World Distribution of Roundworms
Typical nematode morphology. Image obtained courtesy of BIODIDAC: http://www.ucmp.berkeley.edu/phyla/ecdysozoa/nematoda.html
Phylum Nematoda or roundworms are found across the globe almost anywhere there is organic matter. Roundworm habitats include but are not limited to seas, fresh water, soil, and almost every species of plant and animal. Around 20,000 species of roundworms are known and have been classified (Malakhov xi). Because there are very few scientists looking for new roundworm species, the discovery of new species can be rather slow, especially in regards to free living nematodes (Malakhov xi). Moreover, nematodes all share basic morphologies and are difficult to distinguish between (Malakhov 175).
Roundworms are either parasitic or free living. Parasitic roundworms are much more likely to be discovered and classified because they are of more concern to humans. In fact, for some time only parasitic roundworms were known, and today they are much more likely to be studied. This provides skewed knowledge of nematodes because there are more free living species than parasitic ones; around 65% of classified nematode species are free living (Malakhov xii).
All nematodes, however, show incredible ability to reproduce. There are certain species that can carry more than 27 million eggs at once. These species can lay up to 200,000 eggs in one day (Waggoner 2004). So, needless to say, nematodes are extremely abundant in the world. Unfortunately, the amount of classified nematode species can be no more than 20% of the total number existing nematode species. Some scientists, taking into account the problems with finding and classifying roundworms and their relative abundance, have estimated the amount of undiscovered nematodes to be anywhere from 100,000 to 1 million (Malakhov xi). If this statement has any merit then nematodes would be second only to arthropods as the most diverse group of animals (Waggoner 2004).
Evolution & Phylogenetic Relationships
Features Shared by Nematodes with Related Groups