The natural diet of a hexactinellid sponge: Benthic–pelagic coupling in a deep-sea microbial food web

Dense communities of shallow-water suspension feeders are known to sidestep the microbial loop by grazing on ultraplankton at its base. We quantified the diet, rates of water processing, and abundance of the deep-sea hexactinellid sponge Sericolophus hawaiicus, and found that, like their demosponge relatives in shallow water, hexactinellids are a significant sink for ultraplankton. S. hawaiicus forms a dense bed of sponges on the Big Island of Hawaii between 360 and 460 m depth, with a mean density of 4.7 sponges m−2. Grazing of S. hawaiicus on ultraplankton was quantified from in situ samples using flow cytometry, and was found to be unselective. Rates of water processing were determined with dye visualization and ranged from 1.62 to 3.57 cm s−1, resulting in a processing rate of 7.9±2.4 ml sponge−1 s−1. The large amount of water processed by these benthic suspension feeders results in the transfer of approximately 55 mg carbon and 7.3 mg N d−1 m−2 from the water column to the benthos. The magnitude of this flux places S. hawaiicus squarely within the functional group of organisms that link the pelagic microbial food web to the benthos.