Bootlace Worms Can Grow Longer Than Most Whales

By: Mark Mancini  | 
bootlace worm
The bootlace worm is one of the longest animals on Earth. They are usually about 15 to 50 feet (5 to 15 meters) long, but they are claims of them growing up to 180 feet (55 meters) long. Wikimedia/(CC BY-SA 4.0)

It's a skinny creature, not much wider than a No. 2 pencil. Yet it's one of the longest animals alive today, sometimes rivaling (or perhaps surpassing) the mighty blue whale in length, which can grow a little more than 100 feet (30 meters) long.

It has no heart, no spine, no body segments. It lurks in mud, sand and rocky crevices around the North Atlantic. Its mucus is surprisingly toxic — and when hunger strikes, it summons a winding, branch-like appendage that gets turned inside-out.


Its name is Lineus longissimus, or the bootlace worm. And someday, the marine oddity might be enlisted to help farmers protect their crops.

Blood Relatives

In order to make any sense of the bootlace worm, you have to step back and take a look at its peculiar branch on the great tree of life.

Lineus longissimus is the largest member of the animal phylum Nemertea. Also known as the "ribbon worms" or the "proboscis worms," this group includes about 1,200 documented species. Unlike the more familiar earthworms, which aerate our soil and keep bait shops in business, bootlace worms do not have segmented bodies.


The blood of a ribbon worm is held in a series of vessels. When the walls of these constrict, the blood inside is pushed in one direction or another, allowing it to circulate through the body. The normal muscle contractions associated with swimming and crawling help this process; no heart is required to keep the blood flowing.

bootlace worm
Bootlace worms use a tubular feeding structure called the "proboscis" that they force from a special internal pouch when needed.
Rachel Koning/(CC0 1.0)


Probing for Supper

One defining trait of the Nemertea phylum is a tubular feeding structure called the "proboscis."

Usually, it's tucked away in a specialized pouch. However, when the need arises, a ribbon worm applies pressure to the area. That force drives the proboscis tube outside the body by — quite literally — flipping it inside-out. All this can happen in a matter of seconds.


OK, but why?

Ribbon worms eat a variety of different things, like crabs, snails and animal carcasses. A few species are even herbivorous. Having a quick-draw proboscis really helps these legless animals catch and manipulate food. On many ribbon worms, albeit not the bootlace, this elaborate tube is fitted with sharp little barbs.

Sometimes, the proboscis is also used as a digging tool. And it can definitely freak out predators who try to eat the worms.

You can't blame other creatures for feeling confused — or even a bit intimidated — by the display. Certain ribbon worms can double their body length just by whipping out their proboscis.

If John Carpenter ever remakes "The Thing" again, they should audition.


Ribbons of the Sea

Like most (not all, but most) ribbon worms, Lineus longissimus occurs in marine habitats.

Bootlace worms are indigenous to the northeastern Atlantic Ocean. They live around the coastlines of Iceland, Germany, the Netherlands, Scandinavia, Ireland, Great Britain and the Baltic Sea.


The worms like to bundle themselves up underneath large boulders by the shore. Other hangout spots include rock fissures, beds of kelp and natural seaside pools. You might also see them slithering around on muddy beach sand.

Offshore, bootlace worms frequent sunlit parts of the ocean floor, winding their sinuous bodies through beds of muck and seashells. (Divers sometimes find them adrift in the water as well.)

Noted for its dark complexion, Lineus longissimus comes in shades of black and chocolatey brown. The skin may appear iridescent or striped, at least to our fancy human eyes. Ribbon worms cannot "see" images like we can. Instead, they detect changes in light conditions through primitive, sensory "eyespots."

bootlace worm
Bootlance worms can be found bundled up beneath large boulders by the shore or by rock fissures, beds of kelp and natural seaside pools.
Inaturalist/Christine Morrow/(CC BY-NC 4.0)


A Giant in Scotland?

Though the bootlace worm is only 0.2 to 0.4 inches (5 to 10 millimeters) wide, it's one of the longest known animals on the planet, full stop.

Bootlace worms are usually in the neighborhood of 16.4 to 32.8 feet (5 to 10 meters) in length. Pretty respectable for a worm, but that's just the tip of the iceberg.


In a 2008 article for the journal Zoologische Mededelingen, biologists Adriaan Gittenberger and Cor Schipper explain that bootlace worms measuring up to 98.4 feet (30 meters) long "have been encountered repeatedly."

If you're squeamish and the image of a wriggling worm longer than a New York City bus disturbs you, skip the next few sentences. Things are about to get even more mind-blowing.

The carcass of a monstrous bootlace worm reportedly washed ashore in Scotland during the year 1864. From end to end, it was said to measure... wait for it... more than 180 feet (55 meters) long!

Claims like that should be taken with a grain of salt. As Gittenberger and Schipper's article observes, the scientific community doesn't have any preserved specimens of a bootlace worm measuring anywhere near this size.

Which means we cannot confirm the existence of a 180-foot-long (55-meter-long) Lineus longissimus. Also, because ribbon worms in general are stretchable, elastic critters with bodies prone to distortion, it can be hard to pin down their maximum lengths anyway. Sorry for the buzzkill.


The Benefits of Toxins

Handling bootlace worms is not a pleasant experience. To ward off predators — and grabby people — the invertebrates release huge amounts of thick, foul-smelling mucus when they feel threatened.

And there's more to this substance than meets the eye (or nose).


Naturalists have learned that the bootlace worm's defensive mucus is loaded with peptide toxins. Indeed, when a team examined the stuff in 2018, they discovered an entirely new group of said toxins, hitherto unknown to science.

Researchers who participated in that 2018 study have said the most common of these peptides (called "nemertide α-1") probably isn't poisonous to human beings or other mammals. But cockroaches should keep their distance.

Lab tests showed that exposure to nemertide α-1 interferes with nerve and muscle function in green crabs (Carcinus maenas) and Dubia roaches (Blaptica dubia). That can leave the critters dead or permanently paralyzed.

"Nemertide α-1 has a very powerful effect on crustaceans and cockroaches, which is why it could serve as a very effective insecticide," Ulf Göransson, the study's lead author, told SciNews in 2018.

Who knows? In the near future, products derived from the bootlace worm's stinky mucus may keep pest insects from ruining farms and cash crops. Stranger things have happened.