WHAT YOU NEVER KNEW ABOUT EARTHWORMS
Strong, smart and sexy in spades… It’s time to reappraise our most undervalued invertebrate, says Sally Coulthard
So, an earthworm’s just an earthworm, right?
Well, no, not exactly. There are actually thousands of different species around the world and Britain has 26 of its very own. The most numerous group in the UK are the reddish-brown ‘surface dwellers’, such as tiger worms (Eisenia fetida), who rummage around in decomposing leaves and other organic matter. Then there’s ‘shallow burrowing’ worms, such as the grey worm (Aporrectodea caliginosa), who dig no deeper than 30cm.
Red and grey? I thought worms were pink?
Ah yes, you’re thinking of the big, juicy ‘deep burrowers’, like the common earthworm (Lumbricus terrestris), who can tunnel down to a depth of three metres. Despite their name, they actually only account for one in every 80 earthworms under our feet.
Big? How big are we talking here?
Oh, there have been some absolute whoppers. The largest common earthworm ever recorded in the UK measured 40cm long and was discovered in an English vegetable patch.
Is it really true you can chop an earthworm in two and both halves survive?
Well, to answer the question, you first need to know where the front end of an earthworm is. The easiest way to tell is to look for its saddle or clitellum, a fleshy ring around its middle that sits closer to its head end. The common earthworm may survive having its tail cut off below the saddle, as you’ll miss most of its major organs, but it’ll never quite return to its former glory. So, where possible, be careful when wielding your garden spade!
Aren’t they just a wriggly tube with a mouth and a bottom?
Yes and no. While earthworms are essentially one long digestive system, designed to break down organic material and release those nutrients back into the soil, they’re also wonderfully, remarkably complex. Charles Darwin discovered that while they don’t have ears, they still ‘hear’. When he placed them on his piano, in a pot of soil, they retreated underground as soon as a note was played. Darwin rightly concluded that they were acutely aware of sound vibrations – a mechanism that helps them escape from underground
predators such as moles.
Okay, so they respond to sound but they’re not that smart… are they?
Ooh, this is where it gets really interesting. Darwin also suspected the earthworm had some kind of basic intelligence. After watching them choose leaves by their shape, he set up an experiment with tiny triangles of paper. As he predicted, the earthworms were tugging the paper triangles from their apexes, the ends most likely to fit into their burrows. Other experiments have shown that they can learn to navigate mazes and get faster each time they attempt them. A different test also demonstrated that, if threatened, earthworms ‘herd’ together underground, communicating by touch and crawling over each other.
All brains and no brawn, eh? Can’t have everything, I suppose.
Not so fast. Earthworms might be small, but they punch well above their weight. To move through the earth, they burrow by forcefully enlarging tiny crevices and cracks in the soil. This can take an enormous amount of pressure and strength – experiments have shown they can push ten times their own body weight. That’s the equivalent of a
human pushing a large polar bear out of his or her way. What’s even more extraordinary is that tiny hatchlings – baby earthworms – can push five hundred times their own body weight – that’s the same as a person casually shoving a humpback whale to one side.
If earthworms need to push through the earth, why are they so slimy? Surely the soil sticks to them?
You’d think so, wouldn’t you? But, actually, earthworm mucus is so slippy that it actively helps them glide through the soil. This glorious gooey slime also helps them to breathe: earthworms diffuse oxygen and carbon dioxide through their skin and, for this to happen, the surface has to be moist. During the mating process, earthworms also produce lots of slime so they can stick together and swap sperm.
Did someone mention mating? How does that work?
Believe it or not, earthworm reproduction is both lengthy (no pun intended) and rather sweet. The courtship ritual of the common earthworm, for example, is a tender affair, with initial ‘getting to know you’ sessions. Under the cover of darkness, an earthworm will stretch out and attempt to poke its head into a neighbouring burrow – the amount of times an earthworm visits a potential mate’s burrow varies – sometimes only once or twice, sometimes more than a dozen. The ‘courted’ earthworm will then reciprocate the visit, both moving back and forth between each other’s burrows like giddy teenagers. When they’re ready to mate, the two earthworms lie next to each other, facing in opposite directions – head to tail – glued together in a tight embrace. There, they’ll stay stuck together in a sexual marathon that can last anything from one to three hours.
I’m starting to get earthworm envy. Tell me something they don’t do brilliantly.
This should help. Ever noticed that earthworms head to the surface after a rainstorm? We used to think they raced upwards to prevent drowning but now theories suggest that they’re actually mistaking the sound of raindrops for the vibration of moles digging through the soil. To avoid becoming the mole’s lunch, they head to the surface but then tend to get eaten or die from exposure to sunlight.
Actually, that’s made me feel worse. Give me more good earthworm news.
Well, if nothing else, take a moment to marvel at how brilliant earthworms are. We all take them for granted but without them life would stop. Our gardens, fields and farms wouldn’t be able to grow the food and support the crops and animals we need to survive. Recent research even suggests they can help clean up polluted land, turning it back into rich, fertile ground. What’s really incredible is that we don’t even know how many earthworms the land has. Estimates vary wildly from a conservative 250,000 per acre to a writhing 1.75 million. From where I’m standing, we’ve not even scratched the surface of how amazing these miniature engineers of the soil truly are.