Author: Ibon Cancio, UPV/EHU Associated Professor in Cell Biology; researcher in the ‘Cell Biology & Environmental Toxicology’ (CBET) research group of the Plentzia Marine Station (PiE-UPV/EHU); Spanish scientific representative in the EMBRC Committee of Nodes

Ernst Johannes Schmidt, born in Jaegerspris, Denmark on 2 January 1877, untangled one of the biggest historical riddles in marine biology. His scientific and personal career can be said to have revolved around the eel but also around beer. Schmidt worked on the fascinating history of eel reproductive migration through the Carlsberg Foundation (an important patron of science) and the Carlsberg Physiological Laboratory. In turn, for his migration of life, he had the company of Ingeborg Kühle (1880-1958), daughter of the chief director of the Carlsberg Brewery,whom he married as a young doctor in biology and botany.

Schmidt working in a laboratory
Johannes Schmidt in the laboratory on board of the research vessel Dana during one of his expeditions (Photo kindly provided by the Carlsberg Archives).

Schmidt carried out his studies of natural history at the University of Copenhagen, obtaining a degree in biology in 1898. Through a Carlsberg Foundation grant, the young biologist explored coastal areas of Thailand, studying mangrove trees and microalgae. He received his doctoral degree in biology and botany in 1903 with a thesis on the shoot architecture of mangrove trees.

From 1902 to 1909, Schmidt worked part-time as a botanist at the Botanical Institute of the University of Copenhagen and part-time as a marine zoologist, first at the Danish Marine Station and later at the Danish Commission for Investigation of the Sea (Danish section of ICES). In 1909, he became the head of the department of physiology at the Carlsberg Laboratory, a position that he maintained until his death of influenza in 1933. As a botanist, he described a species of cyanobacteria living inside diatoms (his background in botany would be useful later in his role as hop master of Carlsberg), as a zoologist he focused his attention on fish, nearly exclusively on eels.

Carlsberg Foundation logo
Scientists always acknowledge their funding bodies, and the Carlsberg Foundation and Schmidt cannot be separated.

Eel fisheries are, or have been, important in some European countries such as Denmark, but eel biology and life cycles had been a riddle for more than two thousand years. The problem was always knowing how and where they reproduced. There was no way to see an eel in reproductive maturity or to see any spawning taking place! Aristotle wrote the first conclusions: eels come from the Earth’s guts, and that was the belief for centuries. Max Schultze, a German microscopic anatomist, has been said to have confessed on his deathbed in 1874 that everything was already known to science with the exception of the Eel question… poor man!

Eel larvae had already been found in 1856 but they were misidentified and classified as a new species, Leptocephalus brevirostris. Giovanni Battista Grassi, working in the marine station that Nikolaus Kleinenberg had established in Messina, found some small leaf-shaped and translucent L. brevirostris in the Strait of Messina, and discovered that they had 115 vertebrae. This was only compatible with the anatomy of European eels. His ‘The reproduction and metamorphosis of the common eel (Anguilla vulgaris)’ was published in 1896. Moreover, he described a mature male silver eel with maturing testis. With this clear proof of their reproduction and metamorphosis, and after finding the youngest/smallest larvae ever described, he came to the conclusion that they reproduced in the deep waters of the Mediterranean. European eels were Italian!

Schmidt’s mentor at the Danish Biological Station – a naval transport vessel (The Biologen Ark) that, starting in 1889, was anchored in a new location every spring for summer sampling campaigns – was the ichthyologist and father of Danish fisheries research, C. G. Johannes Petersen (1860-1928). Petersen, director of the station, had observed the silvering process of yellow eels (part of the metamorphosis that accompanies eel maturation) in captivity. Schmidt, working like a detective on the ‘Eel question’ from 1904 to 1923, would finally discover that European eels reproduce far away in the Sargasso Sea, after a long reproductive migration from European continental waters.

The life cycle between freshwater and ocean eels
The life cycle between freshwater and ocean eels (Photo from creative commons)

In May 1904, sampling cod-fry close to the sea surface west of the Faroe Islands, in the Atlantic and on board of the research vessel ‘Thor’, Schmidt captured an eel Leptocephalus larva. It had a length of 75 mm, approaching the full-grown size, like the larvae that Grassi found in the Strait of Messina. Schmidt was then appointed cruise leader of two expeditions on board the Thor in the Mediterranean Sea. One took place from 1908-1909 and the second one in 1910, and both were funded by the Carlsberg Foundation. They found that fewer eel larvae were present the deeper they went into the Mediterranean, and that they also grew larger deeper down. In no way were they able to find larvae smaller than those captured in Faroe. The logical conclusion was that the spawning grounds were not in the Mediterranean, but in the Atlantic Ocean. In 1912, Schmidt published a paper on the life history of the eel in Germany after rejection by the Royal Society in London on the grounds that Grassi’s 1896 work was definitive. Take- home message: referees and editors should not always be trusted!

Schmidt on boat
Johannes Schmidt on board of a research vessel with one of the plankton nets used to fish the small leptochepali eel larvae for the oceans (Photo kindly provided by the Carlsberg Archives).

Schmidt continued searching for the eel spawning grounds. He did this by systematically trawling the deep sea of the Atlantic, always moving in the direction where he found the smallest larvae (necessarily west of the Faroes). In 1921, on an expedition on the ‘Dana’, he finally located the spawning grounds in the Sargasso Sea, locating the smallest leptocephali larvae. There, Schmidt found newly-hatched larvae of European eel, discovering that their area of distribution ranged from the north-east of St. Thomas to the south-east of Bermuda. In this way, the breeding place for our eel was mapped, establishing that the eels from European continental waters travel for thousands of nautical miles to spawn in the spring. In the same way, the larvae take advantage of the North Atlantic current to make the opposite journey back to Europe. The fully developed larvae found off the west-coast of Europe during the summer are around two years old, and when metamorphosing into eel elvers in estuaries, are about three years old. North American eels (A. rostrata) reproduced in the same place but migrated towards the American continent. By examining samples of eels captured in rivers of different European countries, Schmidt had established in 1913 that all eels found in Europe belong to the same species, Anguilla vulgaris (now called Anguilla anguilla).

Eel larvae
Leptocephali larvae of European and North-American eels as published in Schmidt’s ‘The breeding places of eel’ (1923). Curiously enough, American eels are displayed looking left while European eels are shown looking right, like facing their respective continent of destination from the Sargasso Sea.

After World War I, which stopped any further research, and beginning in 1922, Schmidt analysed thousands of eels from all parts of the globe at the Carlsberg Laboratory. Besides the two Atlantic species, the European and the North American, there are six species in the Indian Ocean and about twelve in the Pacific. Trying to study some of those species, he made a journey to Australia, New Zealand and Tahiti in 1926 and from 1928-1930, he led the third Dana expedition (on the Dana II) in a two-year circumnavigation voyage funded by the Carlsberg Foundation. He found for instance that the Indo Malayan Eels all seem to reproduce in the Mentawei Deep.

In conclusion, Schmidt untangled one of the most complicated life cycles in the animal kingdom, one that lured humans for years. He would be awarded the Darwin medal in 1930, the Royal Society in London correcting its earlier error of 1912. Previously, Grassi had obtained his Darwin medal in 1896. This means that two Darwin medals have been awarded to the solution of the ‘Eel question’. May Max Schultze rest in peace!

Research vessels
Research vessels Thor and Dana II. Thor was a steam trawler used for oceanographic research from 1903 to 1927, and was used by Schmidt to capture his Faroe Island eel larva. Dana II was commanded by Schmidt during his 1928-1930 circumnavigation expedition.

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