A typical nematode or round worm, as seen under a microscope. (Courtesy photo | Bob Armstrong)

A typical nematode or round worm, as seen under a microscope. (Courtesy photo | Bob Armstrong)

The story behind these ubiquitous round worms

There are thousands of species of nematodes.

There are thousands of species of nematodes or round worms, found in virtually every habitat on earth, from the poles to the tropics, and on the insides and outsides of animals and plants.

Most of them are less than two inches long, although there are some notable exceptions (see below), and some are microscopic in size. They have only longitudinal muscles for locomotion, so most of the free-living species move by up-and-down undulation, pressing against particles of the substrate or the surface tension of water films (including those around soil particles). A few can swim short distances or move like an inchworm; one even jumps by flicking its tail. Nematodes feed by puncturing, piercing or scraping the surface of plant cells; a muscular pharynx then sucks in the nutritious fluids. The brain is a ring of nerves around the pharynx; from there, some nerves go forward to sense organs, and others go back to connect with motor muscles.

Nematodes are unusual in that they are often built of a fixed number of cells. There might be, for example, exactly 200 nerve cells, or 172 cells in the digestive tract; a famous nematode used in lab experiments is reported to have exactly 959 cells when fully grown. Juveniles have to grow, of course, which they do by adding some cells but mostly by cell expansion. Many nematodes have wonderfully complex life cycles, sometimes involving multiple hosts of very different taxonomic groups — so complex that one has to wonder how they came about.

They are among the most numerous animals on the planet — there can be millions or even billions of them per square meter. One scientist who studied nematodes famously claimed (maybe half joking) that if all the other organisms on the earth and all the mountains, valleys, lakes and oceans disappeared, we would still be able to discern a dim outline of all of those things, depicted in a thin film of nematodes.

One species of nematode has proved extremely useful to humans: Caenorhabditis elegans. It normally lives in soil, feeding on dead plant material, but has proved to be a good laboratory animal. It’s almost invisible, being transparent and only about a millimeter long. Nevertheless, scientists have mapped its entire genome and traced the course of development for every cell, contributing to our understanding of developmental processes and pathways — not only for the worm itself but for many other organisms, including humans, that share many of the same genes. Those shared genes also make it useful for studies of neurobiology and toxicology of other animals.

However, our attention is more often drawn to nematodes by their nefarious activity as parasites. Some species are parasites of plants, sucking juices from cells near the root surface; others bore into the plant and cause galls, sometimes killing tissue. Many nematodes parasitize animals in various ingenious ways.

Here are just a few examples: some large species can reach the relatively prodigious length of more than a meter; among these are Ascaris and its relatives that live in the intestines of vertebrates, feeding on the gut contents but somehow not being digested themselves. Hookworms also infest vertebrate digestive tracts, mostly feeding on blood; they get there when eggs deposited on the ground hatch and are picked up by bare feet, get carried in the blood stream to the lungs, and the juveniles migrate to the intestine. Some thread-like nematodes in a group called filarioids infest the lymphatic system of vertebrates; a serious infection over a long time can result in grotesque enlargement of various body parts (elephantiasis).

Another filarioid is the heartworm of dogs and foxes. In a different taxonomic group, the guinea worm, lives in body cavities and connective tissue and can reach a length of two meters. When it is time for a female to release her larvae, she moves toward the skin and creates an ulcerated opening, so the larvae can exit when the ulcer contacts water. This worm is famous for the ancient but still used method of extraction: winding it out very slowly, assiduously avoiding breakage, on a small stick. Some say that method is the basis for the traditional symbol of the medical profession, the caduceus – it’s not snakes on a staff but worms on a stick!

Many free-living nematodes are carnivorous, capturing other small multicellular animals, including other nematodes. Lots of aquatic nematodes, in both salt and fresh water, feed on diatoms, algae, fungi and bacteria; others feed on debris deposits. Some terrestrial nematodes live on lichens or in mosses, but more commonly live in soil, feeding on detritus or the bacteria and fungi that live on debris. They are an important part of the decomposer food chain. Although fungi are often food for nematodes, there are some fungi that turn the tables, snaring the worms in a loop of filamentous hyphae and digesting them.

Recent research has discovered amazing nematodes in a most unusual place: Mono Lake in California is known for its extreme salinity, much higher than the ocean, high alkalinity, and arsenic content. It was previously thought that only brine shrimp (Artemia) and alkali flies (Ephydra) lived there; they provided important food for many migratory birds (shorebirds, a grebe, phalaropes) and for nesting California gulls and some snowy plovers.

Reportedly just for fun, some nematode researchers decided to see if any nematodes could be found there. To their (and everyone else’s) surprise, they found eight species of tiny nematodes, in several different taxonomic groups, in the water, sediments and surrounding sands. Some are mini-grazers on algae and bacteria; some are likely to be predators or parasites. One of these surprising nematodes is a new species of Auanema, with close relatives elsewhere, and all of them are highly tolerant of prodigious levels of arsenic (hundreds of times what would kill a human). Part of the surprise is that the new species is so flexible that it also tolerates laboratory conditions, so it can readily be studied in detail, and the genome has already been mapped.

Although some accounts say that the new Auanema comes in three sexes, this is not quite true; there are males, females and hermaphrodites (that is, both male and female, and capable of self-fertilization), so it would be better to say that there are three forms of sex expression.


• Mary F. Willson is a retired professor of ecology. “On The Trails” is a weekly column that appears every Wednesday.


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