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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