Ultrastructure and Ecology of Calycomonas ovalis Wulff, 1919, (Chrysophyceae) and Its Redescription as a Testate Rhizopod, Paulinella ovalis N. Comb. (Filosea: Euglyphina)¹

An exceptional bloom (10 9 cells/liter) of a unicellular alga, Aureococcus anophagefferens , in Narragansett Bay, RI, was accompanied by unusually high numbers (6 times 10 6 tests/liter) of vase‐shaped cells of Calycomonas ovalis Wulff, hitherto considered a flagellated chrysophyte. Phase contrast light microscopy showed the characteristic oval lorica or test of C. ovalis , ca. 3.0 times 4.5 μm. Transmission electron microscopy of thin sections of natural populations revealed, however, that C. ovalis is actually a rhizopod within a test of electron‐dense scales. This estuarine, planktonk testacean extends a pseudopodium through a narrow pseudostome and lacks chloroplasts or flagella. It is a bacterivore and feeds primarily on unicellular cyanobacteria. The morphology, ultrastructure, and cell division of C. ovalis are described. These characteristics place C. ovalis with the genus Paulinella (Sarcodina: Filosea: Euglyphina), which at present contains the single species, P. chromatophora Lauterbom. Calycomonas ovalis is thus assigned to Paulinella as P. ovalis n. comb.