Taxonomy and phylogeny of two poorly studied genera of marine oligotrich ciliates including descriptions of two new species: Cyrtostrombidium paraboreale sp. n. and Apostrombidium orientale sp. n. (Ciliophora: Spirotrichea)

Although it is widely recognized that oligotrich ciliates are the dominant constituent of microzooplankton communities and perform key functions in energy flow and material cycling in marine microbial food webs, knowledge of their diversity is scant. In the present study, we investigate the oligotrich genera, Cyrtostrombidium and Apostrombidium, with emphasis on their morphology and evolutionary relationships. Three isolates were collected from coastal waters of northern and southern China including two new species, viz., Cyrtostrombidium paraboreale sp. n., Apostrombidium orientate sp. n., and Apostrombidium pseudokielum Xu et al., 2009. Cyrtostrombidium paraboreale sp. n. is characterized by possessing 64-98 cytopharyngeal rods and two macronuclear nodules. Apostrombidium orientate sp. n. is characterized by its somatic kinety consisting of five fragments including a horizontally orientated subterminal fragment and possessing conspicuously long dorsal cilia. Apostrombidium pseudokielum is redescribed based on the new population and a re-examination of the type material. Phylogenetic analyses were performed for the subclass Oligotrichia, incorporating SSU rRNA gene sequences of the three species investigated here. The results indicate that the genus Cyrtostrombidium is monophyletic with C. paraboreale sp. n. occupying the basal position. The genus Apostrombidium is not monophyletic as Varistrombidium kielum is nested within it. (C) 2019 Published by Elsevier GmbH.