Thiriotia euchirellae n. sp., a new gregarine species (Apicomplexa: Eugregarinorida) from the mesopelagic copepod Euchirella rostrata in Sagami Bay, Japan

Abstract

A new gregarine species, Thiriotia euchirellae n. sp. (Apicomplexa: Eugregarinorida), is described from the gut lumen of a mesopelagic copepod, Euchirella rostrata (Calanoida: Aetideidae), collected in Sagami Bay, Japan. Thiriotia euchirellae n. sp. has an elongated and cylindrical body with an average length of 174 μm, and is distinguished from its congeners by its shorter and thicker body, the rounded anterior end with thicker folds than for the rest of the body, and the posterior end covered by epicytic folds. Molecular phylogenetic analysis based on nuclear 18S rDNA placed T. euchirellae n. sp. in the clade of eugregarines infecting marine crustaceans, and in the same subclade as T. pugettiae. Thiriotia euchirellae n. sp. infected 95 % of the E. rostrata population, but was not detected in the other mesopelagic copepod species examined in the present study. Euchirella rostrata has an elongated gut with a swollen and sigmoidal anterior part and a gradually narrowing posterior part connected through a narrow path. Thiriotia euchirellae n. sp. were mostly observed in the posterior part of the gut. The high infection rate of E. rostrata by T. euchirellae n. sp. and the characteristic gut morphology of the host suggest a specific host–parasite relationship between these species.