Soil dwelling earthworms fall into three main niche groupings:
- Epigeic earthworms live in compost and near the soil surface.
- Endogeic earthworms live in the topsoil (top 20 cm).
- Anecic earthworms live in the subsoil (as deep as 3 m).
To the untrained eye, one earthworm often resembles another. Once you take a careful look, you’ll notice that, although earthworms have much in common, there are many differences too.
New Zealand has both native and introduced earthworms. Unless you are in areas of native forest or tussockland, you are more likely to find the introduced lumbricid species. These useful but accidental imports tend to live in pasture and other grassy areas.
One of the first differences you might notice amongst earthworms is size. Native earthworms make up both the largest and smallest earthworm species in New Zealand. Some of the leaf-mould dwellers are barely more than a centimetre in length. Several species, like O. multiporus, grow to 30 cm or more. The longest native species is the 1.3 m subsoil dweller Spenceriella gigantea. Introduced earthworms are not as extreme in size. The pink worm is one of the smallest, whereas nightcrawlers can grow up to 30 cm in length.
Earthworms also vary in colour. Epigeic earthworms – those that live at or near the soil surface – tend to have dark skin colour. This acts as camouflage as they move through the leaf litter. The darker skin also helps to protect them from UV rays. Tiger worms – an epigeic species – get their name through their distinctive bright red and yellow-banded skin. Earthworms living further down the soil profile are often paler in colour. They still have some distinct markings that help us visually identify the species. For example, although the yellow tail is mostly a light grey, it does have a yellow tip. The blackhead earthworm gets its name from the dark grey-brown head.
Earthworms also differ in the roles they play within the ecosystem. Epigeic earthworms such as tiger worms and dung worms are effective composters. They help to break down organic matter and recycle nutrients. Other earthworm species are known as earth workers. The grey worm is one of a number of endogeic earthworms that improve soil structure and fertility as they burrow underground. Their burrowing helps aerate the soil and allows water and soluble nutrients to filter down to plant roots.
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