The oligotrichid ciliate Strombidium coronatum (Leegaard, 1915) Kahl, 1932 is redescribed from plankton samples taken in the Irish Sea, using live observation, protargol impregnation, and scanning electron microscopy. The species is characterized by a conspicuous, uniquelly shaped peristome, which is flat and roughly triangular and extends in the sagittal plane. The Irish Sea specimens measure ~ 45 × 25 µm in vivo and ~ 40 × 24 µm after protargol impregnation. The girdle kinety is equatorial, ostensibly continuous, and composed of ~ 100 dikinetids. The ventral kinety extends longitudinally on the posterior fifth of the cell and is composed of about five dikinetids. The adoral zone of membranelles is widely open and composed of ~ 18 collar and ~ 12 buccal membranelles; the collar portion is disconnected from the buccal portion. The shape and orientation of the opisthe’s adoral zone of membranelles are apparently extraordinary, i.e., the membranelles form an inverted L-shaped stripe extending longitudinally in the elongated posterior cell portion of dividers.

Two new species of centrohelid heliozoans Acanthocystis costata and Choanocystis symna from a freshwater lake on Valamo Island and a freshwater pool in St. Petersburg (North-Western Russia) were studied with light and electron microscopy. Sequences of 18S rDNA were obtained for both species.Choanocystis symna has dumbbell-shaped plate scales (4.4–5.0 × 1.62–1.90 µm) and spine scales (3.9–6.7 µm) bearing from 3 to 5 (usually four) short teeth on the distal end. Acanthocystis costata has oval plate scales (1.3–3.7 × 0.9–1.9 µm), bearing numerous granules as well as radial slits and spine scales (2.1–9.5 µm) with 4–6 teeth on the distal end. Acanthocystis costata and Acanthocystis nichollsi are similar in having slit-bearing plate scales and group together on the 18S rDNA tree. The presence of large particles of unknown nature was observed in food vacuoles of Acanthocystis costata.

Epibiosis is a facultative association between two organisms: the epibiont, which colonizes the surface of living substrates, and the basibiont, which hosts the epibionts. Among protists, while numerous accounts exist for ciliates as epibionts of zooplankton, little information is available about the occurrence of this relationship between flagellates and microcrustaceans in the neotropics. During a survey on the epibiotic relationship between ciliated protists and planktonic copepods in a tropical floodplain, we reported for the first time the occurrence of Colacium vesiculosum as an epibiont onThermocyclops minutus and Notodiaptomus amazonicus from southern Brazil. The mean infestation prevalence was significantly higher on adults 30.53% (± 2.85) in comparison with copepodites 0.87% (± 0.55). When hosts were separated by order (Calanoida and Cyclopoida) copepodites presented a mean infestation prevalence of 0.92% (± 0.85) and 0.83% (± 0.60) respectively, while adults had a mean prevalence of 29.55% (± 6.8) and 30.13% (± 5.83), which could be explained by the fact that adult copepods provide a more stable substrate for epibionts.