Contributions to the morphology, infraciliature and ecology of the planktonic ciliates Strombidium pelagicum n.sp., Pelagostrombidium mirabile (Penard, 1916) n.g., n. comb., and Pelagostrombidium fallax (Zacharias, 1896) n.g., n. comb. (Ciliophora, Oligotrichida).

Freshwater samples from excavated groundwater ponds (Styria, Austria) were examined for planktonic ciliates. Among these, the following 3 common species were investigated by live observations and protargol impregnations: Strombidium pelagicum n.sp., Pelagostrombidium mirabile (Penard, 1916) n.g., n. comb., and Pelagostrombidium fallax (Zacharias, 1896) n.g., n. comb. The new genus Pelagostrombidium is characterized by a spheroidal neoformation organelle (embryonic body) which has a winding exit canal, opening out close to the cytopharynx. The somatic ciliature is reduced to a supraequatorially located circumferential paratene; closely posterior to the latter is a girdle of argyrophilic granules. Pelagostrombidium mirabile differs mainly from P. fallax in shape, cell colour and numbers of buccal adorai membranelles. Strombidium pelagicum has a tubelike neoformation organelle which opens into the circumferential paratene. The somatic ciliature consists of an equatorially located circumferential paratene, a ciliated girdle kinety and a longitudinally oriented, ciliated postequatorial kinety. Abundance and biomass were recorded over 2 annual cycles (1986/87). Keys to the genera of the family Strombidiidae and to the freshwater strombidiids are proposed. The structure and function of the circumferential paratene and neoformation organelles are discussed.