The ultrastructure of Oscillatoria spongeliae, the blue-green algal endosymbiont of the sponge Dysidea herbacea

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Abstract
A number of collections of the tropical sponge Dysidea herbacea (Dictyoceratida, Dysideidae) have been made on the Great Barrier Reef, Australia and the ultrastructure of the filamentous blue-green algal endosymbiont Oscillatoria spongeliae studied in detail. Oscillatoria spongeliae occurs in the mesohyl ofthe ectosome in close association with sponge archaeocytes. Digestion of the algae occurs only rarely, but algae in various stages of digestion have been observed within archaeocytes. The sheathless filaments of O. spongeliae are from four to twenty cells long. Some of the cells of the filament are specialized as necridia, and breakage of filaments along necridial cell walls has been observed. The thylakoids are usually arranged more or less at right angles to the longitudinal cell wall and are evenly spaced around the cell. The algal cells have a typical blue-green algal cell wall and contain various inclusion bodies characteristic of these algae, suggesting that their metabolism must be simil...