Maturing eels break down their skeleton to fuel reproduction

Press Release

To reach their spawning grounds in the Sargasso Sea, European eels migrate several thousand kilometers across the Atlantic Ocean. The fish undergo drastic changes on their way: they stop feeding, shrink their guts, and break down their skeletons to fuel reproduction. An international research team around Marko Freese, Larissa Rizzo, and Markus Brinkmann now discovered that this process also leads to the transfer of a number of potentially toxic metals – cadmium, copper, manganese, and mercury – from bones and soft tissues to the ovaries of gravid silver eels, suggesting that these metals might hamper the reproductive success of this critically endangered species.

Side view on computed tomography scans of female eels in different maturation stages, displaying successive loss of bone structure along progressing maturation (© Freese et al.)

European eels (Anguilla anguilla) are a fish species with a unique reproductive strategy. While eels spend many years in European rivers, lakes and coastal areas to grow and mature, they migrate several thousand kilometers across the Atlantic Ocean to reach their spawning grounds in the Sargasso Sea. Once reached their destination, the eels spawn and die. Researchers from the Thünen Institute of Fisheries Ecology (Bremerhaven, Germany), RWTH Aachen University (Aachen, Germany), Ghent University (Ghent, Belgium) and the University of Saskatchewan (Saskatoon, Canada) now studied the drastic changes that the animals undergo during their maturation. They found that alongside minerals that are needed to fuel reproduction, potentially toxic metals are redistributed from bones and soft tissues to the ovaries. There, they can be transferred into the eggs and might impair the reproductive success of the critically endangered eels.

Underwater picture of a swimming, yellow-stage European eel (© Marko Freese)

With onset of puberty, a maturation process termed silvering, the eels change in appearance from pale yellow to gleaming silver, they return to spawn in the ocean, undergoing further drastic changes along the way. The eels stop feeding, shrink their guts, break down their skeletons, and build up their gonads. An interdisciplinary research team around Marko Freese (Thünen Institute), Larissa Rizzo (RWTH Aachen University) and Markus Brinkmann (University of Saskatchewan) used hormone treatments to artificially mature eels in the laboratory and analyzed the breakdown of the eels' skeletons and depicted the redistribution of mobilized minerals during maturation. Using analytical and imaging techniques (computed tomography, MRI), the authors found that the eels' average bone mass and mineral content declined with the onset of silvering, potentially compromising mechanical support by the vertebrae.

A similar process is observed in humans with the disease osteoporosis. 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. According to the researcher team, these finding provide evidence for an adaptation of eels to use their bones and muscles as energy and mineral stores for locomotion and spawning.

Moreover, the authors detected the transfer of an array of metals – cadmium, copper, manganese, and mercury – from bones and soft tissues to the ovaries of gravid silver eels, suggesting that these metals, known to generate toxic free radicals, might hamper the eels' reproductive success. “Because of the dramatic declines 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 generate toxic free radicals,“ says Dr. Larissa Rizzo from RWTH Aachen University. Dr. Markus Brinkmann from the University of Saskatchewan summarizes: “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 observed population declines.”

The molecular processes that occur during the drastic changes of the eels‘ bodies during migration have not been characterized at this level of detail before. The researchers have published the results of their study in the renowned Proceedings of the National Academy of Sciences of the United States of America (PNAS).


Freese et al. (2019): Bone resorption and body reorganization during maturation induce maternal transfer of toxic metals in anguillid eels.