Roundworms

Roundworms, also called threadworms or nematodes, constitute the phylum Nematoda. Between twenty and thirty thousand species of nematodes have been described, but scientists believe there may be over one million species. Four out of every five animals on Earth are roundworms. Nematodes occupy almost every environment imaginable, ranging from soil to fresh and saltwater and even vinegar and beer malts. One handful of soil generally contains thousands of free-living nematodes. Some species are also notably hardy; for example, in 2023, two roundworms were revived after lying dormant for 46,000 years in the Siberian permafrost. Researchers named the species Panagrolaimus kolymaensis.

Most roundworms are harmless to humans, and many are highly beneficial to their ecosystems. However, others are notable for the huge economic impact they have as crop and animal pests. Some species of nematodes are also human parasites. Nematode parasites and their hosts may have simple associations, in which the nematode uses the host to transport from one food source to another, or more complex associations, in which the nematode is essentially a predator devouring the host from the inside.

Roundworm Facts

Classification:

Kingdom: Animalia

Subkingdom: Bilateria

Phylum: Nematoda

Classes: Chromadorea, Enoplea, Dorylaimida

Geographical location: Worldwide

Habitat: Soil, freshwater, and saltwater; extreme habitats such as decaying cacti and vinegar malts; several species are plant or animal parasites

Gestational period: Varies with species

Life span: Varies with species; some can enter a dormant state and survive for extended periods under adverse environmental conditions, even tens of thousands of years; most like twelve to twenty months

Special anatomy: External cuticle made of collagen; cylindrical bilaterally symmetrical organisms with a pseudocoelom; exchange of oxygen and carbon dioxide occurs across the body wall

Anatomy

Nematodes are unsegmented worms with a pseudocoelom and are round in cross-section, hence their common name roundworms. The worms range in length from one-tenth of a millimeter to nine meters. Most roundworms are transparent. All nematodes share a basic body plan of two concentric tubes separated by a fluid-filled space called a pseudocoelom. The outer tube is covered by a noncellular cuticle composed of the protein collagen, which is secreted by the cells immediately underneath. The inner comprises the pharynx and digestive canal and includes the nervous system.

Most of the cells of the nervous system have their cell bodies clustered in a nerve ring that surrounds the pharynx. The pharynx is a muscular structure that pumps food into the worm’s intestine. Roundworms have no specialized organs for circulation or excretion. The exchange of oxygen and carbon dioxide occurs across the body wall.

Roundworms move using a wavelike motion that relies on the contraction of the four sets of muscles that run the length of the worm. The worms have no circular muscles, so they cannot expand and contract like segmented worms.

Reproduction

The reproduction of nematodes is sexual through internal fertilization. Most nematode species are dioecioushaving both males and femaleswith the notable exception of the genetic model system, Caenorhabditis elegans. Males have specialized organs in the tail, copulatory spicules, which aid in depositing sperm into the vulva of a female. Nematode sperm do not swim using a flagellum but instead are amoeboid and move in a crawling motion using a pseudopod.

Nematodes reproduce in massive quantities. Scientists have documented the production by the females of some species of up to twenty-seven million eggs and the laying of as many as one million fertilized eggs per day. Some parasitic species, such as the cotton root-knot nematode parasite Meloidogyne incognita, can reproduce via parthenogenesis. Sex determination can be influenced by environmental factors in some species.

Roundworms hatch directly from eggs and undergo four molts before they become adults capable of reproducing. The life span and generation time vary. However, the well-studied, free-living species, C. elegans, has a normal life span of two weeks, with a new generation taking only three days to develop from egg to adult.

Parasitic Nematodes

Some nematodes infect humans, causing filarial diseases that affect 120 million people, mainly in tropical areas. With the disease elephantiasis, failure of the lymphatic system results in gross swelling of the limbs and genitals. Sometimes called the pork worm, Trichinella spiralis is the causative agent of trichinosis, a disease acquired by eating undercooked, infected pork or other meat. The worms encyst in the meat, hatch in the consumer's gut, and migrate to the skeletal muscle of the infected individual, resulting in muscle pain and an itching sensation for the infected person.

Nematodes are also important parasites of domestic animals. Dog and cat heartworm is caused by Dirofilaria immitis. Pigs are often infected by Ascaris species. Haemonchus contortus is a gut parasite of sheep that can result in anemia, weight loss, and even death of the host.

In addition to animal parasites, plant parasitic nematodes pose a serious agricultural problem. The root-knot nematodes, Meloidogyne species, are among the most serious plant pests, infecting more than two thousand plant species. These worms hatch in the soil as second-stage larvae. From the soil, they then invade the root tip of the host plant and feed directly from the plant's cells.

Caenorhabditis elegans

Despite the diverse parasites of this phylum, most roundworms are beneficial, aerating the soil and breaking down decaying matter. One free-living soil nematode that has provided much information about development and genetics is Caenorhabditis elegans. This millimeter-long worm is the subject of intense investigation on a cellular and molecular level. Studies in C. elegans have led to a greater understanding of genes mutated in Alzheimer’s disease, an understanding of programmed cell death as a normal part of development in multicellular organisms, and the development of the technologies used to sequence the human genome. Studies have also indicated that C. elegans itself is highly diverse and greatly affected by geographical location.

Principal Terms

Cuticle: the outer, noncellular covering of the nematode

Pharynx: a muscular organ that is used to pump food into the digestive system of the nematode

Phasmid: sense organs located in the tail region of roundworms that are important for detecting chemical signals in the environment

Pseudocoel: the unlined body cavity of the roundworm

Bibliography

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