THE CURIOUS CASE OF THE BOTTLED SEA SERPENT – OR, WHEN IS A GIANT EEL LARVA NOT A GIANT EEL LARVA?

An anguilline Norwegian sea serpent (sea-orm) depicted on Swedish cleric Olaus Magnus's famous antiquarian maritime map of 1539, the Carta Marina

One of cryptozoology's most enigmatic episodes is undoubtedly the very curious (and confusing) case of the bottled sea serpent.

This had attracted particular attention in 1965, when sea monsters enjoyed a renaissance in scientific respectability - thanks to the publication that year of a now-classic tome by cryptozoologist Dr Bernard Heuvelmans, entitled Le Grand Serpent-de-Mer (a somewhat different English version, In the Wake of the Sea-Serpents, appeared in 1968, also incorporating a greatly condensed version of another of his books, dealing with the giant squid and alleged giant octopuses).

First edition of Le Grand Serpent-de-Mer (© Dr Bernard Heuvelmans/Plon Publishing)

Within his book, Heuvelmans proffered evidence for believing that 'the great sea serpent', one of cryptozoology's most celebrated creatures, might actually be a non-existent composite - i.e. it had been 'created' via the erroneous lumping together (by previous investigators) of eyewitness reports that in reality feature a number of totally different types of animal.

In short, there was no single, morphologically heterogeneous species wholly responsible for all sea serpent reports on record. Instead, there were several well-defined, separate species collectively responsible for those reports.

Heuvelmans's 'super eel' category of sea serpent (© Tim Morris)

Some of them, according to Heuvelmans, were species still unknown to science, and included various unusual seals and whales, a giant turtle, a marine crocodile-like reptile, and a giant-sized 'super eel'. If his hypothesis was correct, this would have profound ichthyological implications.

For as a result of a chance discovery made over 30 years earlier, it meant that at least one bona fide sea serpent had already been captured - a sea serpent whose remains, moreover, were preserved, bottled, and available for scientific scrutiny!

Common European eel Anguilla anguilla, 1837 painting

On 31 January 1930, the Danish research vessel Dana unexpectedly captured an exceptionally long eel larva (leptocephalus) at a depth of about 900 ft, west of the Agulhas Bank and south of the Cape of Good Hope, South Africa. Whereas leptocephali of the common European eel Anguilla anguilla measure a mere 3 in long at most, and even those of the formidable conger eel Conger conger only reach 4 in, the Dana's remarkable specimen was a colossal 6 ft 1.5 in! This in itself was quite staggering, but its implications were even more astounding.

The Dana giant leptocephalus as a preserved, bottled specimen (© Prof. Jørgen Nielsen/Zoological Museum of Copenhagen/courtesy of Lars Thomas)

During their metamorphosis from leptocephalus to adult, true eels (anguillids) greatly increase their total length - the precise index of growth varying between species. In the common eel, the increase is generally eighteen-fold, producing adults measuring around 4.5 ft; in the conger, it can be as much as thirty-fold, yielding adults up to 10 ft.

19^th-Century engraving of a conger eel

Consequently, as conceded by Dana ichthyologist Dr Anton Bruun, in the case of the Dana leptocephalus, which was already 6 ft long, there existed the incredible possibility that this would have metamorphosed into a monstrous adult measuring anything between 108-180 ft, with a length of 50 ft seemingly the very minimum (less than a nine-fold increase) that even the most prudent estimator might expect of such a larva! Needless to say, any species of eel attaining such stupendous lengths as these would make an excellent candidate for those sea serpents grouped within Heuvelmans's 'super eel' category.

Artistic representation of an adult super eel's possible appearance in life (© Thomas Finley)

After its capture, the Dana leptocephalus was preserved in alcohol and has since resided in a specimen bottle within the collections of Copenhagen University's Zoological Museum. Periodically, it has been taken out of its bottle to be examined, and as a result it has gradually shrunk, but it remained a notable riddle in need of an answer – especially when, as the years progressed, a few other inordinately long leptocephali were obtained.

In 1959, an anatomically similar but somewhat shorter specimen, collected on 16 July 1958 in shallow water at Westland, South Island, New Zealand, was described by ichthyologist Peter Castle as a new species, which he formally dubbed Leptocephalus giganteus – and to which the Dana specimen was later assigned. 

The holotype (type specimen) of Leptocephalus giganteus - full provenance details given above (© Museum of New Zealand Te Papa Tongarewa (P.002603)/Creative Commons BY-NC-ND licence)

Interestingly, the Danish research vessel Galathea supposedly obtained a 6 ft leptocephalus during its voyages in the early 1950s, but no formal record of this (let alone the specimen itself) appears to exist. Even so, two specimens of L. giganteus certainly did exist, and the reality of the infamously elusive sea serpent, or at least one of its constituent members, seemed at last to have been fully endorsed. Inevitably, however, the truth proved very different.

In 1966, two much smaller specimens of L. giganteus were documented. Measuring just under 4 in and 11 in respectively, they had been sifted from the stomach contents of an Alepisaurus lancet fish captured in the western Atlantic. Except for their modest lengths, they corresponded very closely to the New Zealand example, and were carefully studied by Miami University ichthyologist Dr David G. Smith, in a bid to pinpoint conclusively the taxonomic affinities of L. giganteus in relation to the many other species of eel known to science.

An adult specimen of the snubnosed spiny eel Notacanthus chemnitzii, a typical notacanthid (public domain)

In March 1970, he exploded the sea serpent scenario for L. giganteus - by announcing that its two smaller specimens were the larvae of a notacanthid (spiny eel), not of an anguillid (true eel). This spelled doom for their species' claim to fame as (in its adult phase) a genuine sea serpent - because in stark contrast to the leptocephali of true eels, those of notacanthids do not greatly increase their length during metamorphosis from larva to adult.

Consequently, predictions that mature specimens of L. giganteus would measure over 100 ft were totally unfounded. Instead, when the unknown adult phase of this species was finally collected, it would be very little longer than the leptocephalus, i.e. a mere 6 ft or so.

An adult specimen of the froghead worm eel Coloconger raniceps, a typical short-tailed eel or colocongrid (public domain)

More recently, however, this reclassification of L. giganteus as a notacanthid has itself been challenged, so that nowadays it is popularly classed instead as a species of short-tailed eel (aka worm eel or colocongrid), within the family Colocongridae, housed in turn within Anguilliformes, the order of true eels. Accordingly, it has been renamed Coloconger giganteus (although some researchers still deem it to be a notacanthid).

In any event, just like the notacanthids, the short-tailed eels do not display a sizeable increase of length during larva-to-adult transformation. So the identification of C. (or L.) giganteus as a sea serpent remains null and void.

Konrad Gesner's version of Olaus Magnus's anguilline sea serpent, as included in Gesner's Historiae Animalium, 1558

Of course, there may indeed be eels of gigantic length still eluding scientific detection in the vastness of the oceans - giant anguillids, for example, that are compatible with Heuvelmans's concept of the 'super eel' category of sea serpent - but unlike C. (or L.) giganteus, these have yet to be captured, preserved, and bottled.

This ShukerNature blog article is excerpted and updated from my book The Encyclopaedia of New and Rediscovered Animals (2012).