Which is the longest animal in the world?

Last year, the PeerJ journal published an article about the largest marine animals. The neat infographic accompanying their tweet immediately got my attention (if the figure is not showing up, check it in full resolution here):

For a couple of milliseconds, I thought the scale bar below the whales was a worm. Why? Because here in Norway, at the Sars Centre, we study a ribbon worm named *Lineus longissimus – *the longest animal on Earth.

Or at least it’s supposed to be. That is what’s in the Guinness World Records (Cawardine 1995), on Wikipedia, in books and papers, and recently on the BBC. Every year on Bergen’s Research Day we tell the kids that this thin dark-brownish worm they are looking under the scope can reach 60 meters long! But how do we know?

## Finding Lineus longissimus

We collect live specimens of *Lineus longissimus* by dredging the bottom of the Norwegian fjords near Bergen. They are active, bear a good dose of charisma and survive well in the laboratory. They are also voracious predators and like to feed on annelids, which are usually swallowed whole:

But so far, the longest individual we’ve found is near two meters long. How could it reach sixty?

## Digging up old literature

Here is what the most-cited description says:

The longest known worm is the bootlace worm (

Cawardine 1995Lineus longissimus), a kind of ribbon worm or nemertine (Nemertea) found in the shallow waters of the North Sea. A specimen which washed ashore after a severe storm at St. Andrews, Fife, UK, in 1864, measured more than 55 m (ca. 180 ft) in length.

In 1864, a near-sixty meters worm was found in Scotland. Found by whom? Measured how? Preserved where? Who wrote the original report? All the links mentioned above converge to one source: Cawardine (1995). But to my surprise, his book does not cite the original report. For this reason, I decided to check out what humanity knows about the length of *Lineus longissimus*.

I knew the date. The location suggested that the report must be in some British literature. To find the original sixty-meter observation, I resorted to the great Biodiversity Heritage Library (BHL), which helped me several times to access the classical literature during my doctorate work. BHL has a neat feature where you can search the literature by a species name, and the bootlace worm *Lineus longissimus* is present in many publications.

Here is an almost exhaustive compilation of reports with the length of *Lineus longissimus*:

Reference | Maximum length | Comment |

Sowerby (1806) | Many fathom | This is an estimated length, not a direct measurement (1 fathom = 1.83 meters). |

Pennant (1812) | 9.1 m | – |

Davies (1816) | 28 m | Direct observation: 6.7 meters after fixation. At least four times this size when alive. Estimated between 22–28 meters (12–15 fathom). |

Schweigger (1820) | 1.2–4.5 m | – |

Edwards (1846) | 27 m | – |

Newman (1848) | 27 m | – |

Thompson (1849) | 4.0 m | Direct observation: 1 meter after fixation. Estimated 4 meters (12 feet) when alive. |

Thompson (1856) | 3.7 m | – |

Leuckart (1859) | 0.5 m | – |

Johnston (1865) | 4.3 m | Direct observation. |

McIntosh (1873–1874) | 4.5–9.1 m | Estimated (15-30 feet). |

Claus (1876) | 1.5 m | – |

Carus (1885) | 4.5–13.7 m | – |

Knauer (1887) | 2.0 m | – |

Lacaze-Duthiers (1890) | 7.0 m | – |

Claus (1891) | 4.6 m | – |

Oudemans (1892) | 12 m | – |

Feuille des jeunes naturalistes (1893–1894) | 25 m | – |

Liverpool Biological Society (1893–1894) | 0.6–0.9 m | – |

Bürger (1895) | 11 m | – |

Bürger (1895) | 27 m | – |

Bürger (1895) | 30 m | – |

Duncan (1896) | 4.3 m | – |

Haeckel (1896) | 12 m | – |

Société zoologique de France (1896) | 1.0 m | – |

Page (1906) | 6.1 m | – |

Schmidt (1912) | 15 m | – |

Société de biologie (1914) | 2.5 m | – |

Thomson (1916) | 25 m | – |

Brehm (1918) | 30 m | – |

Boulenger (1936) | 27 m | – |

Wieman (1938) | 30 m | – |

Field Museum of Natural History bulletin (1977) | 55 m | Cites St. Andrews specimen as length estimated to 180 feet. |

Academia de Ciencias de Cuba (1994) | 30 m | – |

What I found is that nobody has ever captured (and reported) a live *Lineus longissimus* more than 10 meters long (but see below!). The longest length is reported by Davies (1816) as 28 meters, and this is an approximation based on how the animal shrinks upon fixation. All subsequent reports simply reproduce this number as a maximum length.

What about the sixty-meter worm from St. Andrews?

## An ocean’s giant

I initially missed it and months went by… until Jon Noremburg gave the answer in a comment to the above feeding video: the St. Andrews report comes from McIntosh.

This is unquestionably the giant of the race, and even now I am not quite satisfied about the limit of its growth, for after a severe storm in the spring of 1864 a specimen was thrown on shore at St. Andrews which half filled a dissecting jar eight inches wide and five inches deep. Thirty yards were measured without rupture, and yet the mass was not half uncoiled.

McIntosh (1873–1874: pg. 183)

Thirty yards (=90 feet or 27 meters) is the measured length of a half-worm. Therefore, a whole-worm measures the double, 60 yards (=180 feet or 55 meters). Right? Well… if the worm was partly in knots, how do we know that the measured part is actually half of the length? We don’t.

## Calculating the volume of a worm

But the report has a crucial piece of information. The worm was said to fill half of an 8 by 5 inches jar (ca. 13 cm). With some basic math, we can calculate the volume of the worm and estimate its length from that. Assuming that the jar was cylindrical:

Radius (r) is 4 inches and the height is 2.5 inches (half of the 5 inches jar). Thus, the volume of the worm equals:

Or converting to meters:

We can then assume the worm is a cylinder and estimate its height for any given diameter. The height will be the length of the worm.

The width of *L. longissimus* ranges from 2 to 10 mm. I used the formula above to calculate the estimated lengths of the famous St. Andrews worm:

Width (millimeters) | Length (meters) |

2 | 666 |

3 | 296 |

4 | 166 |

5 | 106 |

6 | 74 |

7 | 54 |

8 | 42 |

9 | 33 |

10 | 27 |

The most conservative estimate (10 mm of diameter) results in an almost 30 m long worm, suggesting that the St. Andrews specimen might indeed have been over 30 m! How much longer is hard to say.

The specimens we have in the lab are between 1 and 4 mm wide, but most are below or near a meter long. It would be interesting to know how the body width scales with the body length in *Lineus longissimus*.

## A worm as long as a blue whale

In any case, the St. Andrews specimen was at least +30 meters, which is a comparable size to the longest ocean giants *–* the Lion’s Mane Jellyfish (36.6 m) and the Blue Whale (33 m) (McClain et al. 2015). In favor of the nemertean, the volume estimation seems to corroborate his majestic length.

McClain and collaborators (2015) stress the difficulty of estimating the size of large marine animals. Lack of data, biased sampling or simply feasibly measuring a 30 m animal complicates body size assessment. Nemerteans have an additional aggravating factor: they shorten or elongate with ease. Possibly, volume is a more accurate measure of body size for these slim worms.

The authors also highlight that the greatest reported size is quite different from the mean population size. This finding seems true for *Lineus longissimus* as well. Despite the largest 30–55 m estimate, most of the reports describe lengths not longer than 10 m, often ranging from 1 to 5 m. Which might show best the normal size distribution of *Lineus longissimus* populations.

The longest animal on Earth?

Sixty meters might be far-fetched, but there is relatively good evidence that this nemertean can reach – and maybe surpass – the impressive length of a blue whale, thus placing *Lineus longissimus* indeed within the world’s ocean giants.

## References

Carwardine, M. (1995). *The Guinness Book of Animal Records*. Guinness World Records Limited.

McClain, C.R., Balk, M.A., Benfield, M.C., Branch, T.A., Chen, C., Cosgrove, J., Dove, A.D.M., Gaskins, L.C., Helm, R.R., Hochberg, F.G., Lee, F.B., Marshall, A., McMurray, S.E., Schanche, C., Stone, S.N. & Thaler, A.D. (2015). Sizing ocean giants: patterns of intraspecific size variation in marine megafauna. *PeerJ* **3**, e715. https://doi.org/10.7717/peerj.715