Occurrence of vent-derived organic falls and associated fauna at hydrothermal vents on the Indian ridge: Implications for an underestimated food source at hydrothermal vent fields

Organic falls, hydrothermal vent fields and seeps rely primarily on chemosynthetic organic production leading to a carbon enriched habitat set against the oligotrophic deep-sea background. These habitats are typically inhabited by novel faunal assemblages with similar functional characteristics and often show harsh environmental gradients over relatively small scales with severe effects on diversity and density compared to the benthic background fauna. While the importance of particulate organic matter (POM) has been pointed out, the presence of POM as local accumulated organic deposits within hydrothermal vent fields has not been studied. Here, we present the first observations of organic deposits within active vents in the Indian Ocean including observations of giant capitellid worms inhabiting these organic mats with a length of up to 30 cm. This potential new species occurred in shrimp carcasses, the exuvia of Rimicaris kairei , on two hydrothermal vent sites from the Indian Ocean. We used imagery to build a high-resolution 3D reconstruction model of one side of a chimney complex, including the 4.8 m 2 surface area of the exuvia deposit. The capitellids occurred in dense aggregations and may contribute significantly to the food web of active hydrothermal vent fields, representing a possible overlooked food source for benthic and demersal predators. In addition, we report on observations on further organic deposit types and compare their associated epifauna along the mid-ocean ridges of the Indian Ocean. The exuvia deposits where capitellids were present appeared to be compacted and compressed implicating a potential role in stabilizing seafloor habitat. This is the first record of the family Capitellidae in organic deposits at hydrothermal vent fields, where they may serve as ecosystem engineers. • First report of organic deposits and associated megafauna at hydrothermal vents. • Potential new species of polychaetes (Capitellidae). • Capitellids may act as engineering species and increase taxonomic richness. • High frequency of capitellids may influence food-web dynamics within active vents.