Denys Tucker was a brilliant if maverick zoologist and principal scientific officer at the British Museum (Natural History) — at the Natural History Museum in South Kensington — for 11 years until June 1960 when he was summarily dismissed for “long, continued, vexatious, insubordinate and generally offensive conduct towards the museum’s director and other senior staff”. He was 39.

He spent the next seven years in legal challenges to the decision, taking the matter to the High Court and the Court of Appeal as well as having his case raised by his MP in the House of Commons. But it was all to no avail and despite making 100 or so applications in the ensuing years for jobs of one kind and another he never worked again. A gifted but contrary zoologist and taxonomic ichthyologist, he remained defiant to the end.

By his own admission, Denys Tucker was born of humble origins, the only child of a church brass engraver and a shoemaker’s daughter. He was brought up in Exeter and won a scholarship to that city’s Hele’s School, where he was secretary of the school’s natural history society and editor of its magazine. In 1939, at 18, he was awarded a gold medal by the Zoological Society of London, the presentation being made by Julian Huxley, its Secretary. The following year he won an open scholarship in zoology to University College of the South-West, where he read marine ecology with botany.

From 1940-42 he was a member of the Home Guard and a firewatcher on the roof of the university building during the “Baedeker” air raids. His degree course was then interrupted by war service from 1942 to 1946 with the RAF in India where he rose to the rank of flight lieutenant.

On being demobilised he completed his degree, after which he was a research assistant in the Department of Zoology, University of Liverpool, where he worked on ecology and the reproduction of marine molluscs.

In 1949 he joined the staff of the Natural History Museum, and he worked his way up to become in 1958 principal scientific officer, curating and researching the taxonomy and biology of marine fishes, especially deep-sea species. In that year he was awarded a DSc by London University.

The coelacanth, the primitive marine bony fish thought to be extinct until a living specimen was discovered in 1938, was one of his early interests. But he also became absorbed by the freshwater eel and wrote an article for the journal Nature in 1959 in which he challenged the long-held belief that all freshwater eels spawned in the Sargasso Sea. It caused a furore.

The orthodoxy, as propounded in 1904 by the Danish biologist Johannes Schmidt, was that all freshwater eels migrated across the Atlantic to the Sargasso, mated and died, and their offspring then made their way to their ancestral feeding grounds in the rivers of Europe. Tucker challenged this notion, proclaiming that eels migrating from Europe would never have made it as far as the Sargasso because they would have been too debilitated. In any case, they did not have the sensory means to find their way through the myriad ocean currents en route. No one had ever found an adult eel in the Sargasso. It was all theory. And why would any creature travel 3,000 miles or so to one particular place merely in order to breed?

Tucker’s contention was that the eels found in European rivers were the progeny of American eels, the same species, he said, which did indeed spawn in the Sargasso, some of which made their way to American rivers and others to Europe. But Schmidt had observed that in general American eels had 108 vertebrae compared with 115 for European eels. In other words, they were two distinct species. Tucker’s explanation of the difference was ingenious if complicated. The Sargasso, he said, could be divided latitudinally into two areas. In the north the temperature rose gradually from 8C at a depth of 800 metres to 20C at the surface. In the south there was a sudden jump of 4 degrees between 200 and 100 metres, reaching 25C at the surface. Newly hatched larvae rising to the surface of the southern sector would, on encountering the sudden rise in temperature, cease developing the muscle segments which later become vertebrae. Those in the north, untroubled by the sudden increase in temperature, would develop more vertebrae. The southern thin-heads (baby eels) turned westwards; the northern ones headed east.

Jan Boetius, another Danish biologist who had devoted much of his life to the study of eels, told the journalist and author Tom Fort years later: “Tucker was a very, very clever man. It was a very, very clever piece of work, and it could have been right.”

What was extraordinary was that Tucker had never so much as held an eel in his hands. His theory was based entirely upon data he had read in textbooks. But his hypothesis was presented in such elegant and forceful prose that it appeared plausible and prompted an excited and prolonged debate in the world of experts on Anguillidae (freshwater eels). As Fort wrote in his The Book of Eels: On the Trail of the Thin-heads (2003), Tucker’s “haughty impatience was calculated to enrage”.

Experts worldwide responded to Tucker’s article with indignation and, in his own words, his theory was “shot to smithereens” with a deluge of evidence supporting Schmidt’s longestablished view. In a moment of candour he later confessed that his hypothesis was without foundation, was baseless and merely an intellectual exercise.