Tropical seamounts as stepping-stones for coral reef fishes: range extensions and new regional distributions from mesophotic ecosystems in the Coral Sea, Australia

Seamounts and remote oceanic islands serve as valuable natural laboratories in which to study patterns and processes in marine biodiversity. A central hypothesis arising from studies of these systems is the ecological function of seamounts as stepping-stones for dispersal and population connectivity. Evidence of this mechanism exists for a range of taxa, including coral reef fishes, but is still lacking from many tropical seamounts in remote regions. In this study, we used remotely operated vehicles and baited remote underwater video systems to survey fish and benthic communities between 1 and 100 m on seamounts in the Coral Sea Marine Park (CSMP), Australia. We found evidence to support the stepping-stone model of ecological connectivity from new observations of 16 coral reef fishes which have previously not been recorded by quantitative surveys in the region. The widespread distribution of many of these species throughout the full latitudinal extent of the CSMP suggests that there is greater connectivity between mesophotic habitats in the Coral Sea and surrounding biogeographic regions than previously known. We also found a wide variety of mesophotic habitats and recorded significant depth range extensions for 78 fishes in these habitats. This further highlights the potential role of increased habitat area and heterogeneity in a stepping-stone effect throughout the region. Four of the fish occurrence records represent significant range extensions into the Coral Sea from adjacent biogeographic regions, and 13 fishes recorded by this study in the CSMP are not known from the neighbouring Great Barrier Reef, despite its close proximity. Although the Coral Sea remains relatively understudied, these findings suggest that larger-scale models of marine biogeography are relevant to communities in the region, particularly at mesophotic depths. Given the extent and the spatial arrangement of seamounts in the Coral Sea, our findings emphasise that the region is an important link between the centre of marine biodiversity in the Coral Triangle and the Southwest Pacific. Greater mesophotic sampling effort and genetic studies are necessary to understand the nature of connectivity and to establish the role of regional seamount chains, like the Coral Sea reefs, in broader marine biogeographic processes.