Nature: Buried in the sand

The riddle of the wormlike coils of sand on the beach is solved by Éanna Ní Lamhna, but there are plenty more mysteries about the lugworm

Strolling along Sandymount Strand the other day when the tide was out I was struck, not by the enormity of the strand (as was Stephen Dedalus, the hero of Ulysses, “Am I walking to eternity along Sandymount Strand?”), but by the coils of sand underfoot that looked for all the world like coiled-up sandy worms. There were lots and lots of them exposed by the retreating sand. These are in fact the only visible sign of the vast colony of creatures that live in burrows under the sand and feed on the molecules of detritus brought in by the tide. Such a large, bare strand looks barren when the tide is out and yet the sandy mud contains a very dense collection of invertebrate wildlife.

But trying to dig them up with your bare hands is a mug's game. If you look closely at a coil of sand, you will observe a small depression in the sand several centimetres away. Between that depression and the coiled up sand lies a U-shaped tunnel in which resides a lugworm, which can be up to 20cm long. It excavated the tunnel by eating sand in through its mouth and expelling it again out of the other end, having digested any organic matter that was in the sand along the way. But if you use your hand as a shovel and try to dig down into the sand to capture the lugworm, it feels the movement of the sand and tunnels much deeper at tremendous speed, leaving you with just an empty handful of sand. More drastic measures are called for.

I espied some fishermen digging for bait. They were equipped with forks – big gardening forks. By plunging these deeply and quickly into the sand around the casts, they were able to remove a large sod of sand with the worm intact. So I was able to extract one for this photograph. The worm rewarded me by leaving a large stain of iodine on my hand.

It is a most peculiar looking worm. It has a really thin skin and contains red blood, which flows from it if you damage it in any way. The body has bristles on its upper third, feathery gills around the middle and a much slimmer and completely smooth lower third. It has a large proboscis, like a mini elephant trunk, which it can extend or withdraw at its front end (most disconcerting when you see it close up!) and it is with this that it swallows the sand and tunnels along. Well yes – it's a way of life and you could see how it would work.

However there is only one worm per burrow and they never leave it voluntarily. They may dig deeper in a hurry if they feel under attack, but they never emerge, even in the dark of night under a full moon when the tide is in to meet their neighbours from surrounding tunnels. So how do they reproduce? Wait till you hear!

During the first fortnight of every October, they wait for a period of rough weather. Though how they know it's October down in their burrows or what the weather is like above remains a mystery. When this rough weather comes, they experience what is called in literature a “genital crisis”. I cod you not. This unimaginable “crisis” lasts for two days, after which all the worms without exception liberate their genital products into the water above so that fertilisation of the eggs is ensured. No “do you come here often?” chat-up lines for lugworms then!  

The newly-created fertilised embryos settle down, burrow into the sand and continue the cycle again. Such a perfect and highly-adapted life cycle for what we have the nerve to describe as a primitive invertebrate.