International study of larval dispersal and population connectivity at hydrothermal vents in the U.S. Marianas Trench Marine National Monument

Deep-sea, chemosynthesis-based ecosystems, such as hydrothermal vents, have received increasing attention in the past decade for protection of biodiversity and ecosystem function. For these spatially discrete habitats, dispersal of larvae in the plankton, settlement to the seafloor, and recruitment are processes that connect populations of benthic fauna. Knowledge of these processes is vital to understanding the resilience of vent ecosystems to disturbance or removal of sources of propagules. In particular, hydrothermal vents may be impacted by human activities including scientific research, eco-tourism, bioprospecting, and polymetallic sulfide mining. In 2009 the first Marine Protected Area (MPA) for vents in the U.S. EEZ was established as part of the Marianas Trench Marine National Monument (MTMNM). The vents in this region are located along the Mariana back-arc spreading center (BASC) and volcanic arc. In 2010 we conducted the first cruise dedicated to the study of vent-endemic fauna at the southern Mariana BASC. Cruise YK10-11 on R/V Yokosuka was one of several to this region as part of the Japanese multi-disciplinary program TAIGA (Trans-crustal Advection and In-situ biogeochemical processes of Global sub-sea floor Aquifer). Larval, juvenile, and adult specimens of benthic invertebrates were collected at vents on-axis (Snail, Yamanaka) and off-axis (Archaean, Urashima, Pika) at ~3000-m depth. Our objectives included characterization of benthic communities, assessments of larval abundance and diversity, estimates of local-scale larval dispersal (on the order of 10 km), shipboard visualization of larval swimming behavior, and genetic diversity of populations at each vent site and genetic deviation between sites. Operations during the cruise included nine dives with the submersible Shinkai 6500 and seven deployments of sub- surface moorings. These vents on the southern Mariana BASC appear to have similar macrofaunal community composition to the better- studied B- SC vents ~600-km to the north. Benthic collections included gastropods (e.g., Alviniconcha hessleri, Phymorhynchus aff. starmeri, Shinkailepas aff. kaikatensis), barnacles (Neoverruca brachylepadoformis), shrimp (Chorocaris aff. vandoverae), and polychaetes (e.g., Paralvinella hessleri); larvae were also collected near-bottom with a slurp sampler. To further assess larval abundance and diversity, we deployed large-volume plankton pumps near three of the vents, filtering five samples of -40000 L each over a 63-μm mesh. Larvae were sorted morphologically, and selected morphotypes were identified via genetic bar-coding to phylotype and, when possible, to species. The local-scale larval dispersal study included a current meter recording near-bottom flow on-axis during the cruise. For the larval behavioral study, individual polychaete, barnacle, and gastropod larvae of several species were visualized in horizontally and vertically-oriented chambers to measure swimming speeds at 1 atm. Local-scale population connectivity is being determined via molecular genetic analyses of mainly the benthic specimens. The population genetic study may be extended to the regional scale when linked to other analyses for specimens collected at vents elsewhere in the MTMNM, including those relatively nearby along the Mariana arc and further north on the BASC. Results of our study of larval dispersal and population connectivity at these deep-sea vents will be considered in the context of ecosystem management and protection in the MTMNM. Our work will also be considered in the broader context of connectivity of deep-sea, chemosynthesis-based ecosystems, through our involvement with the International Network for Scientific Investigations of Deep-Sea Ecosystems (INDEEP) working group on population connectivity.