Nematodes, also known as roundworms, are one of 37 living animal phyla, and one of the most numerous and diverse. Over 80,000 species have been named by science, and by numbers, nematodes are one of the most numerous animals in existence. Nematodes are found in large numbers in all terrestrial, marine, and freshwater environments, where they outnumber all other animals, including insects. It is estimated there are between 1018 (one quintillion) and 1021 (one sextillion) nematodes worldwide.
Nematodes have adapted to live in every possible environment, consuming bacteria, fungi, protozoans, and the contents of other animals or plants. Of the 80,000 scientifically named nematode species, over 18,000 are parasitic. Scientists believe the total number of nematode species may exceed 500,000. Different parasite nematodes are adapted to different hosts, and vary in length accordingly. Nematodes are called "roundworms" especially when found in the human intestine, where they may be several inches long and cause diseases. The largest roundworms ever found were 8.5 m (28 ft) in length, with 32 ovaries, found in the intestines of sperm whales.
Nematodes are thought to make up 90% of all life on the sea floor, and are found in the deepest ocean trenches, where the pressure is 100 times greater than at the surface. Three species of nematode are found in the McMurdo Dry Valleys of Antarctica, one of the harshest environments on Earth, where temperatures reach −60 °C (-76 °F) in the winter and wind speeds exceed 320 km/h (200 mph), stripping away almost all moisture. Nematodes can survive in little drops of moisture within rocks, consuming bacteria.
Nematodes can be a plague to farmers, causing billions of dollars of crop damage every year. On the plus side, one nematode, the 1-mm long Caenorhabditis elegans is a model organism in biology, and extensive study of its structure, life cycle, and genetics has been a boon to the field. Every cell in C. elegans (959 in the adult hermaphrodite; 1031 in the adult male) has been mapped, their entire developmental fate known. Using genetic manipulation techniques, scientists have been able to extend the lifespan of C. elegans tenfold, from about 19 days at the most to 190 days. Some of these techniques may find future applications in medicine.