Nematode

80% of all animals on Earth are nematodes.

Nematode Summary

The most numerous animal on earth is the nematode or roundworm. Actually, this is a name for the whole phylum Nematoda, which includes anywhere from 25,000 to 1 million species. Nematodes are simple creatures that are on average made of about 1,000 cells and can live in environments with very low oxygen levels. As a result, they have been able to adapt to every habitat on earth, from the polar ice caps, the depths of ocean trenches, the soil in every habitat, and indeed the entire lithosphere of the planet down to a depth of 12,000 feet. There are so many of them, it has been said that if all the matter in our world except nematodes disappeared, we would still be able to see all the geographic features of the planet, its plants, animals, and people as a thin film of nematodes!

Nematode Facts

• Nematodes are the most numerous animals on earth, making up 8 out of 10 individual animals.
• They have a simple structure, made up of only 1,000 or so cells.
• Most nematodes can be seen only with a microscope, but the largest ones can reach 28 feet long and an inch in diameter.
• They have digestive, nervous, and reproductive systems but not circulatory or respiration systems.
• They live in every habitat on the planet.
• Some nematodes improve the soil quality and eat insect pests; others are themselves harmful parasites in crops, animals, or people.
• Nematodes can invade the human body through soil, water, or food. Heavy infestations can be harmful, but lighter ones can be unnoticeable.

Nematode Scientific Name

The word “nematoda” comes from the Greek word “nema” or “nematos” meaning “thread.” Clearly, it is a reference to the thread-like appearance of the species.

Nematode Appearance

Nematodes are very similar to each other so it is difficult to distinguish all of the species. They have been called a “tube within a tube” because of their very basic structure, with an opening at each end for taking in food and eliminating waste. They are often made up of only about 1,000 cells. Nematodes have nervous, digestive, and reproductive systems but not circulatory or respiratory systems. Their surface is smooth and unsegmented. Most species are microscopic, transparent, and invisible to the human eye. The visible species can be a variety of colors: white, brown, yellow, green, etc. The presence of harmful nematodes is most often detected by the damage they inflict. For example, root-knot nematodes cause swollen “galls” to appear on the roots of plants.

Nematode Evolution and History

Fossilized nematodes date back as far as 400 million years ago (the Devonian period), but some researchers believe they actually evolved about 1 billion years ago (the Precambrian era). If so, it means that they are among the oldest lifeforms on Earth, evolving right after bacteria, fungi, and protozoa. They probably evolved first in the sea as a parasite in marine invertebrates. Over time they infected new species, both plants and animals, as they evolved.

There are so many species of nematodes, most have not been studied or understood very well, and many are so close to one another in appearance that they can be virtually indistinguishable. Using not only direct observation but DNA evidence and computer modeling, researchers estimate there are anywhere from 25,000-1,000,000 species of nematodes.

Nematode Behavior

Nematodes in Agriculture

Farmers and gardeners apply nematodes to crops to control grubs and caterpillars. They are an environmentally friendly alternative to chemical pesticides. Species used for this purpose are not harmful to humans, animals, or plants. They target grubs, weevils, root maggots, and other pests. They work their way through moist soil until they find an insect host to infest. Along with the bacteria they bring with them, they kill their host within a few days.

Plant and Animal Infestation

In plants, they often attack roots in large numbers creating swollen nodules and interfering with the plant’s water supply and nutrition. In animals and people, they can infest various parts of the body and create dangerous secondary effects. For example, heartworms can create fatal damage in dogs. The largest nematode is a 28-foot parasite in the reproductive system of the sperm whale.

Human Infestation

About 138 species infect people. Some of these are harmless parasites that go undetected while others can create serious symptoms. For example, hookworms infect the gastrointestinal system and can cause abdominal pain, diarrhea, weight loss, fatigue, and anemia. Another species that is harmful to humans is onchocerca volvulus, a nematode responsible for river blindness. This disease has taken the eyesight of millions of people in South America and Sub-Saharan Africa.

Nematode Habitat

Nematodes live in all habitats on Earth: the polar ice caps, every body of saltwater and freshwater, the land surface of every continent and the ocean floor to a depth of up to 12,000 feet, in plants, animals, and people. Their simple structure and ability to survive in low oxygen conditions have enabled them to adapt and specialize to a level that no other creature on Earth has achieved. There are so many of them, it has been said that if all the matter in our world except nematodes disappeared, we would still be able to see all the geographic features of the planet, its plants, animals, and people as a thin film of nematodes!

Nematode Diet

Nematodes can be herbivorous or carnivorous, eating any kind of organic matter, dead or alive. They also eat small animals like other worms, algae, fungi, bacteria, or diatoms. Many species eat by inserting a straw-like “stylet” to drink fluid from their host. Other species open their mouths to swallow small food particles whole. Some of these have teeth-like structures in their pharynx to crush these particles into smaller bits for digestion.

Nematode Predators and Threats

Some of the main predators of the nematode are other nematodes. Some insects also eat them. Examples of these are beetle and fly larvae, centipedes, tardigrades, and mites. Human beings kill harmful nematodes in the environment with pesticides and in people and animals with antibiotics.

Nematode Reproduction and Life Cycle

A nematode goes through a three-stage life cycle from an egg to a larva to an adult. Females lay eggs that hatch into a larval stage. The larvae go through several metamorphoses and finally grow into an adult worm. Nematodes in animals are often spread by eggs expelled in animal feces.

Because they are often adapted to a specific host, many species of nematode die when their host does. For example, those that feed on plant roots become immobile after they affix themselves to their host. After they have killed the plant they have no way to move to a new food source and die.

There is a great variation in the lifespan of nematode species. They can live anywhere from 3 days to 15 years.

Nematode Population

There are 4.4 × 10²⁰ (100 million million million) nematodes in the earth’s topsoil This is 80% of all individual animals on Earth. There are 60 billion nematodes for each human being on earth. They live in every habitat and climate on the planet and all parts of the earth’s lithosphere down to a depth of 12,000 feet below the surface. They are the most successful species on the planet and face no danger of extinction.

Related Animals

• Worm
• Hammerhead worm
• Antarctic scale worm

1. Wikipedia, Available here: https://en.wikipedia.org/wiki/Nematode
2. National Library of Medicine, Available here: https://www.ncbi.nlm.nih.gov/books/NBK8261/#:~:text=Most%20enteric%20nematodes%20have%20established,increase%20in%20disease%20usually%20occurs.
3. Biogerontology, Available here: https://link.springer.com/article/10.1023/A:1026546719091#:~:text=Like%20insects%2C%20nematode%20species%20exhibit,the%20filarial%20parasite%20Loa%20loa.
4. Oregon State University, Available here: https://today.oregonstate.edu/archives/2011/apr/research-outlines-mysterious-evolution-nematodes-%E2%80%93-one-earth%E2%80%99s-first-animals