After a tumultuous storm in the North Sea, a dark mass found along the wrack line may resemble abandoned nylon rope. Should it exhibit movement, it could be *Lineus longissimus*, also known as the bootlace worm. A specimen that washed up at St Andrews in 1864 was noted to measure over 55 metres, surpassing a blue whale and thereby making the species [potentially the longest animal on the planet](https://www.discoverwildlife.com/animal-facts/marine-animals/bootlace-worm).
The body responsible for that remarkable length is typically only about five millimetres in width. Additionally, the worm is exceptionally elastic, complicating any comparison with a whale beyond mere numerical value.
Another extraordinary attribute is the mucus that envelops its body. Upon being disturbed, the worm secretes large amounts of thick fluid containing peptide toxins, one of which was known to paralyse and eliminate crabs and cockroaches during laboratory experiments where it was isolated and injected.
## The 55-metre dilemma
The renowned length record comes with a significant disclaimer. A [Marine Life Information Network species account](https://www.marlin.ac.uk/species/detail/2075) indicates the standard length ranges from five to 15 metres, though some individuals can exceed 30 metres. The historical specimen measuring 55 metres was not preserved, leaving its condition and the impact of stretching unverified.
A [2018 Sci.News summary](https://www.sci.news/biology/bootlace-worm-toxin-05852.html) similarly states a typical length of five to 15 metres with a maximum reported length of 55 metres. This maximum remains an unusual historical measurement instead of a commonly observed size.
If the 1864 measurement is validated, then the bootlace worm surpasses a blue whale in linear length, though certainly not in mass. The lion’s mane jellyfish, whose tentacles can reach several metres, is likewise found on listings of [the longest living animals](https://a-z-animals.com/blog/the-longest-animals-still-on-earth/). Consequently, “arguably” remains the most appropriate term regarding the bootlace worm’s claim.
## What the creature genuinely is
The bootlace worm is classified in Nemertea, the phylum containing ribbon worms. This group includes about 1,300 identified species, most of which are marine and many significantly smaller than *Lineus longissimus*. Their characteristic feature is an eversible proboscis, a muscular tube that can swiftly extend from the head to seize prey.
Some species of nemerteans possess a stylet on the proboscis capable of piercing prey, but *Lineus longissimus* is part of a group with an unarmed proboscis. It feeds on small marine creatures, including crustaceans and other worms, utilizing the proboscis and sticky mucus to immobilize them.
The species is found along the Atlantic, North Sea, and Baltic coasts in Europe. It is typically spotted coiled under stones, in muddy sand, within rock crevices, or entangled among kelp holdfasts.
Long before its chemical properties were understood, the worm’s mucus had drawn interest. A sixteenth-century account reviewed by the [Wellcome Sanger Institute](https://sangerinstitute.blog/2021/10/18/marine-medicines-from-the-slimiest-of-worms/) is believed to describe a bootlace worm causing swelling upon handling. Researchers started isolating toxins from various nemerteans in the twentieth century, yet the peptide toxins in *Lineus longissimus* were not completely characterized until 2018.
## The toxin within the mucus
In 2018, investigators from Uppsala University, Linnaeus University, and the Swedish Species Information Centre at the Swedish University of Agricultural Sciences collaborated with peers in Belgium and Australia to study the worm’s mucus. Their [research in *Scientific Reports*](https://www.nature.com/articles/s41598-018-22305-w) identified a previously unrecognized family of peptide toxins named alpha-nemertides. Ulf Göransson, a pharmacognosy professor at Uppsala University, spearheaded the study.
The most prevalent toxin, nemertide alpha-1, consists of 31 amino-acid residues and forms structures around three disulphide bonds. These bonds create an inhibitor cystine knot, a compact structural motif also present in various other animal toxins.
The researchers injected isolated alpha-1 into green crabs, where doses beginning at one microgram per kilogram resulted in paralysis. At a dosage of 10 micrograms per kilogram, the crabs perished within minutes. In a separate analysis involving 50 juvenile orange-spotted cockroaches, all subjects receiving over 7.1 micrograms per kilogram were either dead or permanently paralysed when evaluated after 24 hours.
Cellular experiments demonstrated that alpha-1 disrupted voltage-gated sodium channels from German cockroaches, fruit flies, and Var