Microarchicotylus ochroleucus gen. et sp. n. (Plathelminthes, Tricladida, Paludicola): A new genus and species of dwarf planarians in Lake Baikal

Abstract

This paper gives an illustrated description of a new genus and species of endemic Baikal planarians, Microarchicotylus ochroleucus gen. et sp. n. The morphological similarity in the structure of the pharynx, male copulative organs, and external features allowed the authors to include two more species, M. stringulatus (Korotneff 1912) and M. elegans (Porfiriev et Timoshkin 2009), formerly belonging to a poorly investigated genus Archicotylus, into the new genus. Microarchicotylus gen. et sp. n. planarians are attributed to dwarf forms, since their maximal body length does not exceed 5–6 mm. One more distinctive external feature of these Microarchicotylus species is a bright and uneven coloration of the dorsal part of the body, sometimes being dark brown, light brown, or ochreous with distinct white transverse strokes or without them. The anterior part of the body is bright orange or blackish, its color always being different from the remaining body, separated by a white transverse collar behind the pair of eyes. The pharynx is either short cylindrical barrelshaped (length-to-width ratio of ~1) or elongated cylindrical (ratio, 3: 1). The copulative organ is compact and resides behind the pharynx, with its apical part directed downwards. The seminal receptacle, its canal, and the male and common atriums are asymmetrically located relative to the medial longitudinal body axis and centrally located gonopore.