Seasonal changes in bacteria and phytoplankton biomass control the condition index of the demosponge Halichondria panicea in temperate Danish waters

Sponges are sessile filter-feeders, and the basic principles for water pumping and particle retention are the same among all demosponges. Phytoplankton cells smaller than the inhalant openings (ostia) are retained and phagocytosed in the inhalant canal system before the water is sieved through the collar filter of the choanocytes, which efficiently retain bacteria. However, the importance of bacteria versus phytoplankton as the main diet is unclear. Sponges must cope with seasonal changes in food availability in temperate waters, and during winter, low phytoplankton and bacterial biomasses may result in starvation. In this study, the lower threshold of suspended biomass resulting in starvation of the demosponge Halichondria panicea was determined. We measured the seasonal changes in phytoplankton and bacterial biomasses along with the sponge condition index (CI, the ratio of organic to inorganic matter). A low CI during winter reflected starvation, and based on total available carbon concentrations (phytoplankton plus bacterial carbon), we conclude that a concentration of ca. 30 mu g C l(-1) was not sufficient to cover the maintenance costs of H. panicea. Bacteria constitute a minor, but consistent part of the diet, around 20%. A field growth experiment revealed a positive correlation between CI and sponge weight. Measured volume-specific clearance rate of sponges, density of choanocyte chambers and inter-choanocyte chamber-distance were not significantly different in low and high CI sponges, and it remains unknown how sponges withstand long periods with low suspended food particle concentrations.