5 things you should know about earthworms

Aristotle called earthworms “the intestines of the Earth” and Charles Darwin was a fan, he wrote his last book about the humble earthworm just before he died. They are hermaphrodites with a crop and gizzard digestion system, similar to chickens, and there are between 10 and 400 of them in every square metre of soil beneath our feet [1].

Earthworms are vital to the structure and health of soil – they improve aeration and drainage, they feed other soil-dwellers with their poly-saccharide mucus (and moles in a more direct way) and they bring organic matter from the surface and integrate it into the soil. Emma Sherlock, the Natural History Museum’s worm guru calls them ‘little engineers’.

My fascination with earthworms started at an early age – I would have been eight or nine when my class teacher brought in a wormery to show us how they burrowed in soil. The wormery had a glass front covered with a wooden slide to keep the light out. When this slide was removed, we could see the tunnels made by the worms with trails of sand taken from the surface and down into the depths. I tried to make my own at home but the few worms that I imprisoned either escaped or died. An early lesson for me in the fragility of our natural world!

In honour of World Earthworm Day, we have gathered five facts that you may or may not want to know about the little engineers below our feet:

1. What is the biggest earthworm found in the World?

The Giant Australian Earthworm can reach 3m in length – imagine meeting that on a dark night? However, that is nothing to the South African Microchaetus Rappi that holds the official Guinness Book of Records title as the longest earthworm. In 1967, this specimen was measured at 6.7 metres long with a diameter of 20mm.

2. How much soil can earthworms (normal size) process each day?

Typical populations of earthworms can process about 5 tonnes of soil per year per hectare (about 2 tonnes/acre per year)

3. How quickly can they breed?

In good conditions, earthworms can lay about 10 eggs a month. With eggs typically having 10 baby earthworms inside, that results in one earthworm becoming 101 earthworms in a month! It takes sixty days for a worm to reach maturity so, at the end of three months, that one earthworm will have led to a population of 10,000 descendants, each one capable of doing the same thing for several years.

4. Why do you sometimes find worms tied up in knots?

When conditions are not right for worms, it’s too dry or too cold, they can enter a state of estivation (also known as a diapause). They turn off all unnecessary functions, tie themselves into a ball and cover themselves in a mucus that prevents water evaporation from their skin.

5. How long have earthworms been around?

About 209 million years is the best estimate [2]. That is about 35 times longer than humans who appeared on Earth a measly 6 million years ago.

I hope that you will start to think more about the earthworms moving beneath your feet next time you are out for a walk (especially if you live in Australia or South Africa)!

There are over 6,000 species in the World, some glow in the dark, some even climb trees. They can survive for a while in oxygenated water and, at various times in the last 209 million years, they have been aquatic.

Most importantly, we need them for healthy soil, without them and all of their microbial and fungal friends, soil is just dirt.

References:

[1] Lee, K.E. (1985) Earthworms: their ecology and relationships with soils and land use. Sydney: Academic Press.

[2] F.E.Andersen et al. (2017) Phylogenomic analyses of Crassiclitellata support major Northern and Southern Hemisphere clades and a Pangaean origin for earthworms. BMC Ecology and Evolution.