Nematodes

Definition, Examples, Classification and Characteristics


Definition - What are Nematodes?




Commonly known as roundworms, nematodes are a group of worms that make up the phylum Nematoda. With well over 15,000 species identified today, they can be found in different habitats ranging from terrestrial to marine environments. Nematodes exist as free living organisms in terrestrial and aquatic environments or as parasites of both plants and animals. 


Some of the worms belonging to the phylum Nematoda include:


  • Ascarids
  • Filarias
  • Whipworms
  • Adenophorea
  • Enoplea

Some of the general characteristics of roundworms include:


  • A round cross section
  • Elongated body
  • They are very small and can be microscopic. However, some can measure up to one meter long.
  • They reproduce sexually (separate sexes)



Classification


Kingdom - Animalia

Like many other organisms in kingdom Animalia (also referred to as metazoa) nematodes are eukaryotic, multicellular organisms that obtain nutrients from organic sources.

Phylum - Nematoda

Nematodes/roundworms make up the phylum Nematoda.


This phylum is composed of organisms with the following characteristics:


  • Thin and elongated (hair-like)
  • Tubular in appearance
  • Majority are microscopic while a few can be viewed with the naked eye.  However, Placentonema gigantissima, a nematode that infects sperm whales has been shown to grow up to 9 meters (30 ft) in length
  • Head and tail are not visually distinct
  • They have a cuticle
  • They have a nervous system that controls the muscle cells


Classes of Phylum Nematoda


Nematodes are further divided into three major classes and subclasses that possess several different characteristics including:


Class Rhabditea


Class Rhabditea is composed of both parasitic and free-living nematodes. However, the majority of nematodes that exist as parasites are found in this class.

Parasitic Rhabditea

Parasitic nematodes found in class Rhabditea include Ascaris, Enterobius (e.g. human pinworm), Necator species as well as Wuchereria species. These species infect and cause a variety of diseases in human beings ranging from minor to very serious conditions.

For instance, Wuchereria bancrofti, a species belonging to the genus Wuchereria causes a serious condition known as Lymphatic filariasis characterized by the swelling of arms and legs. This is a serious condition that is not only painful, but also causes the disfiguration of arms and/or legs. Compared to other worms in this class that can be found in many regions across the world, this species of nematodes are common in the tropics. On the other hand, intestinal infections caused by such nematodes as Enterobius and Ascaris can be treated and do not usually result in serious health problems.

Free-Living Nematodes in Class Rhabditea

One of the best examples of free-living nematodes in class Rhabditea are members of the genus Caenorhabditis.

Caenorhabditis elegans, a species of Caenorhabditis is a small, free-living worm found in temperate environments. Apart from nutrition sources in decaying matter, these worms also obtain nutrients from rotting fruits. In particular, Caenorhabditis have been shown to live in bacteria-rich habitats such as compost where they obtain their nutrients and only depend on other organisms (insects etc) for transport from one location to another.


General Characteristics of Rhabditea


Free-living nematodes in class Rhabditea can be found in various terrestrial environments between soil particles as well as in fresh water. In these environments, they also feed on bacteria as a source of energy. 


With regards to morphology, members of class Rhabditea have the following traits:


  • Many are unsegmented
  • Possess a cylindrical body
  • Tapered at either end
  • Possess a cuticle and hyperdermis
  • Have an inner and outer tube with the inner tube consisting of intestine, pharynx, and gonads among the adult species
  • They have amphids - Invaginated cuticles with nerves

Rhabditea is further divided into two subclasses that include:

  • Rhabditia - Characterized by well-developed posterior sensory structures known as Phasmids but poorly developed amphids
  •  Tylenchia - Parasitic forms are often found in plants.


Class Enoplea


Enoplea, like Chromadorea, also makes up the phylum Nematoda. However, compared to Chromadorea, researchers have described Enopleans as ancestrally diverged nematodes. As such, they are the more ancestral group of nematodes that have not diverged as much as members of the other classes.  Some of the worms belonging to this class include Trichuris, Diotyphyme, and Diotyphyme.

Trichuris Trichula

Also known as the human whipworm, Trichuris trichula are roundworms that are responsible for trichuriasis.


Apart from being human parasites, Trichuris trichiura have the following characteristics:


  • Female T. trichiura can produce as many as 20,000 eggs per day which enter the infective stage after two or three weeks. Eggs of T. trichura are characterized by a prominent bipolar plug
  • Pinkish-white in color
  • Their anterior esophageal end is narrow in appearance while the posterior end is thicker
  • Obtain their nutrients from tissue secretions
  • They measure between 35 and 50 mm in length
  • They are mostly found in the tropics (particularly Asia)

Some of the other worms affect other types of animals. For instance, Trichinella spp. infect Black Bear, Dingo, and Polar Bears among others. As well, some members of Dioctophyme can affect both human beings and various carnivores and survive as parasites.

Compared to other classes of nematodes, Enopleans have the following general characteristics:


  • Possess a cylindrical esophagus. For some of the species, however, the esophagus is bottle-shaped
  • They have amphid (well-developed Amphids) of Enoplea resembling a pocket. Whereas the Amphids (innerved invaginations among nematode species) of the members of chromadorean may be slit, pore or spiral-like. Enopleans are pocket like which may be used to differentiate between the two classes.
  • Compared to chromadorean species, Enopleans are also characterized by a smooth appearance or fine lines on the surface of some of the species
  • Possess a simple excretory system that is lacking lateral canals (also made up of one or a few ventral or glandular cells)
  • Lack phasmids


Some Characteristics of Enopleans


  • Some are plant-parasites e.g. members of order Dorylaimida
  • Free-living members can be found in fresh water environments as well as given terrestrial environments. A good example of these is Oncholaimus moanae sp., has been shown to live freely in the fine sands of southern New Zealand.

Class Enoplea is also made up of the following subclasses:

Enoplia - This subclass is characterized by oval or pouch-like amphids, cylindrical esophagus, and smooth body. It includes the members of Orders Trefusiida and Enoplida

Dorylaimia - Members of this sub-class can be found in different terrestrial and freshwater environments. A majority of these species are free-living organisms that either exist as predators or omnivorous in their respective environments. However, some, like Trichinellida live as parasites.


Some of the characteristics specific to members of this subclass include:

  • Do not live in marine environments
  • May possess odontostyle or stoma consisting of teeth-like structures
  • Posterior region of the esophagus consists of esophageal glands

The subclass also consists of the following orders:

Order 1: Dorylaimida and Longidoridae

Order 2: Mermithida

Order 3: Mononchida

Order 4: Dioctophymatida

Order 5: Trichinellida

Order 6: Isolaimida

Order 7: Muspiceida

Order 8: Marimermithida


Class Chromadorea


Characteristics

Chromadorea, a class of Nematoda only consists of a single subclass (Chromadoria) which in turn consists of several orders. Members of this class are characterized by three esophageal glands, spiral amphids as well as having annules.

The amphid of Chromadorea are pore-like and may range from labial to post-labial coil or spiral-like apertures. Compared to members of Enoplea, Chromadorea possess annulated cuticles that may be decollated with projections and setae. If present, phasmids are located at the posterior end while their excretory systems may either be glandular of tubular in appearance.

Compared to Anoplia, Chromadoria is currently said to consist of about four distinct lineages. As a result, this class of nematodes can be found in a wider range of habitats.

For instance, while some species can be found in marine environments, others can be found in various terrestrial habitats including on such plants as mosses and lichens to different types of soils. Their cuticle has also been shown to have gone through significant evolutionary modifications, which may help explain their ability to live and survive in different habitats as well as ornamentations.

Among members of the order Rhabditida, researchers discovered a chemically impermeable cuticle which enables the worms to survive in various extreme habitats and also thrive as parasites. Apart from the cuticle, modifications identified in the pharynx of these worms have also been attributed to evolutional developments. This has allowed these organisms to develop a more compact body design when compared to members of class Enoplia. 

This class is also composed of both free-living and parasitic members. As compared to Enoplia and Dorylaimia worms, free-living Chromadorea are smaller in size. However, they also reproduce at a higher rate and are therefore greater in number in their habitats.

For some of the species, compressed generation time has been shown to play an important role in their ability to effectively colonize their habitats. Apart from this evolutionary advantage, they have also been shown to have other adaptations that enhance rapid dispersal to other habitats.

Among members of order Rhabditida (in subclass Chromadoria) long-term survival has been shown to be the result of a special, modified juvenile stage. Whereas the proto-dauer stage is evident among some species, the true dauer stage among some members of order Rhabditida are capable of finding and using other larger animals for their transportation.

At the same time, these organisms in this state are also capable of invading and living in the internal organs of various higher animals. This, in addition to their ability to survive various extreme environments over the course of their lifespan, has allowed various species of order Rhabditida to compete successfully in their environments.

The following is the classification of Class Chromadorea

Phylum- Nematoda

Class - Chromadorea

Subclass - Chromadoria

Subclass Chromadoria is further divided into the following orders:

Order 1 -  Rhabditida

Order 2 - Plectida

Order 3 - Araeolaimida

Order 4 - Monhysterida

Order 5 - Desmodorida

Order 6 - Desmoscolecida

Order 7 - Chromadorida


General Characteristics of Nematodes


Nematodes are some of the most abundant invertebrates on earth.

According to research studies, nematodes rival Arthropoda both in biodiversity and the abundance of species. For instance, while as many as 20,000 species of nematodes have been described, diversity among these species are estimated to range between 100, 000 and 10 million.

As a result of their evolutional path that has resulted in their diverse structures, physiology, and varying reproductive patterns, nematodes have been shown to be among the most successful organisms invading many more habitats than other multicellular organisms. 

Despite there being many diverse organisms, nematodes have a canalized body plan that is often attributed to selection pressure over many millions of years. While a majority of species reproduce through bisexual and amphimictic means, they can also use several other means of production including autotoky, hermaphroditism, and Pseudogamy where the zygote develops as a result of stimulation rather than fertilization.

With regards to morphology, nematodes are mostly non-segmented with a cylindrical cross-section with most of them being microscopic (less than one millimeter in length). A majority of the species also have a complete alimentary tract and a well developed nervous system that controls the cell muscles.

Apart from parasitic species which obtain cell and tissue secretions of the host, nematodes also have a number of different feeding strategies. For instance, whereas some of the species feed on various plants, algae and detritus, others are carnivores that not only feed on other smaller organisms (fungi and bacteria), but also on other nematodes. 

While a good number of nematodes are free-living organisms, there are many parasitic species. Parasitic nematodes infect different types of animals and human beings. For some of the species that are parasitic to human beings, such insects as mosquitoes act as intermediate hosts.

Through these insects, nematodes develop before infecting human beings with the potential of causing various diseases. Filaria, which causes Onchocerciasis (eye and skin disease) in human beings is often transmitted by black flies.

In their animal hosts, most of the roundworms move to and live in the intestines. In this part of the body, they lay thousands of eggs each day that reach the infective stage within a period of two weeks. In their life cycle, the eggs may develop in other hosts once they are ingested. When ingested, the eggs hatch to produce larvae that move to the intestinal tract where they develop further and continue reproducing. In human beings, some of the nematodes may enter the bloodstream where they cause tissue damage and they continue spreading to other parts of the body.

Apart from animals and human beings, nematodes also infect plants (especially plant roots). In trees, they can cause a significant decline by affecting fruit production as well as overall growth. However, for other types of plants, they can completely destroy the crops or cause significant crop reduction. For example, nematodes can cause as much as a 50 percent reduction of lemon and orange yield.


Benefits of Nematodes


One of the main characteristics of some species of nematodes is that they are natural predators. This is a big advantage given that these species have been shown to hunt and kill well over 200 species of pests. As such, these nematodes act as biological control of various pest insects that affect human beings and plants.

As biological control, one of the biggest advantages of these beneficial nematodes is the fact that they do not negatively affect the health of human beings and animals as is the case with various chemicals. While they may infect certain insects with bacteria that ultimately kills the insect, some of the nematodes also live as endoparasites in some of the agricultural pests and kill them in the process.

* Apart from the fact that they do not affect human beings and animals in the process, they are also beneficial to the soil given that they contribute to decomposition and do not pollute the soil.


Also, the beneficial nematodes play an important role as decomposers. Given that they can attack and kill various organisms and also feed on various plants, some of the nematodes act as intermediate decomposers that are responsible for the breakdown of organic material that are then further broken down by other smaller microorganisms such as bacteria.



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References


Alan F. Bird and Jean Bird. (2012). The Structure of Nematodes. 

Donald Lee. (2002). The Biology of Nematodes

Terry Niblack. Nematodes.

Daniel Leduc. (2008). Description of Oncholaimus moanae sp. nov. (Nematoda: Oncholaimidae), with notes on feeding ecology based on isotopic and fatty acid composition

Paul De Ley. (2006). A quick tour of nematode diversity and the backbone of nematode phylogeny. 

Links 

https://www.ncbi.nlm.nih.gov/books/NBK19684/

https://www.sasa.gov.uk/seed-ware-potatoes/nematology/what-are-nematodes


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