Nematode

Nematodes, also known as roundworms or eelworms, are a large group of animals called the phylum Nematoda. They live in many different places, such as soil, water, and even inside other organisms. While most nematodes are harmless and help break down organic matter, some can be parasites, living off their hosts and causing diseases like soil-transmitted helminthiases.

Nematodes are classified alongside arthropods, tardigrades, and other moulting animals. They have a simple body shape with a tubular digestive system, unlike flatworms. Scientists believe there may be over 25,000 known species, with some estimates suggesting up to a million different types exist, though this number is still being studied.

These tiny creatures have adapted to nearly every ecosystem on Earth, from the deepest ocean trenches to mountaintops and even deep underground. They are extremely numerous, with some areas of soil containing millions of nematodes per square meter. In fact, nematodes make up about 80% of all individual animals on the planet and play important roles in maintaining healthy ecosystems and supporting plant and animal life.

Etymology

The word nematode comes from a combination of two parts in Modern Latin. The first part, nema-, means "thread" and comes from an ancient Greek word. The second part, -odes, means "like" or "of the nature of." Together, they create the word "threadlike," which describes these worms very well.

Taxonomy and systematics

See also: List of nematode families

Nematodes, also known as roundworms, have a long history of study. In 1758, Carl Linnaeus described some early species. The name "Nematoda" comes from ancient words meaning "thread" and "species."

The way scientists group nematodes has changed over time. Early classifications divided them into two main groups, but newer studies using DNA have shown more complex relationships. Today, we know there are many different groups of nematodes, each with its own special traits. Scientists continue to learn more about how these tiny creatures are related to each other.

Fossil record

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

Anatomy

Nematodes, also known as roundworms, are very small and slender worms. Most of them live freely in soil or water and are often less than 2.5 mm long, with some being microscopic. However, some parasitic nematodes can grow up to 50 cm long.

Their bodies are often marked with ridges, rings, or bristles. The head stands out from the rest of the body, featuring sensory bristles and sometimes hard shields 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 includes a mouth with teeth or a sharp tool called a stylet, a muscular pharynx, and a simple intestine that absorbs nutrients. They also have a basic nervous system with a nerve ring near the head and various sensory bristles to detect their surroundings.

Reproduction

Most nematodes have separate male and female individuals, though some can be hermaphrodites. They reproduce mainly through sexual means, with males and females having special organs for creating offspring. During mating, males use special structures to transfer sperm to the female.

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

Aging

The nematode Caenorhabditis elegans is commonly used to study how living things age at a tiny, molecular level. Research shows that as these creatures get older, their ability to fix damage in their DNA gets weaker. Special long-lived versions of Caenorhabditis elegans have been found to be better at repairing DNA, suggesting that how well an organism can repair its DNA might be linked to how long it lives. In female Caenorhabditis elegans, the processes that help fix DNA and form structures needed for creating new cells also get weaker as they age.

Free-living species

Free-living nematodes eat many different things, such as bacteria, algae, fungi, small animals, dead organisms, and even fecal matter. In the ocean, these roundworms are very important because they 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 very 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, and they are used by people 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 big problems for farmers by reducing crop yields.

Soil ecosystems

Further information: Soil ecology

Most nematodes live in the top part of the soil, where they help control bacterial populations by eating them. They can eat up to 5,000 bacteria per minute! However, some nematodes harm plants and cause a lot of damage to crops around the world.

Some fungi that live in the soil are natural hunters of nematodes. These fungi use special tricks, like lassos or sticky traps, to catch nematodes. They can also produce toxins to harm the nematodes when they get close.

Survivability

Nematodes are very tough creatures. For example, the nematode Caenorhabditis elegans survived the breakup of the 2003 Space Shuttle Columbia during its re-entry 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, depending on how much it has eaten. In 2023, scientists brought back to life a Panagrolaimus kolymaensis nematode that had been frozen in Siberian permafrost for 46,000 years.