The ecology of colonial radiolarians : their colony morphology, trophic interactions and associations, behavior, distribution, and the photosynthesis of their symbionts

Abstract

Colonial radiolarians (Spumellaria) are among the most common and abundant large zooplankton, but they have been little studied by modern biologists. Colonies were found on 98% of epipelagic diving stations in the period from 1977 to 1979. Measured abundances ranged from .04 to 540 colonies per m3. Colony morphology of common genera and species is described and three new shell-less species which reach a length in excess of 1 m are discussed in detail. Some simple behavioral responses are documented, including control of colony buoyancy and position of algae in the colonies. Radiolarians feed on a wide variety of planktonic organisms including tintinnids, copepods, appendicularians, mollusc larvae and hydromedusae. They are hosts to parasitic hyperiid amphipods, particularly those of the genus Hyperietta. Radiolarians are prey of the amphipod Oxycephaius ciausi, an unidentified turbellarian and possibly the Harpacticoid copepods Miracia efferata and Sapphirina sp. Colonial radiolarians are also hosts to symbiotic dinoflagellates. Experiments were done at sea on the net incorporation of CO2 by these algae using 14C labelled NaHC03. Data from these experiments were related to content of carbon and chlorophyll as a function of colony size (cell number). Carbon content of colonies related well with colony size. Mean values were 50, 85, 100 and 200 ng C per radiolarian cell for coiiozoum inerme, C. iongiforme, Acrosphaera spinosa and coiiozoum radiosum respectively. Chlorophyll content varied widely between colonies and chlorophyll per radiolarian cell decreased with increasing colony size in Acrosphaera spinosa. Net carbon incorporation increased with colony size at given light intensities as did phutosynthetic assimilation (mmoles CO2.mg Chl a -l.hr -1) in A. spinosa. In experiments on the effect of light intensity on photosynthesis, there was no evidence for photoinhibition at high intensities in Acrosphaera spinosa. Replicate pieces of the large colonies of C. longiforme were incubated together, each colony at a different light intensity. Representative pieces were measured and used for chlorophyll carbon and nitrogen analysis and counted for abundance of radiolarian and algal cells and tintinnid prey. Incorporation per unit length varied little within colonies Photosynthetic assimilation followed no predictable pattern as a function of light intensity. However, it related directly to abundance of tintinnid prey remains. This effect apparently overrides that of light intensity. Total photosynthesis incorporation was only 0.1 to 0.8% of the total colony carbon per hour. The contribution of colonial radiolarians to total productivity of the regions studied was insignificant. However, the radiolarians' productivity is available to a unique portion of the planktonic food web. Because of their size and abundance radiolarians are important as substrates in their environment.