Faunal Composition of the Sumisu Caldera Hydrothermal Vent Field as a Key Baseline for Conservation in Light of Deep-Sea Mining

Abstract

Hydrothermal vents are biodiversity hotspots on the deep seafloor powered by chemosynthetic primary production, inhabited by a specially adapted fauna whose composition varies between regions. Sumisu Caldera, located approximately 500 km south of Tokyo, hosts a hot vent with an unusual species composition among the Izu–Ogasawara Arc sites and has been suggested as a priority site for conservation in light of upcoming deep-sea mining for massive sulfides. However, this was based on just five species serendipitously recorded from geological surveys. Here, we carried out the first comprehensive biological sampling of Sumisu using two dives of the human-occupied vehicle Shinkai 6500. Together with literature records, we count a total of 54 animal species of which at least 25 are endemic to chemosynthetic ecosystems, drastically increasing the known faunal diversity—including three sponges, one cnidarian, 12 molluscs, 23 annelids, 13 arthropods, one echinoderm, and one fish. The presence of sediments facilitating the occurrence of burrowing species and three non-endemic predators contribute to the high species richness. Our findings make Sumisu the most species-rich vent community in the entire northwestern Pacific and our work serves as a prime example for detailed faunal surveys to obtain key piece of baseline data for future environmental assessments. Although the apparent lack of massive sulfide deposits means Sumisu is unlikely to be an immediate target of mining, its close distance to Bayonnaise Knoll with a major deposit and the possibility that it acts as a critical stepping-stone population for many species, as well as the presence of several endangered ones, presents a case for its protection.