Earthworms are the
soil-dwelling terrestrial invertebrates that belongs to the phylum Annelida. They were
classified into the order of Opisthopora since the male pores opened
posterior to the female pores. Other common slang names for earthworms are dew-worm, rain worm, night
crawler, and angleworm (due to its use as angling hook baits). Larger terrestrial earthworms are
also known as megadriles (big
worms). The megadriles are characterized by a distinct clitellum and a vascular system with true capillaries (Omodeo, 2000). Earthworms
are the major terrestrial macrofauna, constitute more than 80% of the soil
invertebrate biomass (Senapati and Dash 1981; Sorour and Larink 2001). Earthworms are
commonly found in moist, nutrient-rich soil, eating a wide variety
of organic matters which include detritus living protozoa, rotifers, nematodes, bacteria, fungi and other microorganisms. An earthworm's digestive
system runs the length of its body. They are the most important detritivores and
coprophages in nature and also serve as food for several lower vertebrates (Bonkowski
et al., 2000; Edwards
and Lofty 1977).

Earthworms have an externally
segmented tube-like body with corresponding internal segmentations
(Metameric segmentation) and usually have setae on all
segments (Edwards and Lofty 1977). Double transport
system is present which is made of coelomic
fluid that moves within the fluid-filled coelom and a
simple, closed circulatory system. They generally respire by their lubricated skin,
called cutaneous respiration. As soft-bodied
invertebrates, they lack true external as well as internal skeleton. The
structure and shape of the body are maintained by fluid-filled coeloms that
function as a hydrostatic skeleton. Earthworms have a central nervous system consisting of
two large ganglia (Brain) above the mouth, one on either
side, connected to an axial nerve running along its length to motor
neurons and sensory cells in each segment. Large numbers
of chemoreceptors are located near its mouth.
Circumferential and longitudinal muscles running each segment let the worm to
perform its locomotion. Similar sets of muscles line the gut tube and
their actions propel digested food toward the worm's anus (Cleveland et al., 1984). Earthworms are hermaphrodites
because they have both male and female reproductive organs and genital pores.
When mating, two different earthworms will exchange their sperms and fertilize each other's ova.

Earthworms are also categorized on
the basis of their behavioural, morphological and physiological adaptations
that enable them to screen available resources in the soil. The three main
morpho-ecological categories of earthworms have been reported, called as
epigeic, anecic, and endogeic (Bouche, 1977; Lee, 1985; Edwards and Bohlen
1996).

EPIGEIC: These are soil surface
dwelling earthworms which feed on plant litter. They dwell on the soil surface
or within the litter layer of the soil. These are usually heavily pigmented
earthworms as they are constantly exposed to sunlight. Their size ranges from
small to large. Examples: Eisenia fetida, Eisenia andrei, Eudrilus eugeniae,
Perionyx excavatus and Drawida
modesta.

ANECIC: These are top soil dwelling
earthworms which feed on plant litter and soil. They usually live in vertical
permanent burrows. They are dorsally pigmented and large. Examples: Lampito
mauritii and Octochaetona serrata.

ENDOGEIC: They are deep burrowing earthworms
and are humus and soil feeders. They usually live in extensive horizontal
burrows. They are not heavily pigmented and size ranges from small to large.
Examples: Octochaetona thurstoni, Allolobophora longa. Endogeic
earthworms have been further subcategorized into polyhumic, mesohumic and
oligohumic groups on the basis of their importance of organic rich mineral soil
in their diet and increasing size. Polyhumic endogeic earthworms like to live
in and feed on high organic rich mineral soil. Mesohumic endogeic earthworms
like to live in and feed on soil containing moderate amount of organic rich
minerals. Oligohumic endogeic earthworms like to live in and feed on soil
containing very amount of organic rich minerals.

Out of these three groups of
earthworm species, epigeic earthworms have greater capability of degrading organic
wastes and endogeic earthworms have better capability for protein conservation
and soil turnover, whereas anecic earthworms have capability of both groups
(Dash and Senapati 1980).

SPECIES OF EARTHWORM

Reynolds (1994) recorded the
occurrence of 3,627 terrestrial earthworm species worldwide. The earthworms are well reported fauna in India as compared
to other Asian Countries. About 452 valid species/subspecies of earthworms
under 71 genera are reported from the Indian Territory, including its Islands. The first earthworm species in the Indian subcontinent was
reported by Templeton (1844) when he discovered Megascolex
coeruleus from Sri Lanka. The most diverse families of earthworms in India are Megascolecidae
comprise 191 species of 19 genera; Acanthodrilidae with 123 species of 26
genera and Moniligastridae with 89 species (all native) of 3 genera. The most
diverse genera of earthworm in India belong to Drawida (74
species) and Moniligaster (14 species) of
family Moniligastridae; Perionyx (51
species), Megascolex (32 species)
and Argilophilus (23 species) of family Megascolecidae; Eutyphoeus (27 species), Haplochaetella (19 species) and Octochaetona (15 species) of family
Acanthodrilidae (Earthworms of India).

ECONOMIC IMPORTANCE OF EARTHWORM

Earthworm are the most important
soil fauna which develop and maintain the nutrient and mineral value of soil by
converting biodegradable materials and organic wastes into nutrient rich
vermicompost (Jansirani et al. 2012). They are also called as ecological
engineers (Jones et al., 1994). Their distribution and abundance depend
on several ecological factors such as soil status, nutrients, temperature,
moisture, season, adequate dissolved oxygen, pH and the presence of fertilizers
and pesticides (Lee, 1985; Bhaskaran, 1986; Morgan, 1993; Viswanathan, 1997;
Curry, 1998; Bhattacharjee, 2002). Therefore, earthworms are found in broad
range of soil and play an important role in deposition of about 60%-80% of the
total soil biomass (Luo et al., 1999; Sizmur and Hodson 2009). They are
the most important natural fauna in the terrestrial ecosystem which
significantly regulate various processes such as soil formation, organic matter
breakdown, decomposition process and nutrient-mineral recycling. Indian
earthworms that are currently utilized for vermicomposting of organic wastes
are Perionyx excavatus, Lampito mauritii, Dendrobeena repaensis, and Metaphis
hovlleti (Kaushal et al., 1999).

Earthworms are capable to consume a
wide range of unstable and biodegradable organic matter such as domestic waste,
animal faeces, industrial waste, sewage sludge etc. The feeding and burrowing
activity of earthworms increase the decomposition of organic matter, formation
of humus, and development and maintenance of soil fertility.

Earthworms help in soil
development, soil turnover, increasing the soil aeration, water permeation and
water holding capacity of soil, maintenance of the soil fertility and formation
of the productive soil. Earthworms contribute in the soil forming process by
five ways such as by altering soil pH, by causing physical and chemical
decomposition of organic matter, by humus formation, by improving soil texture
and by enriching the soil with nutrients and minerals. Earthworms are ecosystem
engineers and capability for changing the physical, chemical, and biological
properties of soil. Vermicompost obtained by decomposition of organic waste
through earthworm's gut is quite different from its parental waste material and
popularly known as black gold (Lim et al., 2015b; Patangray,
2014). It boosted the practices of organic farming all over the world.
Earthworms also enhance the physical properties of soil such as hydraulic
conductivity, porosity, bulk density, infiltrability, aggregate stability etc.
(Devkota et al., 2014). Earthworms improve nutrient availability
by ingesting organic residues of different carbon and nitrogen ratios (Patnaik
and Dash 1990). Activities of earthworms also help in enhancing beneficial soil
microbes. The mucus secretion and excretion from the gut of earthworm are known
to enhance the activity of soil microorganisms (Bhaduria and Saxena 2010).
Therefore, earthworms play the beneficial role for soil ecosystem by
maintaining soil structure and fertility. They affect the dynamics of organic
matter, soil texture and microbial fauna in soil. They also increase the
porosity of soil by altering the physical and chemical properties of soil
organic matter and by mixing plant leaf litter with soil to form and stabilize
soil aggregates.

The faecal castings of earthworms
nourish the agricultural land with several nutrients and minerals. They
increase nutrient and mineral contents of the soil biologically such as
nitrogen, phosphorus and potassium. Earthworms help in recycling of nitrogen
through their faecal matter, especially in shifting cultivation (Bhadauria and
Ramkrishnan 1996). Earthworms are the "natural ploughman" because they soften
the soil when they make their burrows in soil. By doing so, they increase the
aeration of the soil that make the soil better for crop plants to grow.
Earthworms participate in the biogenic transfer of soil materials in different
soil strata. All of these are important activities of earthworms for the
maintenance of soil health and are essential to soil ecology and agronomy
(Feller et al., 2003). Earthworms are usually found to live in different
soil strata. Epigeic earthworms feed on the dead and decaying leaf litter and
play important role in both consumption and formation of humus. Endogeic
earthworms make horizontal tunnels and actively aerate the soil that helps in
the growth of plant roots while anecic earthworms make vertical tunnels and
release their faecal castings on the surface of the soil regularly that
increases the fertility of the soil. The collective effect of rain, vegetation
and earthworm's faecal castings cause compaction and decomposition of soil
which aids in pedogenesis (Lavelle, 1988). Earthworms can play an important
role to the migration of pollutants in the soil (Kuzyakov and Blagodatskaya
2015). These incredible services provided by the earthworms to the terrestrial
ecosystem as well as agriculture, are now a days under the threats and this
book chapter is mainly focused on understanding the economic importance of
earthworms and their growing threats.

THREATS TO
EARTHWORM

The economic importance of
earthworms in agricultural practices and soil ecosystem is well known all over
the world. In recent years, the growing use of chemical fertilizers, pesticides,
herbicides and toxic chemicals in agricultural lands have adverse effects on
soil flora and fauna. Earthworms which are popularly known as "Ecological Engineers"
are reported by recent studies to be under threats due to such polluted
environment. The major threats to earthworm population undoubtedly are the chemical
fertilizers, pesticides, herbicides and toxic chemicals that are excessively
used now a day for increasing agricultural crop production. After green
revolution in India, the chemical fertilizers and pesticides were frequently
used by farmers to increase their agricultural crop production. The excess use
of chemical fertilizers and pesticides leads to soil, surface and ground water
pollution that adversely affect to the soil flora and fauna along with
non-target organisms, mainly earthworms. The growing use of chemical
fertilizers and pesticides has been reported to exert severe adverse effects on
soil building process and soil fertility.

According
to some reports, chemical fertilizers, pesticides and toxic chemicals have
harmful effects on earthworm at various levels of body organization such as
change in morphology and behaviour pattern, metabolism and enzymatic
activities, increase mortality, reduce fertility, obstruction in growth and
reproduction (Pelosi et al., 2013). Earthworm population is very sensitive to
these chemical fertilizers and pesticides. Therefore, earthworms are used as
bio-indicator species for the checkup of soil health as well as ecosystem
health. In a study, Jeyaprakasam et al. (2021) found that the earthworm
population was higher in uncontaminated soil as compared to
pesticide-contaminated soil.

        Fig. 1. Excess use of
pesticides in agriculture.                 Fig. 2. Death of earthworms due to
pesticides.

Earthworms are highly sensitive to
changes in ecological factors, essential to the soil and the activity of
earthworms reflects soil pollution. Chemical fertilizers, pesticides,
insecticides, herbicides and fungicides are now the essential components for
the agricultural practices and crop production in the modern world (Stephan et
al., 2011; Pimentel et al., 2011). The use pesticides in Indian
agriculture are increasing year by year after the green revolution. Pesticides
use per unit area is the highest in cotton crop followed by vegetables and rice
(Peshin, 2014). Earthworms are unable to perform their important and vital
roles in the soil ecosystem as they are exposed to various pesticides (Rathore
and Nollet 2012). Some studies reported the weight loss of earthworms due to
their exposer with pesticides such as acetochlor and methamidophos. The
earthworms exposed to contaminated soil (DDT), have greater rates of mortality,
growth inhibition rates, low enzymatic activities (Shi et al., 2016).  Pesticides have harmful effects on the cocoon
formation and their hatchlings, the survivability of newly borne earthworms and
sexual development and reproduction of earthworms. Some pesticides like
Dimethoate (an organophosphate insecticide) have harmful effects on the protein
profile, cellular enzyme system and testicular histomorphology of earthworms
(Mosleh et al., 2003; Lakhani et al., 2012). In recent years,
soil pollution has increased due to the extensive use of inorganic chemicals,
fertilizers and pesticides in agriculture (Kavitha et al., 2020). The extensive use of these inorganic chemicals,
fertilizers and pesticides has depleted soil fertility, eliminated beneficial
microbes and adverse impact on the human health and the environment (Yatoo et al., 2021).