Police had to track down 48 tropical fish before they died after they were mistakenly removed from a train.
The fish were being taken to the University of Hull by post-graduate student Kai Winkelmann on Thursday.
He noticed the case with the fish was missing when he arrived at Doncaster to change trains and assumed they had been stolen.
British Transport Police officers located them in Peterborough and got them back to him within an hour.
Mr Winkelmann said the rare African fish, telmatochromis temporalis, would have died if they had not been found as quickly.
He said: "I had been studying these fish, which have been shown to be tremendously valuable for evolutionary biology, for over three years.
"The fish are tropical so if they stay at less than 23C to 29C for more than a few hours they wouldn't survive so it was vital we tracked down the case.
"Within moments I saw several officers on the phone and like a miracle they found it at Peterborough with the fish still alive."
Sgt Steve Down, from British Transport Police, said "The officers at Doncaster quickly realised the scale of the problem and called BTP and rail staff colleagues at Peterborough and Kings Cross, and the fish were quickly tracked down at Peterborough, still alive.
"We quickly made arrangements to have the fish put on the next northbound service to Doncaster where they were reunited with Mr Winkelmann who managed to continue on this journey without any further incidents."