Redescription and neotypification of a poorly known freshwater ectocommensal ciliate, Larvulina variabilis ( Penard, 1922), and consideration of its systematic position

The morphology and infraciliature of Larvulina variabilis collected from freshwater Gammarus near Boise, Idaho (USA) were investigated using live observation and silver impregnation. Larvulina variabilis was collected from three sites in Idaho and one site in Michigan, indicating a wide distribution. Larvulina variabilis is a small (21–25×14–19 μm in vivo) hymenostome ectocommensal on freshwater amphipods. Distinguishing morphologic features include an inflexible cortex with a prominent notch in the left posterior margin, a single globular macronucleus, a single posterior contractile vacuole, a somatic ciliature with 10 kineties all consisting of ciliated dikinetids, a “6”-shaped paroral membrane of the stichodyad type, long (∼20 μm), immobile cilia arising from three adoral membranelles, a discrete dorsal thigmotactic ciliary field consisting of five kineties, distinctive extrusomes and a coarsely polygonal silverline pattern. The forward-projecting anterior cilium emphasized in the original description is an inconstant finding not essential for identification. An improved diagnosis is provided for the genus and the species. Because no type specimen exists and prior descriptions were conflicting, a specimen from the Idaho population is designated as the neotype. The systematic position of this unusual ciliate remains obscure. Possibly, it belongs to the Pleuronematida or the Thigmotrichida.