A revision of Strombidium paracalkinsi (Ciliophora: Oligotrichea: Oligorichida): Living Morphology, Infraciliature, and Small Subunit Ribosomal DNA

Abstract

Thigmotactic membranelles (TMs) in oligotrich ciliates were considered to be a convergently evolved character to adapt benthic life and thus could not be used as a character to separate different genera, which leads to the transferring of Thigmostrombidium and Heterostrombidium into Strombidium. TMs-bearing species have been recorded among different oligotrich taxa, e.g. Strombidium, Spirostrombidium, Omegastrombidium, and Parallelostrombidium. Strombidium paracalkinsi which has three TMs on its dorsal side were discovered from Korean coastal waters recently and redescribed based on its living morphology, infraciliature and small subunit ribosomal DNA sequence. The Korean population showed some unique characters, i.e. its three TMs were composed of two rows of kinetosomes, respectively and clearly separated from the anterior membranelles (AMs) in protatgol impregnated specimens which makes it different from the other TMs-bearing species. In a dividing cell, the TMs looks like the natural extension of AMs in early-middle stage and the separation of TMs from AMs might happen in late stage of morphogenesis. Furthermore, we sequenced the SSU rDNA of S. paracalkinsi for the first time and in our phylogenetic tree, it grouped out of Strombidium clade which might indicate the potential separation with its congeners from genus Strombidium. Based on the morphological, morphogenesis and DNA sequeof Thigmostrombidium and Heterostrombidium into Strombidium. TMs-bearing species have been recorded among different oligotrich taxa, e.g. Strombidium, Spirostrombidium, Omegastrombidium, and Parallelostrombidium. Strombidium paracalkinsi which has three TMs on its dorsal side were discovered from Korean coastal waters recently and redescribed based on its living morphology, infraciliature and small subunit ribosomal DNA sequence. The Korean population showed some unique characters, i.e. its three TMs were composed of two rows of kinetosomes,