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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.

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

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).

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

Ecology and toxicology

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 partially in freshwater. Haplotaxina species are mainly subservient to freshwater biota, including sediments.

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)