Nematoda: Key Features, Types, and Biological Role

Nematodes are a group of worms. They occur naturally and are very hard to detect visually. These are common soil pests that affect plants. The soil at low levels contains numerous Nematodes. Nematodes can enter the farm through infected transplants. They are parasites of both plants and animals and attack the insects also. However, they cause severe damage to plants. But not all Nematodes are harmful to the plants. Some play an essential role in nutrient recycling.

Commonly known as roundworms, they are unsegmented vermiform pests. They are free-living organisms. Sometimes they enter the plant to extract nutrients from the root cell. They stress tolerance of the plant. Plants abundant with water and nutrients help can tolerate nematode attacks. Once they are present in the soil, It is almost impossible to eliminate. 

History

In 1758, Carolus Linnaeus described some nematode genera (such as Ascaris), then included in the taxon of worms, Vermes. The name of the group Nematoda, also called “nematodes", originally was defined by Karl Rudolphi in 1808. The term came from Nematoidea, defined from Ancient Greek. It was later treated as family Nematodes by Burmeister in the year 1837.

Nematoda - Definition, Characteristics and Classification

Following are the Nematodes Characteristics

1. Tubular in appearance. It has an elongated and thin body (hair-like).

2. The alimentary canal is distinct, but the head and tail are not visually different.

3. The majority of these are tiny and can be microscopic. 

4. They are free-living organisms. 

5. They reproduce sexually. They produce amoeboid sperm cells.

6. They have a nervous system. 

7. They are parasites of both plants and animals.

8. They have cuticles that moult periodically.

Classification of Nematodes 

Approximately 15000 species of Nematoda have been identified till now. Some nematodes live in the roots of plants; some spend their life inside the plants. These are not plant-specific. Following are the main three classes of Nematodes that has been classified further in subclasses and subclasses: 

1. Kingdom Animalia 

2. Phylum Nematoda

1. Class Rhabditea

1. Parasite Rhabditea 

2. Free-Living Rhabditea 

3. Rhabditis 

4. Tylenachia

2. Class Enoplea 

1. Enoplia 

2. Dorylaimia

3. Class Chromadorea

1. Chrimadoria

1. Kingdom - Animalia

Nematodes are multicellular eukaryotic organisms like other organisms (animals, plants, most algae, fungi, Metazoa, and protists) in the kingdom Animalia. Their cells contain a nucleus and other organelles. They obtain nutrients from organic sources, just like other organisms.

2. Phylum - Nematoda

Nematodes, also known as roundworms, make up the phylum Nematoda. 

Phylum Nematoda Classification

Phylum Nematodes are further classified into three major classes and subclasses:

1. Class Rhabditea

Class Rhabditea has both free-living and parasitic nematodes. The majority is of parasite nematodes in this class. Rhabditea Free-living feeds on bacteria as a source of energy. They can be found in between soil particles as well as in water. 

General Characteristics of Rhabditea

• They are unsegmented. 

• They Possess a cylindrical body. 

• They are tapered at either end.

• They possess a cuticle and hypodermis.

• Adult species have intestines, gonads and pharynx. 

• They have invaginated cuticles with nerves.

Rhabditea is Further Classified into Two Subclasses

1. Parasitic Rhabditea

Parasitic Rhabditea nematodes examples are Ascaris species, Enterobius species (e.g. human pinworm), Necator, and Wuchereria species. These species cause many serious diseases in human beings. This species is common in the tropics.

2. Free-Living Rhabditea Nematodes

They are found in temperate environments. They live in bacteria-rich habitats such as compost to obtain nutrients. They depend on other insects for transport from one location to another. Caenorhabditis Elegans worm is the best example of Free-Living Rhabditea Nematodes. 

Further Classification is as Follows

1. Rhabditis - They have well-developed Phasmids and poorly developed invaginated cuticles with nerves called aphids.

2. Tylenchia - Found in plants often in the form of parasites.

II. Class Enoplea

Enoplea makes up the phylum Nematoda. These ancestrally diverge nematodes. Some examples of Enoplea nematodes are Trichuris, Diotyphyme, and Diotyphyme.

Enoplea Nematodes Characteristics:

• Cylindrical or bottle-shaped oesophagus.

• Well-developed Amphids

• The simple excretory system made up of a few ventral or glandular cells

• Do not live in marine environments 

• Possess teeth-like structures

Subclasses of Enoplea Nematodes

1. Enoplia - They have oval or pouch-like amphids, cylindrical oesophagus, and smooth bodies. 

2. Dorylaimida - The majority of this species is a free-living organism. They can be predators or omnivores. Some, like Trichinella, exist as parasites. 

3. Class Chromadorea

This class can be found in a broader range of habitats as it consists of about four distinct lineages. This class also has both free-living and parasitic members. Chromadorea is smaller in size. But they are higher in number in their habitats as they reproduce at a higher rate.

Characteristics

• Three esophageal glands.

• Spiral aphids.

• Pore-like amphid of Chromadorea.

• Possess annulated cuticles.

• Glandular of tubular excretory systems.

Sub-class: (i) Chromadoria

Control Nematoda

The infected plants by nematodes cannot grow well, are paler than normal, often dwarfed, and may wilt in the heat of the day, small leaves. These symptoms can be misunderstood with symptoms of nutrient deficiency. It has been noticed that infected plants can look healthy while growing in fertile soil or during cool weather.

The growth of plants with nematodes will lead to a larger population of it. Try to inspect the roots of plants before placing them into the farm. Because if nematodes are present in the soil, they are almost impossible to abolish altogether, but the damage to plants due to them can be reduced.

Phylogeny

The phylogenetic relationships of the nematodes and their immediate relatives from the Metazoans family still remain unresolved. They were assigned to the group Ecdysozoa during the 1990s together with moulting animals (such as arthropods). However, they were identified quite succinctly with their closest relatives of Nematodes evidently by the morphological characters and molecular phylogenies. They are placed in a sister taxon to the parasitic Nematomorpha, which form Nematoda. The Scalidophora and Nematoda from the clade Cycloneuralia, but with some disagreement between the available morphological and molecular pieces of evidence. Cycloneuralia, with the validity of the available data, often makes the rank of superphylum.

Anatomy

Nematodes are about 5 to 100 µm thick and 0.1 to 2.5 mm long. They could be from the microscopic range to as much as 5 cm, while some could be even larger, reaching over 1m in length. The body has ridges, rings, bristles, or other distinctive structures.

The radially symmetric head of a nematode is relatively distinct, while the rest of the body is bilaterally symmetrical, having sensory bristles protruding outwards around the mouth. The mouth, which often bears teeth, has either three or six lips. The caudal gland is often found at the tip of the tail.

The epidermis is a single layer of cells covered by thick collagenous cuticles. The cuticle is of a complex structure and has distinct layers. Beneath the epidermis, a layer of longitudinal muscle cells is found. The relatively rigid cuticle with the muscles forms a hydroskeleton. Projections originate from the underlines of muscle cells towards the nerve cords in which nerve cells normally extend fibres into the muscles.

Digestive System

In carnivorous species, the oral cavity is lined with cuticles strengthened by ridges. The mouth often includes a sharp stylet to thrust into its prey. The stylet could be hollow and be used for sucking liquids from plants or animals. Digestive glands are found in the pharynx, producing enzymes that start to break down the food. The stomach is absent, with the pharynx connecting directly to an intestine that forms the main gut. This produces further enzymes to absorb the available nutrients with the help of its single-cell thick lining. The rectum is further lined by tiny cuticles to expel the waste generated through the anus just below and at the tail tip.

Excretory System

Nematodes excrete nitrogenous waste in the form of ammonia through the body wall, while salts are excreted by osmoregulation. In many marine nematodes, there are renette glands present. These glands are responsible to excrete salt through a pore present on the animal’s ventral side. The transverse duct opens into a common canal connecting the excretory pore in mostly other nematodes.

Nervous System

Four peripheral nerves are found to run along the length of the body on the dorsal, ventral, and lateral surfaces. The ventral nerve is the largest, while the dorsal nerve is responsible for motor control and the lateral nerves are for sensory actions. The nervous system contains cilia which are all nonmotile with a sensory function. The body of nematodes is shielded with numerous sensory bristles to provide the touch sense. Two small pits or 'amphids' have nerve cells and chemoreception organs.

Agriculture and Horticulture

Depending on its species, a nematode might be useful or detrimental to plant health. Two categories of nematodes are the predatory ones, which kill garden pests, while the pest nematodes (root-knot nematode) attack plants. There are vectors spreading plant viruses between crop plants. Eelworms or plant-parasitic nematodes often attack leaves and buds. Parasitic nematodes can be managed by the rotation of plants of nematode-resistant species. Natural antagonist such as the fungus Gliocladium roseum is used as a treatment method. Chitosan produces plant defence responses to destroy parasitic growth of nematodes on roots of various crops such as soybean, corn, sugar beet etc., without harming beneficial nematodes in the soil. However, soil steaming is an effective method to kill nematodes and eliminate both harmful and beneficial soil microbiota.