After a North Sea storm, a dark tangle in the wrack line can look like discarded nylon rope. If it moves, it may be Lineus longissimus, the bootlace worm. One specimen washed ashore at St Andrews in 1864 was reported to measure more than 55 metres, longer than a blue whale and enough to make the species arguably the longest animal on Earth.
The body producing that extraordinary length is usually only about five millimetres wide. The worm is also remarkably elastic, which makes any comparison with a whale more complicated than the raw number suggests.
Its other remarkable feature is the mucus covering its body. When disturbed, the worm releases large quantities of thick secretion containing peptide toxins, including one that paralysed and killed crabs and cockroaches after being isolated and injected during laboratory experiments.
The 55-metre problem
The famous length record comes with a substantial caveat. A Marine Life Information Network species account gives the usual length as five to 15 metres, although individuals can exceed 30 metres. The historical 55-metre specimen was not preserved, so its condition and the effect of stretching cannot now be checked.
A 2018 Sci.News summary likewise gives a typical range of five to 15 metres and a reported maximum of 55 metres. That maximum remains an exceptional historical measurement rather than a routinely documented size.
If the 1864 figure is accepted, the bootlace worm exceeds a blue whale in linear length, although certainly not in mass. The lion’s mane jellyfish, whose tentacles can extend for tens of metres, also appears on lists of the longest living animals. That is why “arguably” remains the fairest word for the bootlace worm’s claim.
What the animal actually is
The bootlace worm belongs to Nemertea, the phylum of ribbon worms. The group contains approximately 1,300 described species, most of them marine and many far smaller than Lineus longissimus. Their defining structure is an eversible proboscis, a muscular tube that can be rapidly extended from the head to capture prey.
Some nemerteans have a stylet on the proboscis that can pierce prey, but Lineus longissimus belongs to a group with an unarmed proboscis. It feeds on small marine animals, including crustaceans and other worms, using the proboscis and sticky mucus to restrain them.
The species occurs along Atlantic, North Sea and Baltic coasts in Europe. It is commonly found coiled beneath boulders, in muddy sand, inside rock fissures or tangled among kelp holdfasts.
The worm’s mucus had attracted attention long before its chemistry was understood. A sixteenth-century account discussed by the Wellcome Sanger Institute has been interpreted as a description of a bootlace worm causing swelling after being handled. Scientists began characterising toxins from other nemerteans during the twentieth century, but the peptide toxins in Lineus longissimus were not fully described until 2018.
The toxin in the slime
In 2018, researchers from Uppsala University, Linnaeus University and the Swedish Species Information Centre at the Swedish University of Agricultural Sciences worked with colleagues in Belgium and Australia to analyse the worm’s mucus. Their study in Scientific Reports described a previously unknown family of peptide toxins called alpha-nemertides. Ulf Göransson, professor of pharmacognosy at Uppsala University, led the research.
The most abundant toxin, nemertide alpha-1, contains 31 amino-acid residues and folds around three disulphide bonds. Those bonds form an inhibitor cystine knot, a compact structural motif also found in several other animal toxins.
The researchers injected isolated alpha-1 into green crabs, where doses starting at one microgram per kilogram caused paralysis. At 10 micrograms per kilogram, the crabs died within minutes. In a separate experiment involving 50 juvenile orange-spotted cockroaches, every animal receiving more than 7.1 micrograms per kilogram was dead or permanently paralysed when assessed after 24 hours.
Cell experiments showed that alpha-1 interfered with voltage-gated sodium channels from German cockroaches, fruit flies and Varroa mites. It prevented those channels from inactivating normally, producing sustained electrical currents that help explain the paralysis observed in the animals.
The toxin was approximately 100 times more selective for the tested insect channel than for the tested mammalian channels. That does not demonstrate that the mucus is safe to touch or that the toxin is harmless to humans, since the study did not include clinical or whole-animal mammalian safety testing.
In an Uppsala University account of the findings, Göransson called the peptide “the most poisonous substance to have been found in Sweden’s animal kingdom.” The statement referred to its potency in the experimental systems used by the researchers, not to evidence of human poisoning.
Why test it on cockroaches?
The cockroach experiment helped researchers determine whether the peptide’s effect extended beyond aquatic crustaceans. Its strong activity against both cockroaches and invertebrate sodium channels also raised the possibility that alpha-nemertides could provide a starting point for new bioinsecticides.
That possibility remains preliminary. The study established the peptide’s structure, its activity in a small set of laboratory animals and its effects on selected ion channels. It did not produce a commercial insecticide or demonstrate that the molecule would work safely and economically outside a laboratory.
What the paper does not show
The experiments did not examine the toxin’s effects on bees, other beneficial insects or wider aquatic food webs. They also did not establish how long it remains active under field conditions, whether it can be manufactured economically at scale or how quickly target species might develop resistance.
The natural role of alpha-nemertides also remains unsettled. Because crustaceans can be either prey or predators of ribbon worms, the toxins could contribute to hunting, defence or both. The experiments were not designed to distinguish conclusively between those functions.
The rest of the phylum
The researchers analysed transcriptomes from 17 nemertean species and identified eight alpha-nemertide sequences, including one partial sequence. Those 17 species represented only about 1.5 percent of the known phylum, leaving most ribbon worms chemically unexplored.
The small sample makes broad conclusions premature, but it also shows how much remains undiscovered. Animals that appear simple can produce highly specialised compounds with structures and biological effects unlike those previously documented elsewhere.
The bootlace worm is therefore notable for more than its disputed length record. Its mucus contains a family of peptides that was unknown to science before the 2018 investigation, and most of its relatives have yet to receive comparable chemical scrutiny.
What it feels like to find one
A contracted bootlace worm can resemble a knot of dark, wet cord beneath a rock or among kelp. When handled, it produces thick mucus with a faint, pungent smell, making close contact an unpleasant experience even without assuming that the toxin poses a demonstrated danger to people.
The head carries rows of small, deeply set eyes that can be difficult to see against the dark body. Its proboscis has no piercing stylet, so this species captures prey by extending, wrapping and restraining rather than stabbing.
A ribbon worm does not have one rigid length in the way a whale does. Its soft body changes dramatically as it contracts, crawls and extends, which is why a measurement taken from a stranded specimen is difficult to compare with the fixed body length of a vertebrate.
The 55-metre claim may never be confirmed to modern standards, but the ordinary animal is strange enough without embellishment. Beneath a boulder on a cold European shore, a ribbon only millimetres wide can extend for metres while carrying one of the most potent insect-active peptides yet identified.