With a name like Lineus longissimus, it’s no surprise that the bootlace worm is, well, long. The largest specimen on record was said to measure 55 metres: longer than a blue whale (although the individual wasn’t preserved so we can’t be sure).
Despite growing so long, they are very thin – giving them a high surface area-to-volume-ratio – and don’t have any lungs. Instead, they absorb oxygen from the water through the skin.
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What is a bootlace worm?
Bootlace worms are a type of ribbon worm. A visually noticeable different between these worms and earthworms is that they don’t have segmented bodies. These black or brown worms are often found in a messy heap on the seabed or in rockpools, making them seem much smaller than they are. If you unfurled this pile of bootlace-like tissue and laid it out in one long string, it would stretch for several metres.
These long, thin danger noodles produce a stinky, toxic mucus that can kill insects (but is not though to be toxic to humans – although, best not risk it!).
One reason for their extraordinary length could be to help them feed: it’s thought these worms can absorb nutrients directly through their skin.
What do bootlace worms look like?
Like a really, really long worm. They are often tangled in a lump like a pile of old shoelaces. These ribbon worms typically reach lengths of between five and 15 metres – for context, adult sperm whales are around 15 metres long. Despite stretching so far, they’re usually just five to 10 millimetres wide (that’s similar to a strand of tagliatelle pasta).
But these worms can grow to much more epic proportions. The longest specimen ever recorded washed up on the coastline of St Andrews in Fyfe, Scotland, after a massive storm in 1864. The 55-metre creature broke records and put this elongated species into the record books as the longest solitary animal on the planet. (The brilliantly named long stringy stingy thingy, Apolemia uvaria, is even longer. In 2020, one of these free-living colonial creatures was filmed in Australia measuring a staggering 119 metres.)
Where do they live?
Bootlace worms are usually found in shallow waters and rockpools around the northeast Atlantic Ocean. They’re commonly found around the UK coastline.
What do they eat?
Small marine animals such as tiny crustaceans, snails, molluscs and worms.
How do they hunt?
To consume their prey, bootlace worms stick out their tube-like proboscis and turns it inside out to grab their target. They exude a gross, sticky mucus that paralyses their victim and – once it’s succumbed – suck their meal back in and chow down.
How do they reproduce?
Bootlace worms and other ribbon worms typically reproduce sexually by releasing eggs and sperm into the water. Once fertilised, these develop into larvae and eventually grow into an adult worm.
Are they dangerous to humans?
No. To defend themselves, bootlace worms can produce a sticky, gooey mucus containing a powerful peptide toxin. This toxic mucus is said to smell vile – it’s been compared to iron or sewage.
If you’ve heard of peptide toxins before, it might be because they are also produced by the deadly cone snail. But humans don’t need to worry. Although this neurotoxin is enough to paralyse and kill crabs, fruit flies, mites and cockroaches, lab tests have shown it doesn’t affect mammals in the same way. That said, wash your hands if you happen to touch one as the goo could cause irritation.
This venom could actually be helpful. Because it affects insects but doesn’t seem to be harmful to humans, scientists are exploring whether it could be useful in developing new insecticides.
Are they under threat?
Experts don’t think so, although bootlace worms don’t have a formal classification from the IUCN Red List of Threatened Species. They are commonly found around the UK coastline and their populations are thought to be stable. However, habitat destruction and a changing marine habitat could impact them in the future.
