Morphological diversity of stylets in Kolhymorbis (Mollusca: Pulmonata: Segmentinidae)

Abstract

The taxonomy of freshwater pulmonates (Hygrophila) relies heavily on morphology of their reproductive system, most notably on the characters of the copulatory apparatus. For this reason, taxonomically important components of the copulatory apparatus such as penial stylets have always received much attention in the morphological studies on Hygrophila. The present study describes the stylets of Kolhymorbis bogatovi Zatrawkin et Moskvicheva in Zatrawkin, 1985 and K. shadini Starobogatov et Streletzkaja, 1967 and compares them with the previously described stylet of K. angarensis (Dybowski et Grochmalicki, 1925). The lengths of the stylets vary within the genus from 20 to 30 μm. In K. bogatovi, the stylet is a trough-shaped plate with a distal portion twisted 110° counterclockwise with respect to the proximal portion. The stylet of K. shadini is also a trough-shaped plate, but its walls are curved in such a way that the stylet forms a hollow cone with a beveled tip. The slit between the opposite lateral flanks of the stylet is wide proximally, but narrows down distally. The stylets of all three Kolhymorbis species differ markedly in their shape. The trough-shaped stylet of K. angarensis most likely represents the plesiomorphic state; the stylet of K. bogatovi may have originated from this type of stylet by increasing the curvature of the walls and counterclockwise twisting of the distal portion of the stylet. The cone-shaped stylet of K. shadini may have evolved from the plesiomorphic type by widening of the proximal portion and by even more pronounced curving of the walls throughout the length of the stylet without distal twisting. The penial stylets of Kolhymorbis species are compared with those of Palaearctic genera of the family Segmentinidae Baker, 1945 and the taxonomic utility of the stylet characters is evaluated.