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Earthworm overview.png

Taxonomy and molecular phylogeny

Figure 1 - 1: Cuticule; 2: Circular musculature (striated tissue); 3: Longitudinal musculature (smooth tissue); 4: Dorsal setae; 5: Intestine; 6: Intestinal lumen and typhlosolis; 7: Ventral setae; 8: Ventral vein; 9: Ganglion (nerve cell cluster); 10: Dorsal artery; 11: Nervous cord; 12: Nephridial pinna; 13 Nephridial canal; 14: Nephridial pore.

The sub-order Lumbricina belongs to the phylum Annelida, Class Clitellata and order Haplotaxida. It is considered that this sub-order consists of the so-called "true earthworms". To its part, the family Haplotaxidae, sub-order Haplotaxina, is considered to be phylogenetically a basal group in regard to the classical terrestrial earthworms. The very first earthworm species, Lumbricus terrestris, was described by Carl Linnaeus in 1758. Savigny showed, in 1826, that earthworms were in fact composed of several species. Today, 23 families have been described and more than 5000 valid species are recognized. From the first taxonomical descriptions to the present, earthworm classification has always been unstable due to its complexity as reflected by the recent disparition of the family Octochaetidae, the re-erection of the Rhinodrilidae (James, 2012) described by Benham in 1890, or the description of the Tritogeniidae (Plisko, 2013). Many groups, in particular in temperate areas, contain cryptic species, including genus common in Europe and North America (Lumbricus, Eisenia or Aporrectodea), and for some of them, specialists speak more about species-complexes. However, recent development of molecular technologies, such as New Generation Sequencing (NGS) and DNA barcoding (Huang et al., 2007; Decaëns et al., 2013), may considerably help the usual morphological taxonomy and the understanding of the phylogenetic relationships in earthworms.

Distribution: Earthworms are distributed in a very large range of soil types and climates. Their main limitant factors of survival are temperatures and water content of soils. Thus, they can be found everywhere except dry lands and deserts, and polar areas. Their diversity is by far more important in the tropics, becoming spectacular in tropical rainforests, than in temperate and boreal countries. In this latest, such as in Siberia, only few species can survive through specific adaptations.

Main references: Linnaeus, 1758; Savigny, 1826; Michaelsen, 1900; Stephenson, 1930; Yamaguchi, 1953; Brinkhurst and Jamieson, 1971; Jamieson, 1971; Reynolds and Cook, 1976, 1981, 1989, 1993; Sims, 1980; Jamieson, 1988; Csuzdi, 1995; Gregory and Hebert, 2002; Erseus and Ällersjö, 2004; Erséus, 2005; Blakemore, 2006; James and Davidson, 2012; Decaëns et al., 2013; Plisko, 2013

Morphology and biologie

Figure 2 - Earthworm bisexual reproduction.

Earthworms are triploblastic animals. In other terms, the blastula or earliest stage of embryonic development, contains three primary germ layers: the ectoderm, mesoderm, and endoderm. They belong to the basal group of Protostome Lophotrochozoa, which they share with the other annelids, molluscans, nemerteans (ribbon worms), platyhelminths (flatworms) and rotifers. The nervous cord is ventral and the intestinal region occupies 80% or more of the body length (figure 1). The body wall musculature is divided in 2 tissue types: the longitudinal musculature (striated tissue) and the circular musculature (smooth tissue), which allow body extension and peristaltic movement. They often possess several pairs of hearts and from 2 to several tens of excretory systems (nephridies) per segment, in particular in tropical species, in which symbiotic bacteria have been recently discovered (James and Davidson, 2012).

Reproduction: Most earthworm species have a bisexual reproduction mode, few of them being exclusive or non-exclusive parthenogenetics (i.e. Eiseniella tetraedra or Pontoscolex corethrurus). The copulation is carried out head to toe (figure 2) when male organs are well developped. Spermatozoids are transfered to the partner and stored in spermatecae until the maturation of female organs. Then occurs the fecondation.

Physiology: The immune system is particularly well developped in earthworms and immune cells are found in the coelomic liquid, in the space surrounding the digestive tract and limited by the body wall (Cooper and Roch, 2003).

Parasitism: The earthworm digestive tract, like a large number of invertebrates, are often parasited by gregarines (unicellular apicomplexans) (Omoto and Cartwright, 2003).

Main references: Satchell, 1980; Jamieson, 1981; Gregory and Hebert, 2002

Ecology and toxicology

Figure 3 - Pasture in India with earthworm casts.

Lumbricina species are found in a large range of biota, from costal marine environments (i.e. Pontodrilus littoralis) to freshwaters (i.e. members of the Almidae family) and soils. However, most species are terrestrial, some of them living in mud or very wet soils. Haplotaxina species are mainly subservient to freshwater biota, including sediments.

Functional ecology: In a soil, the microfauna is composed by microscopic animals (<0.2mm), including nematodes, small arthropods and protists (i.e. protozoans) (Lavelle and Spain 2002). This fauna, together with the soil bacteria and fungi, is very active as organic matter decomposers. The mesofauna, between 0.2 and 1mm, includes mainly nematodes, acarians, rotifers and collembolas. This animal guild fractionates large particles of organic matter and greatly facilitates the decomposition activity of the microorganisms. The macrofauna, the most visible fringe of the soil fauna, is mainly composed and massively dominated by earthworms and large arthropods (Lavelle and Spain 2002; Edwards, 2004). This animal fraction is the motor of soil bioturbation, contributing to air and organic matter circulation within soil layers, and fragmentation of the soil litter. The gallery network of earthworms helps the penetration of the root system in depth and the soil drainage. Thus, earthworms are a key element in the soil nutrient cycling.

Habitat disturbance and species invasion: With increasing human migratory movements around the world, always more peregrine or invasive species can be introduced and is susceptible to produce great damages on local soils and soil biodiversity, such as largely reported in the United States today. In another hand, soil fauna and earthworm diversity are strongly affected by habitat disturbance, such as deforestation, for instance. A large portion of the communities will be then replaced by invasive species or by species from secondary forest. The functional equilibrium between species will be thus perturbed. The main trend in earthworms is the loss of anecic and endogeic species to the benefit of epigeics as disturbance intensifies (Lavelle 1997).

Main references: Darwin, 1881; Lee, 1985; Greig-Smith et al., 1992; Edwards and Bohlen, 1996; Lavelle and Spain, 2001; Edwards, 2004; Bienert et al., 2012; Decaëns et al., 2013

Pictures (click to enlarge)