New observations on the tontoniid ciliate Spirotontonia grandis (Suzuki and Han, 2000) Agatha, 2004 (Ciliophora, Oligotrichida, Tontoniidae); comparison with the similar Laboea strobila

Spirotontonia grandis (Suzuki and Han, 2000) Agatha, 2004 belongs to the family Tontoniidae Agatha, 2004 because it has a contractile tail. It is the type of the genus Spirotontonia Agatha, 2004, which is characterized by a sinistrally spiralled girdle kinety. Supplementary observations on protargol-impregnated and, especially, live specimens of S. grandis from the Irish Sea are provided. These new findings are included in an improved species diagnosis. Furthermore, they contribute to a better separation of the tontoniid S. grandis from the strombidiid Laboea strobila, which also has a screw-like appearance, a sinistrally spiralled girdle kinety, and multiple macronuclear nodules, but lacks the tontoniid tail. Since the tail of S. grandis, the main distinguishing feature between the two species, is highly contractile and easily lost, further differences are emphasized, e.g., the lower number of girdle kinety whorls (3.0–3.5 vs. 4.5–5.0), the larger cell size (110–170×55–80 μm vs. 80–110×40–60 μm after protargol impregnation), the relatively larger size of the ventral portion of the membranellar zone (51% vs. 40% of body length), and the overlapping cortical platelets (present vs. absent). The record of S. grandis from the Irish Sea is the first record outside the East China Sea and the second worldwide.