Beginning this #WormWednesday with the beginnings of a worm. The cleavage stages of the annelid Chaetopterus.
At some point in our lives, we have all been through that (=embryonic cleavage).
Paper! How are segment polarity genes expressed in an animal without segments? https://www.nature.com/articles/srep32387
Which is the longest animal on this planet?
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), in Wikipedia, in books and papers, and recently in BBC. Every year in 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?
We collect live specimens of L. 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? Here is what the most-cited description says:
The longest known worm is the bootlace worm (Lineus 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 (180 ft) in length.Cawardine 1995
In 1864, a near-sixty meters worm was found in Scotland. Found by who? 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 L. 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 L. longissimus is present in many publications.
Here is an almost exhaustive compilation of reports with the length of L. longissimus:
|Sowerby (1806)||Many fathom||This is 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 L. 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?
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 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.
But the report has a crucial piece of information. The worm was said to fill half of a 8 by 5 inches jar. With some basic maths 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 length of the worm.
The width of L. longissimus ranges from 2–10 mm. I used the formula above to calculate the estimated lengths of the famous St. Andrews worm:
|Width (millimeters)||Length (meters)|
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 L. longissimus.
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 L. longissimus as well. Despite the largest 30–55 m estimate, most of the reports describe lengths not longer than 10 m, often ranging from 1–5 m. Which might show best the normal size distribution of L. 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 L. longissimus indeed within the world’s ocean giants.
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
Read the previous section: Spiral cleavage, an oblique matter.
Annelids, arthropods and vertebrates show a remarkable morphological diversity (Chipman, 2010). Beneath this multiplicity of shapes and forms lies a common pattern of body organization—a trunk divided into repeated parts. This pattern and the developmental process that generates it are known as segmentation (Minelli and Fusco, 2004). While the vertebrate trunk is divided into somites1 (a portion of the mesoderm), the body of annelids and arthropods is divided into intricate repeated compartments spanning the ectoderm and mesoderm—the segments (Scholtz, 2002). The morphological similarity between these body segments previously was taken as support for a kinship between Annelida and Arthropoda, in a group called Articulata (Scholtz, 2002; Seaver, 2003). In this scenario, segmentation would have evolved only once in the protostomes and once in the deuterostomes (Davis and Patel, 1999; Peel and Akam, 2003; Seaver, 2003).
Analyses arising from the area of molecular phylogenetics have disputed the monophyly of Articulata, suggesting that annelids and arthropods occupy different branches of protostomes, the Lophotrochozoa (=Spiralia) and Ecdysozoa, respectively (Aguinaldo et al., 1997; Eernisse, 1998). This phylogenetic hypothesis indicates that annelids and arthropods are more closely related to groups without body segmentation than to each other (Seaver, 2003); a topology that favors the independent evolution of annelid and arthropod body segmentation, in addition to the independent evolution of the different segmented tissues of vertebrates (Graham et al., 2014). Subsequent phylogenetic studies continue to corroborate the distant relationship between annelids, arthropods and vertebrates (Dunn et al., 2008; Dunn et al., 2014; Edgecombe et al., 2011; Hejnol et al., 2009), reinforcing the homoplasy of their body segmentation.
Remarkably, the molecular mechanisms of body segmentation in arthropods and vertebrates show a number of striking similarities (Damen, 2007; Davis and Patel, 1999; Kimmel, 1996; Patel, 2003; Peel and Akam, 2003; Seaver, 2003; Tautz, 2004). These molecular similarities were taken as evidence to support the homology of bilaterian segmentation (De Robertis, 1997; De Robertis, 2008; Dray et al., 2010; Kimmel, 1996), despite the opposing data from phylogenetics. To reconcile this apparent conflict between developmental and phylogenetic data, we must apply a comprehensive evolutionary approach to the problem.
The concept of segmentation is often used in a typological—and not evolutionary—manner (Budd, 2001). The result is a taxonomic bias, where the evolution of segmentation is regarded from the point of view of the groups considered to be segmented, i.e., annelids, arthropods and vertebrates (Budd, 2001). As a matter of fact, there is no conceptual basis to restrict segmentation to these three groups, because the repetition of parts along the body axis (Budd, 2001; Hannibal and Patel, 2013; Minelli and Fusco, 2004) also occurs in varying degrees in other bilaterians—usually considered to be pseudosegmented or unsegmented (Budd, 2001; Minelli and Fusco, 2004; Scholtz, 2002; Willmer, 1990).
Another aspect to be considered is that segmentation—as much as spiral cleavage—is a complex of characters that ought to be individually compared between taxa (Scholtz, 2010). Breaking down segmentation into comparable traits (Scholtz, 2010), such as seriated nerve chords, segmented mesoderm or ectodermal boundaries, should provide a better overview of their evolutionary history.
Nevertheless, the sole comparison of traits between distantly related groups can still be misleading for understanding the evolution of a character (e.g., trunk segmentation), because the ancestral conditions of closer taxa are unknown. Since developmental mechanisms can be coopted to nonhomologous structures (Shubin et al., 2009), the phylogenetic context of a character is essential to distinguish homology from convergence. A recurrent proposal to better understand the evolution of segmentation is to expand taxonomic sampling (Arthur et al., 1999; Budd, 2001; Couso, 2009; Davis and Patel, 1999; Minelli and Fusco, 2004; Patel, 2003; Peel and Akam, 2003; Seaver, 2003; Tautz, 2004). Thus, examining segmentation traits in a wider range of taxa, including those without obvious segmented features, might help us to grasp the evolution of the developmental mechanisms that form repeated body parts in bilaterians.
This text is the final section of my PhD thesis (published on this blog).
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- In addition to the somites, vertebrates also show segmentation in the rhombomeres and in the pharyngeal archs; segmented structures that likely evolved independently in the deuterostome lineage (Graham et al., 2014).↩