The oceanographic isolation of the Ogasawara Islands and genetic divergence in a reef-building coral

Aim: Due to their spatial isolation, oceanic islands are natural systems to study evolutionary divergence. The Ogasawara Islands belong to the most isolated archipelagos on Earth and are well-known for their high terrestrial endemicity, however, less is known about the marine realm. Here, we analyze the degree of oceanographic isolation of the archipelago based on genetic data of a reef-building coral and a biophysical dispersal model.

Location: North-Western Pacific (Ogasawara, Ryukyu, Daito Islands, Guam)

Taxon: Galaxea fascicularis L.

Method: Three to 15 specimens were sampled at several sites in Ogasawara and its closest potential migration sources in southern Japan and the Mariana Islands (Guam) and RAD-sequenced. 108 specimens from the common Pacific lineages 'L' (Ryukyu- and Daito Islands, Guam) and 'Ogasawara' (Ogasawara) were analyzed with population genetics and demographic modeling. Oceanographic dispersal was investigated by inverse particle tracking using a Lagrangian particle advection simulation based on ROMS and applying biological dispersal parameters of G. fascicularis.

Results: The G. fascicularis population in Ogasawara is genetically highly differentiated from the next closest reefs in the region and has diverged from the Ryukyu Islands under very little, asymmetric, eastward migration. Inverse particle tracking confirmed the oceanographic isolation of Ogasawara and showed that the islands are rarely but most likely reached by settlers from the Ryukyu Islands by long-distance-dispersal of exceptionally long-lived larvae (>44 days), with no dispersal vice versa.  

Main conclusions: Ogasawara is a dispersal sink location and the high degree of genetic differentiation in Galaxea has resulted from strong oceanographic isolation. This research highlights how oceanographic features impact species-level genetic differentiation even in well-dispersed taxa such as broadcast-spawning corals, and they are likely even more pronounced in less vagile organisms. These findings suggest the Ogasawaran Archipelago should be considered an important priority for marine conservation, alongside its high importance for terrestrial conservation.