Kate Priestman shines a spotlight on the humble earthworm…
Earthworms are one of the creatures that we take for granted the most. They, along with our soils, have become ‘background music’ – we’re aware of them, but we don’t pay them much attention. However, earthworms are very much deserving of our attention; they are the engineers of our soils and play an essential role.
Earthworms are invertebrates belonging to the sub-class Oligochaetes, which account for about half of the species within the Annelida phylum. Oligochaetes meaning, ‘few bristles’.
There are around 3,000 earthworms worldwide that have been identified and there are 27 species of earthworm in the UK – each species has its’ own biological niche and requirements. However, earthworms are under-researched and new species (and sub-species) are being identified on an ongoing basis as advances in DNA testing enables better quantification.
Earthworms are found on every continent and in all types of habitat, except for deserts – they require moist conditions to survive due to their permeable skin, which is susceptible to drying out. Earthworms are hermaphrodites i.e. they have both male and female reproductive organs. They are segmented and the majority have a body cavity (coelom) which is used as a hydroskeleton (i.e. a skeleton supported by fluid pressure). Lacking lungs or other specialised respiratory organs, earthworms breathe through their skin. The following diagram identifies the main internal features of an adult earthworm.
Baby worms emerge from eggs fully formed. They grow sex organs within the first two or three months and reach full size in about a year. They can live up to eight years, though one to two years is more typical.
Earthworms move by gripping the surface with retractable bristles. With the exception of the first segment, all segments have eight bristles. Worms are also able to reverse their direction of travel and lead with the tail.
Burrowing is achieved by forcing the front end into a crevice and widening the gap by body expansion. Large quantities of soil are swallowed in the process, which is mixed with mucus as it passes through the gut and is used to plaster the tunnel walls, forming a lining. Excess material is extruded on the ground surface, forming a faecal casting. The burrow may have two entrances and several vertical and horizontal tunnels (Ruppert, Edward E.; Fox, Richard, S.; Barnes, Robert D. (2004). Invertebrate Zoology, 7th edition. Cengage Learning).
Based on the behaviour of the worm, they can be sorted into four groups or ecotypes (Earthworm Society of Britain, 2017):
- Compost earthworms – found in compost, or areas very rich in rotting vegetation. They prefer warm and moist environments with a ready supply of fresh compost material. They can very rapidly consume this material and also reproduce very quickly. Compost earthworms tend to be bright red in colour and stripy. They can also remove contaminants from soil.
- Epigeic earthworms – live on the surface of the soil in leaf litter and tend not to make burrows. They are typically bright red or reddy-brown and are not stripy.
- Endogeic earthworms – live in and feed on the soil. They make horizontal burrows through the soil to move around and to feed and will often reuse these burrows. They are typically pale in colour: grey, pale pink, green or blue. Some can burrow very deeply into the soil.
- Anecic earthworms – make permanent vertical burrows in soil. They feed on leaves on the soil surface that they drag into their burrows. They also cast on the surface (often seen in grasslands) and make middens (piles of casts) around the entrance to their burrows – the casts aid a fine crumbly structure in soils. They are the largest species of earthworms in the UK, are darkly coloured at the head end (red or brown) and have paler tails.
Earthworms ‘engineer’ their environment. The burrows that they create allow oxygen and water to enter the soil, and carbon dioxide to leave, they also mix soil layers. Earthworms play a vital role in processing and breaking down organic matter; releasing nutrients and making them available for use by plants. They also mix the organic matter into the soil, further increasing the availability of nutrients (etc.).
Earthworms aid bacteria and fungi, which are key in releasing nutrients from organic matter. They are also an important source of food for many other animals present in soils.
Earthworms have different features that allow identification between species; some of these features require a microscope/hand lens to see them in detail. The following diagram illustrates the main characteristics used for identification purposes:
Earthworms are under-recorded and there is still much we have to learn about them; to that end, people are encouraged to submit their earthworm records in the UK to the National Earthworm Recording Scheme, which is hosted by the Earthworm Society of Britain. To aid identification, Open Air Laboratories’s (OPAL) have developed a Key to Common British Earthworms and a Key to Common British Earthworms of Amenity Grassland.
About the Author: Kate Priestman (CEnv, MCIEEM) has over sixteen years experience as an ecologist. Prior to setting up her own consultancy business in 2012, Kate worked in London for over a decade, providing the lead ecology role for a number of high profile projects. Kate works as an artist, author, writer and editor.