially if you try to fit the Cambrian forms into the system, that what is a phylum and what is a class seems secondary to me
I think the one thing that's fair to say at this point is that there is no consensus, and has not been one for a long time. Just in the books on my shelves I can find them considered a Class (of Phylum Arthropoda--this is the traditional taxonomy), a Superclass (example), a Subphylum, and a Phylum.
Of course, all such higher taxa are in some sense arbitrary--even an undoubted monophyletic clade can be classified at different levels by different people, and everybody acknowledges that, for example, traditional "Orders" of birds are not at all comparable to traditional "Orders" of, say reptiles. Most systematists these days just use clade-names without specifying a Linnean rank at all. Here , for example, is a pdf of an influential 2001 Nature paper on the subject, using a combined data set of molecular and morphological characters. Note that it found a nearly monophyletic Crustacea, except for one family of barnacles that weirdly grouped with Drosophila(!) and an obscure family of non-insect hexapods. In a more recent combined-evidence paper, however, the same authors use Phylum Arthropoda in an appendix.
Here, Wikipedia shows a cladogram from a 2006 paper based on mDNA alone that has the traditional crustaceans as a paraphyletic group with various groups of hexapods (including insects) nested within--presumably this is what Sanders was talking about.
Clearly, however, this will not be the last word on this most contentious subject!
Thanks for the plug!
As Sven has already indicated, the question's a non-issue - all taxonomic ranks above species are arbitrary constructs*, and whether a given taxon is referred to as "phylum", "class" or whatever is entirely a matter of convenience, tradition and/or ease of communication.
*Just to make sure no-one misunderstands me, the taxa themselves are not generally arbitrary, just the level of significance they're given.
That said, the concept of recognising the arthropod "subphyla" as separate "phyla" was mostly influenced by the influential suggestion in the mid-1900s, primarily by Sidney Manton (IIRC), that "arthropods" were a polyphyletic group that had originated multiple times from an annelid-like ancestor. As far as I know, this hypothesis has been entirely rejected by current researchers.
In respect of the weird position for Drosophila in the Giribet et al. (2001) paper, Drosophila is phylogenetically quite distant from the other insect taxa included, and its position probably represents simple long-branch attraction. Japygidae probably has the same problem.
Hi Chris, I really like your blog--keep procrastinating!
That darn long-branch attraction...always rearing its ugly head when I'm trying to learn about deep relationships of animals.
I'd be interested in your opinion of the Hassanin mDNA paper* that shows insects nested within crustaceans--can that be right??
*Alexandre Hassanin (2006). Phylogeny of Arthropoda inferred from mitochondrial sequences: Strategies for limiting the misleading effects of multiple changes in pattern and rates of substitution. Molecular Phylogenetics and Evolution 38: 100–116.
Re Hassanin et al. paper:
Hmmm... I'll start of with the things that I don't like. The outgroups used (some annelids, some chordates) are all quite distant from the ingroup, which instantly make their appropriateness somewhat suspect. The Myriapoda + Chelicerata clade has turned up in a few analyses, but I currently don't buy it. In this paper, the support for the clade is too low, and it conflicts with the morphological data supporting Mandibulata (Myriapoda + Pancrustacea) such as... well... mandibles.
Crustacea being paraphyletic to Insecta is certainly not impossible. The characters uniting crustaceans are fairly vague, with the group encompassing a wide range of morphologies and many groups having highly derived morphologies. Some potential morphologies uniting malacostracans and insects have been identified in the past, notably in the nervous system (see this paper, for instance.
As for Collembola not being directly related to insects and hexapods being polyphyletic... it's not unprecedented [he says cautiously]. A Russian author suggested a separate origin of Collembola from myriapods back in the 1950s, I believe. A more recent analysis by Giribet et al. (2004) also found a polyphyletic Hexapoda, with the entognathous orders (Collembola, Protura and Diplura) originating separately from the insects, and Collembola also unconnected to a Protura + Diplura clade.
Thanks, Chris. Very interesting stuff. I teach in intro course that includes a survey of animal diversity, so I try to keep up with the latest in phylogeny...it's hard these days!
So with regard to the arthropods, is it safe to say that many deep relationships are presently uncertain, but we can at least kiss the Uniramia good-bye?
So with regard to the arthropods, is it safe to say that many deep relationships are presently uncertain, but we can at least kiss the Uniramia good-bye?
Uniramia in the "polyphyletic Arthropoda" sense, which was Onychophora + Myriapoda + Hexapoda, is a definite non-contender. As for Uniramia in the restricted sense, or Atelocerata (Myriapoda + Hexapoda), I wouldn't want to completely rule out the possibility of a comeback, but it seems unlikely at the present moment. It seems probable that the characters shared by Hexapoda and Myriapoda, such as uniramous limbs, probably represent convergent adaptations to a terrestrial life. The arachnids also convergently evolved uniramous limbs.
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