Frontonia fusca (QUENNERSTEDT, 1869) KAHL, 1931 was rediscovered from brackish water bodies with a salinity range of 4–25 ‰ on the Tyrrhenian and Ligurian coastlines (Mediterranean Sea) at Naples and in Tuscany, Italy. From the morphological point of view it is a typical member of this genus; it is about 100–170 μm long in vivo and has two contractile vacuoles with 6–9 collecting canals and 2–3 excretory pores, and two small micronuclei of the “endosomal” type. A very distinctive pigment spot on the right side of the anterior dorsal cortex is always presented. The dark-greenish pigment of the organelle has no autofluorescence and is located in small vacuoles 0.5–1.2 μm across and equally distributed during cell division between dividers within 45–60 min, again producing a spot in the same location of daughter cells. The ciliate is positive phototactic. Frontonia fusca has a strong food preference to diatoms and can easily survive in pure marine water, but not in non-saline one. Till now the species was found in Europe only. The general morphology and morphometry is redescribed according to observations of living and silver-impregnated cells. The population from the Tyrrhenian Sea is fixed as neotype. According to 18S rRNA molecular data, Frontonia fusca clusters with some other brackish water frontoniids, which clearly separate from the other representatives of the genus.
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