Invertebrates
Bootlace Worm
Lineus longissimus · also called bootlace worm
The bootlace worm is a strong candidate for the longest animal on Earth. A specimen washed onto a Scottish beach in 1864 was recorded at around 55 metres — longer than a blue whale — yet the animal is only a few millimetres wide, dark brown, coated in toxic slime, and usually found knotted under a rock on the shore.
Overview
Bootlace worms live along the rocky and muddy coasts of northern Europe, coiled in dense tangles under boulders and among kelp holdfasts between the tide marks. Ordinary individuals are 5–15 m long but only 5–10 mm wide. The record St Andrews specimen, at about 55 m, would be the longest animal ever measured.
A ribbon worm
It is not an earthworm. The bootlace worm belongs to the nemerteans, or ribbon worms — an unsegmented group whose signature feature is an eversible proboscis kept inside a fluid-filled chamber and fired out explosively to seize prey. It eats crustaceans and other worms, wrapping and paralysing them before swallowing them whole.
Toxic slime
The worm coats itself in copious mucus laced with nerve toxins that rapidly paralyse crabs. On human skin the slime can leave a numb, tingling feeling, and its paralysing peptides are now being studied as models for targeted insecticides.
Hard to measure
The body is extraordinarily elastic, stretching far past its resting length, and a handled worm readily breaks into pieces — which is why the historic 55 m figure is treated with caution and why measuring a live one is nearly impossible.
Status
The bootlace worm has not been assessed. It is locally common on suitable shores, with no particular conservation concern beyond the general sensitivity of coastal life to pollution.
Bootlace Worm: common questions
Is the bootlace worm really the longest animal in the world?
It is one of the two usual answers, along with the lion's mane jellyfish. The famous figure comes from a bootlace worm washed ashore at St Andrews, Scotland, after a storm in 1864, which was recorded at about 55 metres, longer than a blue whale. The measurement is treated with caution because these worms are enormously stretchy and the specimen may have been pulled out well beyond its natural length, and because the animal is almost impossible to measure without it contracting, tangling, or breaking. Even setting the record aside, ordinary bootlace worms of 5 to 15 metres are longer than any other animal of comparable width.
How can something so long be only a few millimetres wide?
The bootlace worm's body is essentially a very long, thin, muscular tube with a simple internal layout, no rigid skeleton, and no segments. It moves by gliding on a carpet of cilia and a layer of slime and by waves of muscular contraction, and it spends most of its time coiled into a compact mass rather than stretched out, so the great length is only apparent when it is disturbed or handled. Being thin keeps every part of the body close to seawater for gas exchange and lets the worm thread itself deep into crevices and kelp holdfasts where thicker animals cannot go.
Is the bootlace worm dangerous to people?
Not seriously. It is not aggressive, has no bite that can hurt a person, and does not sting in the way a jellyfish does. Its defence is the toxic mucus it secretes, which is potent against crabs and other small animals and which can cause a mild numbness, tingling, or irritation if it gets onto sensitive skin or into a cut. Handling one and then washing your hands is generally harmless, but it is sensible not to rub your eyes or mouth afterwards, and people with sensitive skin may prefer to look rather than touch.
What does the bootlace worm eat and how does it catch prey?
It is a predator and scavenger of small invertebrates. Like other ribbon worms it has a proboscis, a long muscular structure that is normally held inside out within a fluid-filled cavity in the body. When prey is detected the worm contracts the cavity and the proboscis shoots out explosively, everting like a finger turning inside out, and wraps around or spears the target. In the bootlace worm the proboscis and the surrounding mucus deliver paralysing toxins, so a crustacean is quickly subdued, after which the worm draws it in and swallows it.