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Written by Administrator   
Sunday, 19 February 2006

Earthworm Biology

Pest Management Research Center

Agriculture & Agri-Food Canada

By Alan D. Tomlin

Introduction

"---the intestines of the soil" - Aristotle (about 330 B.C.)

"It is a marvelous reflection that the whole---expanse has passed, and will again pass, every few

years through the bodies of worms. The plough is one of the most ancient and most valuable of man's inventions; but long before he existed the land was in fact regularly ploughed, and still continues to be thus ploughed, by earthworms. It may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organized creatures."
-Darwin (1881)

Obviously man has been generally aware of the existence of earthworms for a long time. This is not to suggest that man has an accurate, comprehensive knowledge of earthworms. Outside of 2 or 3 species of worms we know little and understand less of the biology of earthworms.

Nineteen species of earthworm are found "in the wild" in Ontario, only 2 of these species are native to North America. Three other species of worms, Eudrilus eugeniae (African night crawler, national night crawler), Amynthas sp. (Georgia jumper, grey night crawler) and Eisenia foetida (manure worm, red wiggler, tiger tail, brandling, also found "in the wild" in Ontario) have been introduced into Ontario for commercial scale culture. Occasionally another species, Aporrectodea trapezoides (garden worm, dug worm) is cultured commercially as a bait worm.

Another common species of worm in Ontario is Lumbricus terrestris (dew worm, Canadian night crawler). It was introduced from Europe to North America by settlers probably several times over the past 350 years. This worm is harvested from golf courses for example and sold as bait. The declared number of dew worms exported to the United States was about 500 million in 1978. This particular species of worm is highly prized as bait, and demand causes prices to fluctuate dramatically during the fishing season.

The Position of Earthworms in the Animal Kingdom

The major subdivisions of the animal kingdom are the phyla. Earthworms belong to the phylum Annelida; for example, insects, spiders, crayfish, crabs belong to the phylum Arthropoda; humans, frogs, birds and fish belong to the phylum Chordata. A phylum, then, can include many apparently different kinds of animals. The phylum Annelida is further divided into the Polychaeta (aquatic and marine worms) and the Oligochaeta (the earthworms). In Ontario all worms but one species fall into the family Lumbricidae, which means Ontario worms are fairly closely related.

The Structure of Earthworms

Earthworms are externally segmented (with corresponding internal segments) with no skeleton. They have a thin coloured cuticle bearing setae (bristles). They are designed as a "tube within a tube" - the digestive tract runs the length of the body from mouth to anus, with the organs of reproduction, water balance control, and nervous system control lying between the 2 tubes. Strong sets of muscles make up much of each of the inner and outer "tubes". A blood-like fluid fills the rest of the space between the two tubes

The Physiology of Earthworms

Different species of earthworms are adapted to different climates (temperature and moisture) and to different soil types (high organic carbon content) to mineral soils (very low carbon content). These adaptations are responsible for a wide range of physiological differences between earthworm species.

However, there are some general comments to be made:

 1. Earthworms respire ("breathe") by absorbing oxygen through their body surface into the body and sending carbon dioxide out. The oxygen dissolves in the mucous on the body surface and then passes into the highly branched capillary blood vessels in the body wall where it is picked up by the respiratory pigment and transported to the internal organs.

2. Earthworms may survive for considerable lengths of time in water if the dissolved oxygen level is high enough.

3. Earthworms cannot maintain a steady body temperature like humans or dogs or cattle.This means their respiration rate rises with increasing temperature. Increasing respiration rates mean increased metabolic rates which means increased energy requirements which means increased feeding rates by worms or food must be "stolen" from reserves in the worms' own body tissues. The reverse is true for decreasing temperatures.

4. Earthworms derive their nutrition from organic matter in a wide variety of forms. So far plant matter (various forms, fresh-decayed), protozoans, rotifers, nematodes, bacteria, fungi and decomposing remains of other animals are known to be worm food.

5. For the manure worm, Eisenia foetida, there is strong evidence that protozoans form the basis of their diet. If this is true, it means successful culture requires that protozoan populations be high in the "food medium" whether that "food medium" be shredded paper, manure or waste foods.

6. Excretion of metabolic products (wastes of digestion, growth and death of cells) is accomplished through the nephridia (similar function to kidneys) and through the gut.

7. Blood circulates by flowing forward along the dorsal collecting vessel into pulsating hearts where it is pumped down to the ventral vessels where it is pumped forward to the head and backward to all parts of the body.

8. There is a large ventral nerve cord with an anterior enlargement in the head ("brain"). The nerve cord connects various sense organs in the body and controls muscle contraction and relaxation. Light sensitive structures have been identified in worm species. Earthworms are also very obviously capable of sensing differences in acidity, relative humidity, touch, foods (chemicals).

9. Same species are able to regenerate amputated segments of the body. Regeneration of tail
segments occurs more readily than head segments. There does seem to be a limit to the number of segments that can be regenerated, but this number varies from species to species. No known worm species, if cut in half, will form 2 viable worms.

The Ecology of Earthworms

Earthworms live in the soil, but the species "mix" and the types of soil they inhabit varies widely.

First of all, there are only 2 native North American earthworm species found "in the wild" in Ontario, and both are relatively rare. The other 17 species found in Ontario were imported from Europe by settlers over the past few centuries. Therefore the natural Ontario earthworm population at the present time looks like European worm populations. The native North American earthworms were wiped out in Ontario (if they were ever here) by the ice sheets covering Canada until about 10,000 years ago. That is why earthworms in Ontario (and Canada) are not found far from human settlements (farms, towns, and cities).

Some worm species occupy their place in the soil by moving vertically in the soil (dew worm). Other species such as Aporrectodea (garden worms) occupy the top 5-10 cm of soil and move horizontally.

Other species such as the manure worm (Eisenia foetida) require soil with a high carbon content (muck soils) or manures to survive.

The density (the number of worms per unit volume of soil) at which different worm species can survive (or increase) varies enormously. Dew worms seem to require a minimum of 100 cubic inches of soil/worm.

Manure worms and African night crawlers thrive at densities of 1-2 cubic inches/worm. This factor obviously plays an important part in determining whether it is economically possible to culture a particular worm species. It just doesn't make economic sense to culture the dew worm - it requires too much space (20 million worms would require a minimum sized room of 100' x 100' x 100' plus ventilation and access). All earthworm species require fairly moist environments (probably to meet their respiratory requirements). Worms cannot survive in very low or no oxygen (anaerobic) environments. Different species have different minimum oxygen requirements though. Most earthworms are very susceptible to toxic chemicals. This means most pesticides (insecticides, fungicides, herbicides) should be kept away from your worm cultures.

In addition, we have found even fertilizers can be toxic to dew worms. Similar caution should be applied to other worm species.

The Reproduction of Earthworms

Earthworms may reproduce biparentally (by exchanging genetic material with another worm of the same species only) or uniparentally (no sexual fertilization by another worm takes place). The method of reproduction is characteristic of the species (that is both methods of reproduction are not normally found in the same species - but relatively little is known about this). Where biparental reproduction occurs (dew worm and manure worm), both male and female organs occur in the same animal and are cross fertilized by the other worm (of the same species only) simultaneously. Both worms will then produce cocoons (capsules). In uniparental worms, some internal mechanism triggers production of an ovum, which is then released as a cocoon, which develops into a mature worm.

Normally each cocoon produces 1 or 2 worms (but as many as 11 in the manure worm). Depending on the species, it takes from 3 weeks to a year to reach reproductive maturity.

A word about "hybrid" worms. It is not technically feasible nor has it been demonstrated that it is possible to hybridize two different species of worms (such as the dew worm and the manure worm) to produce a "hybrid" worm. Any worm claimed to be a 'hybrid" which has been analyzed by an expert, has been proved to be a particular species of (not a "hybrid"). Three worm species are often touted as being "hybrids" - they are the manure worm (E. foetida), the African night crawler (E. eugeniae) and the grey night crawler (A. hawayanis). If you hear people talking about "hybrids", be immediately suspicious .

The Parasites and Predators of Earthworms

A. The Predators

Many species of birds (gulls following ploughs in field), moles, hedgehogs, foxes, toads and snakes are known to eat earthworms. Beetles, leeches, slugs and flatworms also feed on worms. Most of these are unlikely to be problem in earthworm cultures.

B. The Parasites

Bacteria, protozoa (single-celled animals), flatworms, nematodes (roundworms) and dipterous larvae are internal parasites of earthworms. The cluster fly (Pollenia rudis), often a nuisance pest in house attics, parasitizes worms of the species Eisenia rosea which is often a contaminating worm in manure worm cultures. I have had no reports, though, that this parasite has been a problem in manure worm cultures.

There are other fly species which can parasitize worms, but so far I have received no reports from commercial worm growers.

I have had occasional reports of mites (small spider-like animals) causing problems in worm beds. One, Histiostoma murchiei is reported to parasitize cocoons of A. chlorotica worms. Another, Uropoda agitans also attacks earthworm cocoons. Sometimes earthworm beds can become so badly infested with mites that the worms' food supply is endangered and the worm population declines.

HELPFUL BOOKS ABOUT EARTHWORMS

Edwards, C.A. and J.R. Lofty. 1977. Biology of Earthworms. 2nd ed. Chapman and Hall, London.

Laverack, M.S. 1963. The Physiology of Earthworms. Pergamon Press, London.

Reynolds, J.W. 1977. The Earthworms (Lumbricidae and Sparganophilidae) of Ontario. Life Sciences

Misc. Publications, Royal Ontario Museum.

Stephenson, J. 1930. The Oligochaeta. Clarendon Press, Oxford.

The Edwards' and Reynolds' books are quite readable and both have extensive
References.

 
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