Nematode

Nematodes, also known as roundworms or eelworms, are a large group of animals called the phylum Nematoda. They can be found in many places, such as soil, water, and even inside other living things. Most nematodes are harmless and help break down natural materials. However, a few can live on other organisms and cause sickness.

Nematodes are grouped with arthropods, tardigrades, and other moulting animals. They have a simple body shape with a tube-like digestive system, unlike flatworms. Scientists think there are over 25,000 known species, and possibly many more that have not yet been discovered.

These small creatures live in almost every ecosystem on Earth, from deep ocean waters to high mountains and far underground. They are very numerous, with some spots of soil holding millions of nematodes in just one square meter. Nematodes make up a large portion of all animals on the planet and help keep ecosystems healthy by supporting plants and animals.

Etymology

The word nematode is made from two parts in Modern Latin. The first part, nema-, means "thread." It comes from an ancient Greek word. The second part, -odes, means "like" or "of the nature of." Together, they make the word "threadlike," which fits these worms well.

Taxonomy and systematics

See also: List of nematode families

Nematodes, also called roundworms, have been studied for a long time. In 1758, Carl Linnaeus described some of the first species. The name "Nematoda" comes from old words that mean "thread" and "species."

Scientists used to think nematodes belonged to just two groups. But newer studies using DNA have shown that they have more complex relationships. Today, we know there are many different groups of nematodes, each with its own special traits. Scientists are still learning more about how these small creatures are related.

Fossil record

Scientists found very old nematode eggs in ancient poop from a lake that existed long ago in what is now São Paulo. These eggs belonged to different groups of nematodes and show that these tiny worms lived with many other animals a long time ago. Nematodes have also been found in special fossil sites like Burmese amber, the Moltrasio Formation, and the Rhynie chert. These sites hold some of the earliest known fossils of these creatures.

Anatomy

Nematodes, also called roundworms, are small and thin worms. Most live freely in soil or water and are often less than 2.5 mm long, with some so tiny you can only see them with a microscope. But some nematodes that live on other animals can grow up to 50 cm long.

Their bodies often have ridges, rings, or tiny hairs. The head stands out from the rest of the body, with special hairs and sometimes hard parts around the mouth. They have a thick outer layer called a cuticle and muscles that help them move, working together like a water-filled skeleton. Their digestive system has a mouth with teeth or a sharp tool called a stylet, a muscular pharynx, and a simple intestine that takes in food. They also have a basic nervous system with a nerve ring near the head and special hairs to sense their surroundings.

Reproduction

Most nematodes have separate male and female individuals. They reproduce mainly through sexual means. Males and females have special organs for creating offspring. During mating, males use special structures to transfer sperm to the female.

Eggs are protected by a tough shell. They can hatch right away or develop inside the mother before hatching. Some nematodes can reproduce without males, creating clones of themselves. Different types of nematodes have many ways to ensure their species continue to grow.

Aging

The nematode Caenorhabditis elegans is often used to study how living things age. As these tiny creatures get older, they become less able to fix damage in their DNA. Special long-lived versions of Caenorhabditis elegans are better at repairing DNA. This suggests that the ability to repair DNA may be linked to how long an organism lives. In female Caenorhabditis elegans, the processes that help fix DNA and create new cells also get weaker with age.

Free-living species

Free-living nematodes eat many different things, such as bacteria, algae, fungi, small animals, and dead organisms. In the ocean, these roundworms help break down dead material and recycle nutrients. They can also show when the water is polluted because they are sensitive to changes in their environment.

One special roundworm, called C. elegans, lives in soil and is often studied by scientists. It has been helpful for research because scientists have mapped every cell and neuron in its body and have fully sequenced its genes.

Parasitic species

Nematodes that live on or in other organisms are called parasitic nematodes. Some common ones that affect humans include ascarids, hookworms, pinworms, and whipworms. There are also nematodes that live in animals, like Trichinella spiralis, which causes trichinosis, and Dirofilaria immitis, which causes heartworm disease in dogs and cats.

Some nematodes are helpful because they attack harmful insects. For example, certain nematodes can kill garden pests like slugs, snails, and various worms. People use them to control these pests without using harmful chemicals. However, some nematodes can harm plants by feeding on their roots or spreading plant diseases. These plant-parasitic nematodes can cause problems for farmers by reducing crop yields.

Soil ecosystems

Further information: Soil ecology

Most nematodes live in the top part of the soil. They help by eating bacteria. Some nematodes can harm plants and damage crops.

Some fungi in the soil also hunt nematodes. These fungi use tricks like lassos or sticky traps to catch them. They can also make toxins to harm nematodes.

Survivability

Nematodes are very tough creatures. For example, the nematode Caenorhabditis elegans survived the breakup of the 2003 Space Shuttle Columbia when it came back into Earth's atmosphere. This makes it the first known species to survive such a journey with almost no protection. Another nematode, Panagrolaimus davidi, can survive freezing inside its own cells. In 2023, scientists brought back to life a Panagrolaimus kolymaensis nematode that had been frozen in Siberian permafrost for 46,000 years.