Blue mats: faunal composition and food web structure in colonial ciliate (Folliculinopsissp.) mats at Northeast Pacific hydrothermal vents

The present study provides a first description of the faunal composition and food web structure associated with blue mat ciliates (Folliculinopsissp.) at hydrothermal vents on the Juan de Fuca and Explorer Ridges in the NE Pacific. Invertebrates associated with blue mats were identified, quantified and analyzed for stable carbon and nitrogen isotopes. In addition, fatty acid analyses of blue mats were performed. We found a recurrent assemblage of invertebrates associated with the blue mats, and, in all samples, meiofauna were numerically dominant. The harpacticoid copepodAmphiascussp. was far more abundant than any other species within the folliculinid mats. While some of the invertebrates (includingAmphiascussp.) within this assemblage seem exclusively linked to blue mats, others are known from other nearby hydrothermal vent habitats.Folliculinopsissp. ciliates were far more depleted in δ13C than invertebrates within the blue mat assemblage, indicating that the latter do not feed exclusively on the former. At least 2 trophic levels exist within this assemblage, with juvenile macrofauna, ostracod and nematode species occupying higher trophic levels. The lipid profiles indicate that 16:1ω7 and 18:1ω7 (typical of sulfur oxidizers) make up over half the blue mat fatty acids. Similar to tubeworm bushes at hydrothermal vents and mussel beds at cold seeps, blue mats may create a habitat within which meiofaunal and juvenile macrobenthic species can find shelter and food, thus playing an indirect but important role in the ecology of these organisms.