Parapharyngodon curupira is one of about four hundred new species of round worms that will be named this year. This particular worm was discovered in the northern Brazilian Amazon where it parasitizes the frog Osteocephalus taurinus. Only 11 species of the Parapharyngodon are known from the Neotropics, of which 6 are known to be frog parasites.
Many people have never knowingly seen a nematode, a round worm, yet we are all surrounded by them wherever we go. Nematodes vary tremendously in size, from many that are microscopic to a lesser number of comparatively larger ones, including free-living species reaching 2 inches in length and parasitic species as long as 3 feet. To the extent that there is a typical size range for threadlike nematodes it is 5 to 100m thick and 0.1 to 2.5mm in length. For perspective, recall that a micron is just one one-millionth of a meter. This means that the smallest nematodes are smaller in diameter than the finest human hair, while the largest ones are smaller in diameter than the thickest human hair! Various species may be characterized by rings, bristles, ridges or other distinctive surface structures. They are found in the soil where one nematode can eat several thousand bacteria per minute; in both fresh and salt water; from the tops of mountains to the depths of ocean trenches. In recent years, nematodes were found in deep mines, 9 km below the surface, and high in the atmosphere. There are both parasitic and free-living species, the latter with diverse food sources from bacteria to fungi, algae, plants, feces, and both living and dead animals. Some nematodes are helpful to agriculture, attacking garden and crop pests —some are sold commercially as biological control agents; others attack plants directly, or are vectors of plant virus diseases. Their mouth may have stylets that can puncture plant or animal tissues so that fluids may be extracted. It is estimated that 90% of the animals living on the ocean floor are nematodes and that, in topsoil alone, there are 60 billion nematodes for every human on earth. Some have claimed that eight out of every ten living animals on earth are, by headcount of individuals, nematodes.
Born in Spencer, Massachusetts in 1859, Nathan Cobb is considered the father of nematology in the United States. He named more than a thousand species of nematodes and created a taxonomic framework for the group. He explained the abundance and ubiquity of round worms in an eerie scenario published in the 1914 USDA Yearbook:“In short, if all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes. The location of towns would be decipherable since, for every massing of human beings, there would be a corresponding massing of certain nematodes. Trees would still stand in ghostly rows representing our streets and highways. The location of the various plants and animals would still be decipherable, and, had we sufficient knowledge, in many cases event heir species could be determined by an examination of their erstwhile nematode parasites.”
Dr. Mike Hodda of the National Research Collections Australia, recently published an analysis of trends in the discovery and description of nematode species. Looking at data between 2011 and 2019, as a percentage of the total number of species, nematodes ranked at the high end of groups for which new species are being named. About 1/8th of all known nematodes were named during this brief period of time, bringing to total to more than 28,000 species. During the first two hundred years of modern taxonomy, an average of 50 new species of nematode were discovered each year. By the 1950s, the annual number had risen to about 200 per year. In recent years, however, the average has doubled to about 400 species per year. While this is exciting news for round worm fans everywhere, there is no end in sight.
So, how many species of nematodes might there be? Various methods and assumptions have been used to estimate numbers, with predictions varying from 500,000 to 1 million species (and one much higher outlier that is not taken seriously). Even accepting the lower estimate, Hodda points out that, at the current rate of discovery, it would take a thousand years to find them all. As in most groups, the species being described reflect biases of geography (there are more nematologists in some countries than others), economics (far more attention is paid to species that impact our wallets), and methods, such as those used to sample nematodes, so the discovery pattern is unnaturally skewed. Growing economies in Asia, including India and China, have been accompanied by increased nematode discoveries.
With knowledge of fewer than five percent of Earth’s nematode species, drawing any intelligent conclusions about the group as a whole is risky at best. There is so much that we can, and should, learn from a group as successful as this one. Things about evolution, adaptation, and natural history. Cobb’s planetary-scale apparition should be enough to convince us that nematodes play incredibly important roles in all kinds of ecological systems and we ought not simply take them for granted as the world undergoes massive biodiversity changes.
So many nematodes and so little time. Round worms are one of many mega-diverse taxa about which our knowledge is rudimentary. As Cobb said, we must “…conceive of nematodes and their eggs as almost omnipresent, as being carried by the wind and by flying birds and running animals; as floating from place to place in nearly all the waters of the earth; and as shipped from point to point throughout the civilized world in vehicles of traffic.” By increasing the number of nematologists and taking full advantage of modern technologies, from molecular sequencing to cyberinfrastructure, we can vastly accelerate the pace of species discovery, replacing ignorance with knowledge. Even if you prefer not to think of the soup of round worms in which we live (I confess, I try not to dwell on it myself), it is very much in the interests of science and society to cheer on nematologists as they try to give round worms a square deal.
Further Reading
Cobb, Nathan (1914) Nematodes and their relationships. United States Department of Agriculture Yearbook, pp. 457-490 (quote on p. 472).
Hodda, Mike (2022) Phylum Nematoda: Trends in species descriptions, the documentation of diversity, systematics, and the species concept. Zootaxa 5114: 290-317.
Santos, A. N., Jesus, R. F., Macedo, L.C., Santos, J. N., Melo, F.T.V. (2022) New species of Parapharyngodon (Nematoda: Pharyngodonidae) parasite of Osteocephalus taurinus (Anura: Hylidae) from northern Brazilian Amazon region. Systematic Parasitology 99: 437-445.