Fusiforma themisticola n. gen., n. sp., a new genus and species of apostome ciliate infecting the hyperiid amphipod Themisto libellula in the Canadian Beaufort Sea (Arctic Ocean), and establishment of the Pseudocolliniidae (Ciliophora, Apostomatia).

A novel parasitic ciliate Fusiforma themisticola n. gen., n. sp. was discovered infecting 4.4% of the hyperiid amphipod Themisto libellula. Ciliates were isolated from a formaldehyde-fixed whole amphipod and the DNA was extracted for amplification of the small subunit (SSU) rRNA gene. Sequence and phylogenetic analyses showed unambiguously that this ciliate is an apostome and about 2% divergent from the krill-infesting apostome species assigned to the genus Pseudocollinia. Protargol silver impregnation showed a highly unusual infraciliature for an apostome. There are typically 8 (6-9) bipolar somatic kineties covering the banana-shaped body. The anterior end of the oral cavity begins about 1/3 of the body length from the anterior end and is composed of an inpocketing that is lined on its anterior and left wall with an oral field of densely packed ciliated kinetosomes. Stomatogenesis begins with some dedifferentiation of the parental oral field and elongation of its paroral and oral kineties. A new oral field develops midventrally and the paroral and oral kineties break to form the oral apparatus of the opisthe, which completes development by additional kinetosomal proliferation and migration of the paroral. This morphology is novel among apostomes and justifies the establishment of a new genus and species.