Diets of Antarctic sponges: Links between the pelagic microbial loop and benthic metazoan food web

Abstract

Antarctic benthic suspension feeders may consume water column bacteria to buffer the seasonal variation of primary production, yet little is known about consumption of ultraplankton by this fauna. In the present study 3 experiments - fatty acid analysis, stable isotope concentrations, and laboratory-based feeding - addressed the nutritional role of the microbial loop in 4 species of Antarctic sponge: Homaxinella balfourensis, Isodictya setifera, Kirkpatrickia variolosa, and Sphaerotylus antarcticus. Sponges were sampled at distances between 115 and 840 m from the McMurdo Station sewage outfall to investigate local food source variability. The sewage effluent acted as a tracer for particulates larger than bacteria and was identified isotopically and by the biomarker 18:2(n-6). Sponge diets differed between each species: I. setifera consumed mostly bacteria, as indicated by the ratio of bacterial fatty acids to polyunsaturated fatty acids; H. balfourensis consumed larger particles, indicated by an abundance of 22:6(n-3) and an outfall signature; K. variolosa was intermediate between these two, with abundant 22:6(n-3), but an isotopic signature similar to I. setifera. The diet of S. antarcticus was not completely resolved; fatty acid analysis supported its similarity to K. variolosa, yet the isotopic analysis separated it from the other sponges, suggesting that symbionts were abundant enough to confound the results. This study provides the first application of fatty acid analysis to determine diet composition of sponges, the first species-specific stable isotopic analysis of Antarctic sponges, and the first conclusive evidence of differential utilization of microbial loop components by co-occurring sponges. © Inter-Research 2007.