| Earthworm - Nutrition |
Above: the gut of a typical Lumbricid earthworm, such as Lumbricus terrestris. The mouth opens on the underside
of the first true segment (the peristomium or second apparent segment) with the first false segment, or
prostomium forming the overhanging and lobe-like upper lip. The mouth opens into a short buccal cavity, which
may be protrusible, and this in turn opens into the muscular pharynx, which acts as a pump, sucking up the food.
The pharynx leads into the oseophagus, which may be lined by three pairs of calciferous glands (shown in
orange). The oesophagus connects to the crop which is an expandable sac that stores food temporarily, until the
muscular gizzard is ready to receive more food. The gizzard grinds the food up, aided by its thick muscular walls
and soil particles which the earthworm has swallowed. (This is similar in function to the gizzard of birds which
grinds food using stones or grit that the bird has swallowed). The gizzard then empties into the intestine. The
intestine is a long straight tube which continues all the way to the anus on the end segment (pygidium) of the
worm. Note that this picture shows only the first 25 segments, before the clitellum, and since the worm will have
some 150 segments when fully grown, the intestine forms the main part of the gut. In the intestine digestion is
completed and the nutrients are absorbed into the earthworms bloodstream.
Digestion in earthworms is extracellular (at least in large part) with the cells lining the pharynx secreting proteases
(proteolytic enzymes that break down proteins). The pharynx also secretes mucus to lubricate the food and ease
its passage along the gut. The intestine also secretes enzymes - proteases like trypsin and pepsin, diastase
(breaks down starch to sugar) and lipase (breaks down fats and oils). These secretions are alkali and so help to
neutralise the humic acids present in ingested soil. These acids are released into soil as organic matter
decomposes. In all but very chalky soils, the gut contents of the earthworm remain acidic, however.
Peristalsis (waves of muscular contraction) transports food along the intestine. Cilia lining the posterior part of the
oesophagus also help to transport food along it (aided by the mucus lubricant). The intestine has an infolding that
forms a longitudinal ridge, called the typhlosole, along its dorsal surface and increases the surface area of the
lumen (the lumen is the hollow inside of a sac-like or cylindrical organ) to assist in food absorption. The typhlosole
also has cilia along it, inside the intestinal lumen, but these apparently do not aid in the transport of food, but
possibly help to circulate the digestive juices. Bacteria also release enzymes which probably assists in digestion.
Other species of earthworms have different gut arrangements. The crop may be absent and a region may
intervene between the gizzard and intestine, which is then called the stomach (this region contains calciferous
glands and is really the posterior part of the oesophagus, but functions like a stomach as it secretes proteases).
There may be no gizzard, or as many as ten gizzards, one in each of ten segments. The calciferous glands may
be simple folds with little activity of the oesophagus, or they may be oval or cylindrical ducted glands filled with
cells rich in granules of calcium carbonate. The level of activity of the calciferous glands depends upon species
and possibly also on time of year - in winter it is reported that these glands shrink to small folds that are hard to
see and then expand with calcium carbonate at other times of the year.
It was originally thought that the calciferous glands excrete excess calcium, since earthworms living in calcareous
soils ingest huge amounts of calcium carbonate (limestone/chalk) - sometimes too much to digest and absorb and
so it is presumed that they must rid themselves of excess calcium. However, whether or not an earthworm lives in
calcium-rich soil does not seem to correlate with calciferous gland function. The glands have been shown to
contain large amounts of the enzyme carbonic anhydrase, which fixes carbon dioxide gas by reacting it with
calcium to produce calcium carbonate. Carbon dioxide is generated by respiration within the earthworm and must
be excreted since it is acidic. Experimental removal of the calciferous glands has been shown to result in
increased acidity (lowering of pH) in the earthworm's coelomic fluid. This suggests that these glands have an
important role in acidity (pH) regulation. All organisms function best within a certain pH range and the body must
be maintained within this (often very narrow) range. The calcium carbonate excreted by these glands may be so
abundant as to form crystals or concretions that pass out with the worm's faeces (or casts). Perhaps in winter
these glands are not required as much, since the respiration rates of earthworms may drop with a fall in
temperature. Calcium excretion may also help to neutralise the humic acids in the ingested soil.
What do earthworms eat?
Earthworms which dwell in the leaf litter, such as Lumbricus rubellus (red earthworm, red wriggler, or red marsh
worm) are small (usually between 10 and 30 mm long) devour medium-sized soil particles that constitute the
leaf-litter/humus layer. This soil is rich in organic nutrients, especially in woodland soils. These worms form only
shallow burrows or no burrows at all (simply relying on the leaf litter to cover them). Some earthworms, such as
Allolobophora caliginosa form horizontal burrows just beneath the leaf-litter, in the organic-rich layer of topsoil.
These worms are usually weakly pigmented and translucent, variable in size, and ingest small soil particles
(containing organic matter that has been well broken up as it is older than the topmost leaf litter layer).
Deep-burrowing earthworms, such as Lumbricus terrestris (nightcrawler) and the tropical giant earthworms, live in
vertical burrows up to several metres beneath the soil surface and eat large particles of food. These worms are
large and Lumbricus terrestris will surface at night to feed and to fetch a fallen leaf or stem which it then drags
back to its burrow where it can eat it safely during the day (the leaf also serves to plug the burrow).
Most earthworms are saprophages (detritovores) that feed upon the rotting remains of other organisms,
principally dead plant matter, but they are essentially omnivorous. Those that at soil will, of course, also consume
many of the tiny or microscopic organisms in that soil. Agastrodrilus is a carnivorous African (Ivory Coast)
earthworm which feeds upon other earthworms.
Casting
Those earthworms that surface periodically may deposit faecal matter outside their burrows in the form of worm
casts. Not all earthworms produce casts. In Britain, only Allolobophora longa, Allolobophora nocturna and
sometimes Lumbricus terrestris produce worm casts. Tropical and subtropical earthworms will only cast in the wet
season. South African Microchaetid earthworms deposit towering globular casts along the rim of circular or
elliptical cups-like depressions up to 1 m in diameter and 30-100 cm deep. Other species of earthworms produce
granular casts or worm-like (vermiform) casts.
of the first true segment (the peristomium or second apparent segment) with the first false segment, or
prostomium forming the overhanging and lobe-like upper lip. The mouth opens into a short buccal cavity, which
may be protrusible, and this in turn opens into the muscular pharynx, which acts as a pump, sucking up the food.
The pharynx leads into the oseophagus, which may be lined by three pairs of calciferous glands (shown in
orange). The oesophagus connects to the crop which is an expandable sac that stores food temporarily, until the
muscular gizzard is ready to receive more food. The gizzard grinds the food up, aided by its thick muscular walls
and soil particles which the earthworm has swallowed. (This is similar in function to the gizzard of birds which
grinds food using stones or grit that the bird has swallowed). The gizzard then empties into the intestine. The
intestine is a long straight tube which continues all the way to the anus on the end segment (pygidium) of the
worm. Note that this picture shows only the first 25 segments, before the clitellum, and since the worm will have
some 150 segments when fully grown, the intestine forms the main part of the gut. In the intestine digestion is
completed and the nutrients are absorbed into the earthworms bloodstream.
Digestion in earthworms is extracellular (at least in large part) with the cells lining the pharynx secreting proteases
(proteolytic enzymes that break down proteins). The pharynx also secretes mucus to lubricate the food and ease
its passage along the gut. The intestine also secretes enzymes - proteases like trypsin and pepsin, diastase
(breaks down starch to sugar) and lipase (breaks down fats and oils). These secretions are alkali and so help to
neutralise the humic acids present in ingested soil. These acids are released into soil as organic matter
decomposes. In all but very chalky soils, the gut contents of the earthworm remain acidic, however.
Peristalsis (waves of muscular contraction) transports food along the intestine. Cilia lining the posterior part of the
oesophagus also help to transport food along it (aided by the mucus lubricant). The intestine has an infolding that
forms a longitudinal ridge, called the typhlosole, along its dorsal surface and increases the surface area of the
lumen (the lumen is the hollow inside of a sac-like or cylindrical organ) to assist in food absorption. The typhlosole
also has cilia along it, inside the intestinal lumen, but these apparently do not aid in the transport of food, but
possibly help to circulate the digestive juices. Bacteria also release enzymes which probably assists in digestion.
Other species of earthworms have different gut arrangements. The crop may be absent and a region may
intervene between the gizzard and intestine, which is then called the stomach (this region contains calciferous
glands and is really the posterior part of the oesophagus, but functions like a stomach as it secretes proteases).
There may be no gizzard, or as many as ten gizzards, one in each of ten segments. The calciferous glands may
be simple folds with little activity of the oesophagus, or they may be oval or cylindrical ducted glands filled with
cells rich in granules of calcium carbonate. The level of activity of the calciferous glands depends upon species
and possibly also on time of year - in winter it is reported that these glands shrink to small folds that are hard to
see and then expand with calcium carbonate at other times of the year.
It was originally thought that the calciferous glands excrete excess calcium, since earthworms living in calcareous
soils ingest huge amounts of calcium carbonate (limestone/chalk) - sometimes too much to digest and absorb and
so it is presumed that they must rid themselves of excess calcium. However, whether or not an earthworm lives in
calcium-rich soil does not seem to correlate with calciferous gland function. The glands have been shown to
contain large amounts of the enzyme carbonic anhydrase, which fixes carbon dioxide gas by reacting it with
calcium to produce calcium carbonate. Carbon dioxide is generated by respiration within the earthworm and must
be excreted since it is acidic. Experimental removal of the calciferous glands has been shown to result in
increased acidity (lowering of pH) in the earthworm's coelomic fluid. This suggests that these glands have an
important role in acidity (pH) regulation. All organisms function best within a certain pH range and the body must
be maintained within this (often very narrow) range. The calcium carbonate excreted by these glands may be so
abundant as to form crystals or concretions that pass out with the worm's faeces (or casts). Perhaps in winter
these glands are not required as much, since the respiration rates of earthworms may drop with a fall in
temperature. Calcium excretion may also help to neutralise the humic acids in the ingested soil.
What do earthworms eat?
Earthworms which dwell in the leaf litter, such as Lumbricus rubellus (red earthworm, red wriggler, or red marsh
worm) are small (usually between 10 and 30 mm long) devour medium-sized soil particles that constitute the
leaf-litter/humus layer. This soil is rich in organic nutrients, especially in woodland soils. These worms form only
shallow burrows or no burrows at all (simply relying on the leaf litter to cover them). Some earthworms, such as
Allolobophora caliginosa form horizontal burrows just beneath the leaf-litter, in the organic-rich layer of topsoil.
These worms are usually weakly pigmented and translucent, variable in size, and ingest small soil particles
(containing organic matter that has been well broken up as it is older than the topmost leaf litter layer).
Deep-burrowing earthworms, such as Lumbricus terrestris (nightcrawler) and the tropical giant earthworms, live in
vertical burrows up to several metres beneath the soil surface and eat large particles of food. These worms are
large and Lumbricus terrestris will surface at night to feed and to fetch a fallen leaf or stem which it then drags
back to its burrow where it can eat it safely during the day (the leaf also serves to plug the burrow).
Most earthworms are saprophages (detritovores) that feed upon the rotting remains of other organisms,
principally dead plant matter, but they are essentially omnivorous. Those that at soil will, of course, also consume
many of the tiny or microscopic organisms in that soil. Agastrodrilus is a carnivorous African (Ivory Coast)
earthworm which feeds upon other earthworms.
Casting
Those earthworms that surface periodically may deposit faecal matter outside their burrows in the form of worm
casts. Not all earthworms produce casts. In Britain, only Allolobophora longa, Allolobophora nocturna and
sometimes Lumbricus terrestris produce worm casts. Tropical and subtropical earthworms will only cast in the wet
season. South African Microchaetid earthworms deposit towering globular casts along the rim of circular or
elliptical cups-like depressions up to 1 m in diameter and 30-100 cm deep. Other species of earthworms produce
granular casts or worm-like (vermiform) casts.
| Click a link below to learn more about earthworm biology: Structure of the body wall and respiration Movement, support and locomotion Circulatory system Nervous and sensory systems Nutrition Reproduction and development Ecology |
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