Nematode Round Worms are Related to Arthropods
Detailed Studies Reveal Basic Mechanisms, but Relationships Unclear
Mar 19, 2009
Round Worms (Nematodes) are extremely successful animals with over 80,000 species already described and many more awaiting discovery (half a million according to some estimates). They are found in all environments, and in some places their numbers beggar belief (90,000 individuals on a single rotting apple!).
Nematode Biology
Although the Round Worms (nematodes) live in all environments and exploit almost all possible ways of life (predators, parasites, grazers etc.), their structure is relatively simple and they all look very much alike. The identification of individual species is a matter for specialist and often relies on microscopic differences.
- Each species has a set number of cells in the adult form ((959 in the adult hermaphrodite form of Caenorhabditis elegans and 1031 in the male).
- They all move by throwing their bodies into s-shaped curves.
- Nematodes range in size from minute meiofaunal species up to earthworm sized parasites (look at the article image of Ascaris lumbricoides, a common human gut parasite).
Detailed Study of a Single Species - Caenorhabditis elegans
The free-living nematode Caenorhabditis elegans has been studied in great detail. It is easy to keep in the laboratory and its simple structure makes it ideal for researching basic questions about how it works.
- Caenorhabditis elegans was the first animal to have its DNA sequenced (first published in 1998, and fully completed in 2002).
- All 302 nerve cells have been mapped in detail, and the way they function in a variety of behaviours is becoming clear.
- Some cells (131 in the hermaphrodite form) which are initially present in the developing embryo are killed off as the animal matures, and studying the mechanisms involved might have medical significance.
Three Nobel Prizes have been won for basic research with Caenorhabditis elegans (2002, 2006, and 2008).
Molecular Studies and Nematode Classification
It has always been thought, and modern studies have confirmed, that the nematodes are very closely related to the Nematomorpha (Horsehair Worms) - but their relationships with other phyla remain unclear.
Annelids (True Worms) were once thought to be ancestral to the arthropods (crabs, spiders, insects etc.), but there is now the suggestion that this might not be the case and that the nematodes might in fact be closer (with the annelids only distantly related). This new position would have the Nematoda, the Nematomorpha, and also the somewhat mysterious Gastrotricha, closely related to one-another. These would then be seen as very close to the Tardigrada, Onychophora,and Arthropoda.
Interpreting Structure and Molecular information
Traditional interpretations of animal relationships relied heavily on looking at adult body plans. The problem with this is that it is difficult to tell whether similarities are the result of a common ancestry, or the result of ‘convergent evolution’, where the same solutions to problems are arrived at by different groups independently (e.g. bat, bird and insect wings).
Molecular studies go much deeper, but there are still many uncertainties. The details of all animal relationships are still not clear.
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