Key to Australian Freshwater and Terrestrial Invertebrates

Phylum Annelida
Class Clitellata
Order Megadrilacea

Common names: earthworms, megadriles

Overview

Earthworms are elongate, many segmented, terrestrial worms. They are recognisable by the prominent clitellum, a region of thickened tissue near the anterior of the body. Earthworms have an outer slimy, moist muscular body wall surrounding a digestive tract that begins with the mouth in the first segment and ends with the anus in the last segment. Body wall musculature consists of an outer circular layer and an inner longitudinal layer that extends and shortens the body in peristaltic contractions allowing earthworms to push their way through the soil. Each segment except the first bears bundles of setae, small chitinous hairs used for traction in the burrow. Mucous and body fluid is excreted onto the skin to keep it moist for gas exchange. Light-sensitive tissues near the head enable light detection. Some earthworms can repair body damage by regenerating lost segments, but this ability varies between species and depends on the extent of the damage. Most earthworms are less than 250mm long, though some giant species can grow to up to several meters long, such as the Australian giant Gippsland earthworm, Megascolides australis, which is around one metre long and can extend to over two metres long and weigh around 200 grams. The taxonomy and systematics of earthworms have many unsolved issues due to the lack of a fossil record and difficulties in assessing the diagnostic value of the main morphological characters (earthworms as a group lack a defining unique morphological characteristic), although recent molecular work may help resolve some of these problems.

Distribution and diversity

Earthworms are distributed globally but rarely occur in deserts and are absent from regions with permanent snow and ice. The vast majority are found in soil although there are a small number of species that occur in aquatic or semi-aquatic habitats. Worldwide, there are approximately 4000-6,000 species of earthworms from 17-20 described families (depending on the author as the taxonomy and systematics of Clitellata/Oligochaeta is problematic and often controversial). In Australia and New Zealand, as in most parts of world, the earthworm fauna consists of native and introduced species. Families in the region include Moniligastridae, Glossoscolecidae, Lumbricidae, Ocnerodrilidae, Eudrilidae, Benhamiidae, Acanthodrilidae, Octochaetidae and Megascolecidae. Australian native earthworms are estimated to total 1000-2000 species and belong to the latter three of these families, with the first two more common in the tropics and arid regions, while megascolecids are more common in the southern states. There are over 80 introduced species in Australia with representatives from all of the above families. The Megascolecidae have the widest natural distribution, being present on all continents, except Europe, and is the dominant family in the Australasian and Oriental regions. The Glossoscolecidae are confined to tropical South America, Central America, and a few Caribbean islands, while the Eudrilidae are found only in sub-Saharan Africa. The Lumbricidae are mainly in Europe, with a few species native to North America. Around 120 species are widely distributed outside their natural ranges around the world, however, these introductions are man-made and in general, earthworms have limited natural dispersal abilities. Accordingly, many Australian species are endemic, with several displaying high levels of short-range endemism. The Moniligastridae is represented by Drawida barwelli from Queensland. Two families have aquatic representatives in Australia: the Lumbricidae and Megascolecidae.

Life cycle

Earthworms are hermaphrodites, possessing both male and female reproductive organs located in the anterior segments. They do not self-fertilise, however a handful of species are parthenogenetic producing clones from unfertilised eggs. Mating typically occurs when the ground is wet following rain. A mating pair overlaps anterior ends ventrally and each exchanges sperm with the other. The worms then separate and the clitellum secretes the cocoon that forms a ring around the worm into which the eggs are laid and sperm deposited. Egg cocoons are deposited in the soil or moist leaf litter. The fertilised eggs develop directly into small, but fully formed earthworms that emerge into the soil and grow continuously until adulthood.

Feeding

Most earthworms are scavengers/detrivores that feed on dead organic matter. As they move through the substrate they feed on dead plant matter or soil from which bacteria and other microscopic organisms are extracted. This is passed through the gut then excreted and deposited on the surface of the ground as 'castings'. A few species are predatory on other earthworms and small invertebrates. Like terrestrial species, aquatic earthworms are mostly collector/gatherers that ingest large quantities of mud and organic debris as they burrow through the upper layer of soft, fine sediment filtering out and grazing on bacteria, protozoa, algae and dead organic matter.

Ecology

The typical earthworm habitat is soil, particularly in wetter, more heavily vegetated areas. There are also species that occur in leaf litter, manure, under rocks and logs, inside decaying logs, in suspended soils in tree canopies, freshwater mud and saltwater beach sand. The need for moisture for gas exchange restricts them, in general, to damp environments and nocturnal activity. Soil-dwelling earthworms are classified into three main ecophysiological categories depending on what part of the soil profile they inhabit: (1) leaf litter/compost dwelling worms (epigeic); (2) topsoil or subsoil dwelling worms (endogeics); and (3) worms that construct permanent deep burrows through which they visit the surface to obtain dead plant material for food. Earthworm populations depend on both physical and chemical properties of the soil including soil temperature, moisture, pH, salts, aeration and texture. Accordingly, their presence or absence in a habitat and overall species composition are useful indicators of soil pollution. Native species are found in both rich and in nutrient-poor conditions, occurring in mostly undisturbed areas but some can tolerate cultivation and other soil disturbances. A few species also occur in more arid regions. Many are listed as endangered or extinct due to land clearing, the application of chemical fertilisers and/or competition with introduced species. Urban gardens and other similarly disturbed environments are mostly dominated by exotic species. Earthworms, both native and introduced are important members of the soil fauna in many habitats. Their burrowing and feeding assist to aerate and transport nutrients deeper into the soil making earthworms vital to the recycling of soil nutrients and soil fertility. Earthworms also are significant components of many terrestrial and aquatic food webs. They are preyed upon by many species of birds, mammals, reptiles and fish, and are also eaten by many invertebrates such as predacious beetles, snails and slugs. Composting earthworms are cultured for gardeners and used in vermiculture, where worms are fed organic waste to produce compost. Earthworms are also cultured as bait for anglers and fish food for the aquarium industry.