Agriculture Reference
In-Depth Information
Enchytraeidae are hermaphroditic although reproduction is bisexual in most species.
However parthenogenesis is not uncommon and asexual reproduction by fragmentation
occurs in a few (but abundant) species. In species with sexual reproduction, adults lay
cocoons which are mucilaginous bags containing 1 to 48 eggs, depending on the species
(O'Connor, 1967). Some species cover their cocoons with organic and mineral particles
to protect them from drought. Estimates of hatching rates are generally high and vary
from 19 to 97 %. In British meadows, Enchytraeidae are reported to produce 4 to 10
immatures with a developmental period of 4 to 12 months (Brockmeyer
et
al.,
1990). Under optimal conditions, total life cycle from cocoon hatching to maturity
may range from 60 to 120 days.
Asexual reproduction occurs through the fragmentation of individuals in a few species.
In
Enchytraeus fragmentosus,
individuals break into 3 to 14 fragments of about five
segments, each of which develops into a complete worm in 10 days (Dash, 1990).
In
Cognettia sphagnetorum,
a dominant species in acid soils of northern Europe, worms
start fragmenting when they have more than 42 segments. They divide into 2 or 3 frag-
ments and the time taken to regenerate varies from 8 to 26 days. Minimum regeneration
times occur for head fragments and are proportional to temperature (15 °C rather than
10 °C or 5 °C) (Standen, 1984). In natural populations, fragmentation occurs throughout the
entire year with obvious maxima in spring or early summer, especially when relatively dry
periods occur (Standen, 1973; Lundkvist, 1981), or in the middle of winter (Ellenberg
et al.,
1986).
Drought is a major limiting factor for the Enchytraeidae since they cannot survive
water potentials lower than -1.0 MPa (pF 4) (Abrahamsen, 1971; Bell, 1959).
Populations may survive through some combination of colonising favourable microsites
and laying drought-resistant cocoons. Fragmentation permits rapid recolonisation and
conserves energy and nutrients (Standen, 1973). Resistance to low temperature appears
rather limited and the temperature range over which reproduction occurs in temperate
species ranges from 1.5 ° to 25.5 °C for several species, with optimal values between
5 ° and 15 °C (Standen, 1973). Nonetheless, Enchytraeidae have been found in Arctic
tundra soils (Ryan, 1977; McLean, 1980) and in the high altitude South American
paramo (Sturm, 1983)
Ecological categories have not been defined for Enchytraeidae probably because
of the relatively homogeneous way in which they exploit soil and litter resources.
Most Enchytraeidae seem to be hemiedaphic or euedaphic and thus relate to the polyhumic
category defined for earthworms (see Chapter III.4.3.1.2).
Density and biomass of communities
The abundance of Enchytraeidae is clearly determined by soil temperatures and moisture
regimes. While they are virtually absent from 'extreme' tundra environments in Signy
Island (maritime Antarctica), they have mean densities of 46,000 ind. (from 11,400
in 'raised polygons' to 93,600 in 'polygon troughs') in the milder tundra environment
at Point Barrow (Alaska) (Dash, 1990). The highest mean density, 200,000 ind.
(82,000-289,000 ind. ) was observed at Moor House (England) in a
Juncus
moor
(Peachey, 1963; McLean, 1980).
