Bootlace worm

The bootlace worm is one of the most amazing animals in the sea, because it might be the longest creature on the whole planet! It is a kind of ribbon worm, which means its body is soft, smooth, and flat like a long, dark shoelace. It lives along cool European coasts, from Iceland and Norway down to the shores of Britain, tucked away where the waves wash in and out.

How long can it grow? Most bootlace worms stretch about 5 to 15 metres, and some reach over 30 metres. Long ago, after a big storm, one washed up on a beach in Scotland that was measured at more than 55 metres — longer than a blue whale! Its stretchy body can pull much thinner and longer than usual, so these giant lengths are tricky to measure exactly.

The wonderful thing is how skinny it stays. Even when it is metres and metres long, the bootlace worm is only about as wide as a shoelace, around 5 to 10 millimetres. Its body is a dark brownish-black colour, and it winds and loops into a tangled heap when it rests. From a distance, a bundle of bootlace worm can look just like a pile of wet, knotted string.

You will usually find this worm hiding in rockpools, wrapped around the roots of big seaweeds, or tucked into cracks in the rocks. Out in deeper water it slides over muddy, sandy, and stony seabeds. It is a gentle hunter that catches tiny sea creatures using a special sticky part of its body called a proboscis, which it pushes out to grab a meal.

To keep itself safe, the bootlace worm has a clever trick. When something pokes or grabs it, the worm makes lots of thick, slippery slime with a faint funny smell. The slime tastes nasty to crabs and other hungry animals, so they leave the worm alone. Scientists have not measured how many bootlace worms there are, so its conservation status has not yet been worked out — but along many European coasts it is still a common find.