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Genetic isolation despite geographic proximity highlights threat to island fish species

Amphidromous fish—which migrate between freshwater streams and the sea—can move between habitats thanks to ocean currents. Since island streams are generally small and vulnerable to human impact, understanding how exactly fish populations are connected between streams and between islands is important for the conservation of geographically isolated species.

In a study now in Scientific Reports, a team of researchers from the Okinawa Institute of Science and Technology (OIST) and collaborators sought to understand these dynamics by investigating the genetic connectivity among populations of the amphidromous goby Luciogobius ryukyuensis at four islands in the Ryukyu Archipelago in Japan: Okinawa, Kume, Ishigaki, and Iriomote.

The researchers found that the populations are all genetically distinguishable, suggesting that larvae very rarely travel between the islands, despite relative physical proximity: Iriomote and Ishigaki share the same lagoon, and the populations are just 23km apart.

Mitochondrial genome analysis suggested that the two clades—Okinawa and Kume, and Ishigaki and Iriomote—diverged approximately 900,000 years ago, and that there may have been a larval exchange between the two neighboring islands until more recently.

Their findings highlight the challenges faced by amphidromous fish species in island ecosystems. In particular, the study shows the urgent need for increased conservation efforts targeting the highly vulnerable habitats of the goby, which are especially exposed on the smaller Kume, Ishigaki and Iriomote islands. For example, adult gobies have only been found in a 10m² portion of the Urachi River on Kume Island. This habitat is surrounded by urban structures, and the upper watershed is developed for agriculture.

Protecting and maintaining all existing habitats is critical for the conservation of the whole genetic diversity of this species.