Bootlace Worm Wins Invertebrate Profile With 55-Meter Length and Toxic Slime

Q: Is the legendary 55-meter bootlace worm a true biological marvel, or just an optical illusion caused by extreme elasticity?

It is both a real biological marvel and a measurement that comes with a major caveat. The bootlace worm, *Lineus longissimus*, is unquestionably an extreme animal. Even typical individuals are reported at roughly 5 to 15 metres long, with some reaching 30 metres or more, while remaining only 5 to 10 millimetres wide. That alone makes it one of the longest known single animals on Earth. But the famous 55-metre record from St Andrews in 1864 is treated cautiously by scientists because ribbon worms are extraordinarily elastic. Their bodies can stretch, contract and tangle dramatically, so “length” is not as fixed as it is in a whale or snake. The specimen was washed ashore after a storm, and it was not preserved, which makes the old measurement hard to verify by modern standards. So the record is not merely an optical illusion; the worm really can become astonishingly long. The uncertainty is whether 55 metres reflects its normal relaxed biological length or a storm-stretched extreme. That is why many experts describe 55 metres as plausible but unconfirmed, while treating 15 metres or more as a safer, routine scale for the species. What is beyond dispute is that the bootlace worm is remarkable for more than size. It is a predator, can evert a proboscis to subdue prey, and produces toxic mucus containing alpha-nemertides that kill arthropods but appear to have little effect on mammals, making it scientifically valuable as well as visually extraordinary.

1 articles · Updated · The Guardian · Aug 10

A bootlace worm — Lineus longissimus — was spotlighted in the Invertebrate of the Year series for stretching beyond an Olympic pool, with one St Andrews specimen recorded at 55 meters.
The marine ribbon worm is usually about 15 meters long and just 5-10 mm wide, but its other standout trait is poisonous mucus that it ejects when threatened.
Scientists fully described the worm’s alpha-nemertide toxins in 2018, finding they can paralyze crabs and kill cockroaches while having little impact on mammals.
That profile points to possible insecticide uses and underscores the wider scientific value of nemerteans, a little-known phylum with more than 1,250 described species.
The feature is part of a reader-nominated series running Aug. 3-12 before voting opens on favorite invertebrates.