Deep secrets

Abstract

A new study suggests the puzzling freshwater eels have a deep oceanic origin. Nigel Williams reports. A new study suggests the puzzling freshwater eels have a deep oceanic origin. Nigel Williams reports. The European eel, and its other relatives that spend much of their adult life in freshwater habitats, have not only long puzzled locals that wondered about its life cycle but also researchers who were intrigued about these fish. The fish, a target for fishermen for centuries, is becoming increasingly scarce, raising concerns amongst conservationists about its extraordinary life cycle and what best can be done to bolster its numbers. It is only relatively recently that it has been shown that this once-common species migrated across thousands of kilometres of open ocean to breed in the Sargasso Sea off South America, with the young returning to the freshwater haunts of their parents to grow and mature before making the great oceanic journey themselves to breed. Other species of freshwater eels have been shown also to take on similar epic journeys in their life cycles too. There are more than 800 species of eel with only 16 species and 3 subspecies which spend most of their life in freshwater habitats. The spawning areas of these species are located in the open ocean a long distance from these habitats, often thousands of kilometres away. The offspring eventually return to the freshwater habitats of their parents to grow and develop before returning to the ocean. The evolution of such an extraordinary lifestyle has long puzzled researchers hindered by the uncertain evolutionary relationship of these freshwater forms with the vast majority of wholly marine species. But in a new study, published in the Royal Society's Biology Letters (online), Jun Inoue at the Ocean Research Institute at the University of Tokyo and colleagues in Chiba, Hamburg, Osaka and London report a phylogenetic study of whole mitochondrial genomes from 58 species, including 31 newly determined sequences, representing all 19 families of the anguilliform order of eel families, plus two outgroups, to address the evolutionary origin of the freshwater eels in a phylogenetic context. They believe the results suggest the freshwater forms comprise an exclusive clade with diverse oceanic midwater species placed in six families. The team found that analyses of the mitochondrial sequences suggested a relatively clear phylogenetic relationship between the species of freshwater eels. They found that these eels had the closest genetic links with 6 families of eels, comprising 47 species, which are all oceanic midwater dwellers, occurring mainly at tropical and subtropical depths of 200–3,000 metres throughout their adult life with no evidence of any exceptions. “How can we explain why this apparent evolutionary shift of the freshwater eel life history from the oceanic midwater to freshwater occurred?” the authors write. The two environments are remarkably different and require fish to be adapted to remarkably different ecological and physiological constraints. The authors consider whether there was a gradient in productivity between freshwater and marine environments at the time of the evolution of these eels. Another possibility the authors consider is that there was a vacant niche in freshwater when multiple marine species were emerging. “Multiple lineages of marine eels in marine environments, including voracious predators such as moray eels,” may have led to a vacant niche in freshwater, they suggest.“How can we explain why this apparent evolutionary shift of the freshwater eel life history from the oceanic midwater to freshwater occurred?” “How can we explain why this apparent evolutionary shift of the freshwater eel life history from the oceanic midwater to freshwater occurred?” The authors speculate that some ancestral forms may have come to estuaries during their larval or juvenile phases and developed an adaptive behaviour of regularly inhabiting estuaries and occasionally entering freshwater in tropical regions because of higher food availability, better survival or to escape from predators. “Once natural selection resulted in the emergence of eels that regularly used freshwater for growth,” a new life history was established, they suggest. But what is most evolutionarily remarkable is the return to the deep ocean by these species to breed. “Reproductive behaviour is typically conservative and constrained by many ecological and physical factors,” the authors say. So the migration of freshwater eels back to their offshore habitats over an evolutionary timescale represents a remarkable relic of the reproductive behaviour of these enigmatic animals that share a common ancestry with pelagic eels of the deep ocean, they write. “This surprising discovery offers a new perspective on the evolutionary origin of the freshwater eels,” and may provide novel insights into the evolutionary process of their unique migrations, they believe.