Lanthanum anomalies provide constraints on macrofaunal predation at methane seeps

The feeding habits and predation behaviour of organisms can exert significant control on the dynamics of local food webs. Yet, little is known about the effects of predation on the material and trophic transfer dynamics in chemosynthesis-based ecosystems. Here, we investigated the rare earth element (REE) composition of soft tissues and hardparts for thiotrophy-dependent vesicomyid clams (Archivesica marissinica), aerobic methanotrophy-dependent bathymodiolin mussels (Gigantidas haimaensis), in addition to turrid gastropods (Phymorhynchus buccinoides) and parasitic scale worms (Branchipolynoe pettiboneae) from the Haima seeps of the South China Sea; the latter two species are predators feeding on mussels. Our goal was to determine if the specific, microbially-derived, light REE enrichment characteristics of seep mussels can be transferred to the biomass of their predators. The vesicomyid clams were found to exhibit light REE patterns similar to that of seawater. In contrast, the bathymodiolin mussels, turrid gastropods, and scale worms revealed pronounced lanthanum (La) enrichment, agreeing with substantial transfer of La within the local food web. The observed enrichment of La in seep dwelling predators represents an independent method for monitoring the dynamics within seep ecosystems and potentially for assessing faunal interactions in ancient chemosynthesis-based ecosystems.