Experimental and observational studies of radiolarian physiological ecology: 2. Trophic activity and symbiont primary productivity of Spongaster tetras tetras with comparative data on predatory activity of some Nassellarida

The prey of Spongaster tetras tetras, determined by transmission electron microscopic examination of ultrathin sections through food and digestive vacuoles, was predominantly picoplankton (1 to 3 μm, largely monerans) and only occasional vacuoles contained masses of larger digested organic matter. By contrast, Nassellarian specimens collected in the same location and fixed and examined identically contained nanoplankton prey (5-10 μm) including small phytoplankton cells. The differences in prey may reduce competition for nutritional resources and account for the co-existence of these two groups of radiolaria in the same water mass. The primary productivity of the symbionts in intact S. tetras tetras was assessed at varying light levels to more fully document potential sources of carbon compounds for nutrition. Thirty-five μg of carbon were fixed per radiolarian per hour at an intensity as low as 20 μE/m²/s which is approximately one-third the maximum productivity in the intensity range of 170 to 260 μE/m²/s. The mean abundance of S. tetras tetras, density of potential prey, and physico-chemical characteristics of the water are presented for early spring (March-April), mid-summer (June and July), and late summer (August). The mean density of S. tetras tetras varied from c. 12 individuals/m³ during the low productivity periods (spring and late summer) to 37.8 individuals/m³ in July. The concentration of cyanobacteria was at a peak during July while larger autotrophic plankton was at a minimun further suggesting that cyanobacteria can be a significant food source for S. tetras tetras.