Morphology, morphogenesis, and molecular phylogeny of a new brackish water ciliate, Pseudourostyla cristatoides n. sp., from Songjiho lagoon on the coast of East Sea, South Korea

A new brackish water urostylid ciliate, Pseudourostyla cristatoides n. sp. was collected from Songjiho lagoon on the coast of East Sea, South Korea, and investigated based on morphology, morphogenesis, and 18S rRNA gene sequences. Pseudourostyla cristatoides is characterized by the following features: slender to elliptical body shape; colourless to dark grey in colour; size in vivo about 220-265 x 85-125 mu m; flexible and slightly contractile body with narrowly spaced extrusomes (trichocyst type) throughout the whole cell; 2 contractile vacuoles on left side of cell at about 25% and 75% of body length; 84-115 adoral membranelles, 20-30 frontal cirri, 1 buccal cirrus, 2 frontoterminal cirri, 17-25 midventral pairs, 2 pretransverse, 6-12 transverse cirri, 5-7 left and 4-5 right marginal rows, and 10-13 dorsal kineties; 30-106 macronuclear nodules and 3-5 micronuclei; brackish habitat (salinity 3-5%). This new species is very similar to P. cristata, but distinguished primarily by contractile vacuoles (2 vs. 1 in number; positioned 25%, 75% of body length vs. ahead of mid-body), dorsal kineties (10-13 vs. 8), hyaline layer underneath pellicle (inconspicuous vs. conspicuous), habitat (brackish vs. freshwater), and participation of posterior cirri of rear corona in fronto-ventral-transverse cirral anlagen (yes vs. no). Additionally, intra-/inter-specific pairwise genetic distances of the 18S rRNA gene sequences supported that the Songjiho population is a novel species (intra-specific distances of 0.07-0.12% among three P. cristata populations vs. inter-specific distances of 0.93-1.00% between P. cristatoides and three P. cristata populations).