A new trophic link between the pelagic and benthic systems on the Antarctic shelf

Abstract

During the expeditions EASIZ II and EASIZ III carried out off the Antarctic Peninsula with RV ‘Polarstern’, a prey, until now never registered, was observed in the gastrovascular cavities of octocorallian Anthomastus bathyproctus colonies. A. bathyproctus gastrovascular contents contained salps of the species Salpa thompsoni in 83 to 88% of the colonies. Salps represented almost 100% of the prey items found in the octocorallian polyps. Salp chains were observed drifting just above the sediment. These chains undertook vertical migrations down to a depth of 700 m, reaching the seafloor. The captured salps had stomachs full of microplanktic prey, mainly diatoms and other phytoplankton cells. Fatty acids, considered to be of diatom origin, were detected in the stomachs of salps and in their tunica. The presence of such signature lipids was also detected in the octocorallian coenenchyme, although their concentration was considerably lower. These results suggest that salps may play an important role as a direct grazer of the phytoplankton produced in the top layers of the water column which, in turn, would be directly transferred to A. bathyproctus. An important part of the fresh contents of the salps will be assimilated by the octocorallians. Primary production is captured by a benthic suspension feeder through the grazer, bypassing the faecal pellet rain. The combined filtering activity and vertical migration of salps produces an ‘elevator effect’, which reduces the loss of energy through this short food chain, thus making the exchange between top and bottom layers more efficient.