NEMATODE DIVERSITY

Nematode - Definition

A spindle-shaped colourless worm, also called roundworm, belonging to the phylum Nematoda (about 80 000 species). Most nematodes are less than 3 mm long and have a mouth at one end, sometimes containing teeth or stylets, and usually a short muscular pharynx leading to the intestine.

What are Nematodes

Nematodes are the most numerous multicellular animals on earth. A handful of soil will contain thousands of the microscopic worms, many of them parasites of insects, plants or animals. Free-living species are abundant, including nematodes that feed on bacteria, fungi, and other nematodes, yet the vast majority of species encountered are poorly understood biologically. There are nearly 20,000 described species classified in the phylum Nemata.

Nematodes are structurally simple organisms. Adult nematodes are comprised of approximately 1,000 somatic cells, and potentially hundreds of cells associated with the reproductive system . Nematodes have been characterized as a tube within a tube ; referring to the alimentary canal which extends from the mouth on the anterior end, to the anus located near the tail. Nematodes possess digestive, nervous, excretory, and reproductive systems, but lack a discrete circulatory or respiratory system. In size they range from 0.3 mm to over 8 meters.

Phylum Nemata

The word Nematoda comes from the Greek words nematos, meaning thread, and eidos, meaning form. Over the years, nematodes have been classified in four different phyla, not always under the same name. There are two contending names for the phylum of nematodes. In 1919, Cobb named the study of nematodes nematology and therefore wished to rename nematodes nemata. Cobb also placed nematodes in their own phylum, the phylum Nemata. However, when nematodes were placed in the phylum Aschelminthes, they were classified as class Nematoda (along with class Rotifera, class Gastrotricha, class Kinorhyncha, class Priapulida and class Nematomorpha).

Current trends in the study of Nematodes

Nematodes were largely regarded as only of marginal interest not so long ago. But due to recent awareness about their role in agriculture and soil ecosystem, work has began on these animals in a big way in many parts of world, in particular on plant and soil nematodes.

Various characteristics of Nematodes that facilitate their study

They possess transparent bodies that facilitates direct observation of their structures without a real need to stain them. Short life cycles, cell constancy and easy to follow embryonic development makes them one of the finest models for various kinds of biological studies. Some nematode species are regarded as harmful because of threat that they pose to human health and also that of animals and crops. But there are others, which are useful in suppressing pest populations and also in serving as useful bio-indicators. A few entomophagous species are much in demand these days because of their role in controlling insect populations world over.

Distribution and diversity in Nematodes

Basic pattern of body organization may be very similar in all nematodes, but these tiny primitive creatures are structurally very diverse and have in many respects, amazing ways of life. Foremost among these is their numerical superiority, which surpasses all imaginations. A single acre of soil from an arable land is said to contain as many as 3,000,000,000 nematodes, while those from sandy beach nearly half as much. A single grain of wheat parasitized by well known wheat-gall nematode, Anguina tritici may contain up to 100,000 of its juveniles. Those who may be familiar with techniques for their extraction from soils would indeed know that in a single sample it is not unusual to find 10-15 genera of nematodes with as many or more species of different kinds and characteristics. Rarely these numbers could be astounding. For example, in one soil sample, weighing not more than 1 kg from a locality in Malnad tracts of Karnataka, we had ourselves recorded over 60 genera of nematodes. This alone speaks volumes about tremendous diversity of group as also their wide distribution. Leaving aside perhaps protozoans, no other animal group, insects included, is so numerous and widespread. The claim of some nematologists that nearly 90% of all metazoans in the world are nematodes, something about which entomologists would rather be skeptical, is indeed closer to truth than exaggeration. Apart from the numerical superiority of nematodes, species numbers are also unbelievably very high, close on the heels to that of insects.

Nematodes Vs Insects

Common man usually has a very good knowledge of insects as pests of crops and stored grains and also as parasites, carriers, vectors and intermediate hosts of many well-known diseases. Insects such as houseflies, bedbugs, locusts, butterflies, honeybees, silkworms, lac insects etc. are well known to mankind, largely due to their economic importance. But the same cannot be said about nematodes in spite of their almost equal importance.
Insects cause damage to our crops and also parasitize man and higher groups of animals. Nematodes are capable of doing all this and more. Structural, functional and habitat diversity of two groups is also quite comparable. Maybe nematodes have a slight edge over insects in their adaptability and survival under adverse conditions. It is not surprising that many plant protection scientists regard nematology as younger sister of entomology. Principles are same, methodologies and approaches are similar, only organisms and set of scientists are different.