Award: OCE-1634172

The boundary between land and the deep sea is the continental slope, an area now recognized to host thousands of sites where methane emerges from the earth’s crust, termed methane seeps. These are highly productive ecosystems based on chemosynthesis, rather than photosynthesis, hosting increased biodiversity and biomass relative to the surrounding deep sea. Seeps may increase in number and expand with ocean warming. This research explored Costa Rica methane seep ecosystems (Fig. 1), described new species, and examined the form, extent, and nature of the linkages between methane seeps and the surrounding deep-sea ecosystem on the Costa Rica margin. Located approximately 50 km offshore at depths of 400-3600 m, the Costa Rica seeps support habitats including authigenic carbonates, vestimentiferan tubeworm bushes, mussel and clam beds, and microbial mats. A combination of sampling, colonization, and transplant experiments across natural seepage gradients were conducted to understand the influence of methane on species evolution and biodiversity of invertebrates and the rates at which communities respond to changes in methane seepage. Stable carbon and nitrogen isotopes were examined to understand the contributions of methane-derived, chemosynthetic food sources. Key findings: We observed interannual variability in community composition of the invertebrate macrofauna (retained on a 0.3 mm sieve) on in situ carbonate rocks, due to a natural decline in seepage activity from 2017 to 2018, with lower abundances of seep species. This was accompanied by changes in nitrogen isotopes that reflect changing (more recycled) food sources. Experiments with carbonate rocks showed that macrofaunal successional dynamics are affected by availability of microbial chemosynthetic productivity (i.e., food availability), availability of source recruits, and habitat complexity promoted by carbonates. Understanding these interactions between biotic and abiotic factors is important for predicting responses to disturbances. We identified the presence of a chemotone – a transition zone characterized by intermediate macrofaunal biomass, a distinct macrofaunal species composition (including habitat endemics and species from both active and background habitats), high densities of permanent meiofauna (retained on a 0.042 mm sieve) and enhanced variability in species composition among samples. As biodiversity loss accelerates globally, understanding environmental influence over biodiversity–ecosystem functioning (BEF) relationships becomes crucial for ecosystem management. The finding of maximal BEF relationships at intermediate biodiversity under active seepage but no relationship in transition or background (i.e., no visible seepage activity) habitats suggests that absolute biodiversity is not a good metric of ecosystem ‘value’ at methane seeps The Costa Rica seeps support notable animal diversity and offer insights into deep-sea biogeography and phylogeography. We identified more than 2600 specimens occurring at these seeps to the lowest taxonomic level possible, revealing the occurrence of 109 known (named and described) species and at least 42 undescribed species. Of the described species, 37 were newly described from these seeps (Fig. 2) and 27 are not known to occur anywhere else. Other species occurrences reveal new biogeographic and ecological connections, including range extensions for species known from California, Mexico, the Galápagos seamounts, and Chile; new depth records; and the first seep records for species associated with vents and/or organic falls. Further species descriptions and a faunal inventory for the seeps are underway. Furthermore, two groups of worms common in the deep sea (serpulid and sabellid polychaetes; Fig. 1B) were found for the first time to host bacterial symbionts capable of providing them with carbon derived from methane, conferring an unexpected level of trophic flexibility. In synthesizing these examples, we note that no single evolutionary narrative explains the patterns of biodiversity at these seeps. For instance, Costa Rica seep species can have sister taxa in the eastern Pacific, western Pacific, or western Atlantic, reflecting complex connectivity and a history of gene flow prior to the closing of the Isthmus of Panama. Even morphologically indistinguishable species can have distinct biogeographic and depth restrictions, as illustrated by Laminatubus spp. worms and Bathymodiolus spp. mussels. This project has illustrated the value of careful molecular taxonomy for assessing cryptic species, endemism, connectivity, habitat requirements, and other ecological baselines important for conservation and research. Overall, the Costa Rica margin has emerged as a biodiverse and significant site for understanding of deep-sea chemosynthetic ecosystems. Last Modified: 12/28/2021 Submitted by: Lisa A Levin