Maturing Eels Break Down Their Skeletons

Swimming eel © Thünen Institute

International research team is studying the endangered species.


European eels migrate several thousand kilometers across the Atlantic Ocean to reach their spawning grounds. As an international research team has now discovered, the eels undergo drastic changes on their way – breaking down their skeletons and potentially redistributing toxic metals in their systems, which could be problematic for the endangered species.

European eels Anguilla anguilla are a fish species with a unique reproductive strategy. They initially spend many years in European rivers, lakes, and coastal areas, however once they mature, they migrate several thousand kilometers across the Atlantic Ocean to reach their spawning grounds in the Sargasso Sea, where they spawn and then die.

A research team formed of members from the Thünen Institute of Fisheries Ecology in Bremerhaven and RWTH Aachen University in Germany, Ghent University in Belgium, and the University of Saskatchewan in Canada, has now studied the drastic changes that the species undergo during their maturation. They discovered that alongside minerals that are needed to fuel reproduction, potentially toxic metals are redistributed from bones and soft tissues to the ovaries. The researchers are now looking into the potential consequences this may have for the endangered species.

With the onset of puberty, a maturation process termed silvering means the eels change in appearance from pale yellow to gleaming silver. The eels stop feeding, shrink their guts, break down their skeletons, and build up their gonads, or reproductive organs. The interdisciplinary research team is using different analytical and imaging techniques, such as computed tomography and MRT to show how eels use their skeletons as mineral stores during the maturation process to develop their gonads. Over the course of their maturation, the eels’ mineral stores and bone mass declines to such an extent that this can potentially compromise the mechanical stability of their vertebrae. These processes have been observed as considerably stronger in females than in males.

The process is similar to the human disease osteoporosis, which also affects more women than men. However, unlike in humans, the decrease of phosphorus and calcium in the bones of eels leads to an increase in the concentrations of these minerals in soft tissues. The research team’s findings provide evidence that eels have evolved and adapted to use their bones and muscle mass as energy and mineral stores to migrate and reproduce.

The findings also indicated the transfer of an array of potentially toxic metals – cadmium, copper, manganese, and mercury – from soft tissues, the liver, and bones to the ovaries of gravid silver eels. "Due to the dramatic decline in eel populations over the past decades, the species is now considered critically endangered. In this context, our findings are particularly relevant," says Marko Freese from Thünen Institute. "It is known that metals can produce toxic free radicals," says Dr Larissa Rizzo from RWTH. Dr. Markus Brinkmann from the University of Saskatchewan adds: "Our recent findings provide evidence that metals might hamper the eels’ reproductive success, and that contamination of their continental habitats might be a contributing factor to the noted population decline." The construction of dams, fishing practices, and parasites further hamper the European eels’ chances of survival.

This is the first time that the molecular processes occurring during the drastic changes of the eels‘ bodies during migration have been characterized at this level of detail. The research team has published the results of their study in the renowned journal Proceedings of the National Academy of Sciences of the United States of America, or PNAS for short.

"This publication is a good example of a successful cooperation with international research institutes, but also institutes within RWTH," adds Professor Henner Hollert, Head of the Department of Ecosystem Analysis at the University. "The easy collaboration in natural sciences in the biology department, but also the Faculty of Mathematics, Computer Science, and Natural Sciences, and the Faculty of Medicine is certainly fitting with the University’s Excellence Strategy."


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