EARTHWORM BIOLOGY
by Alan D. Tomlin
Pest Management Research Center
Agriculture & Agri-Food Canada
London, Ontario
N5V 4T3
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.
Categories: Earthworm, biology, Parasites, Predators, Ecology
Date: 01/01/2001