Sperm ultrastructure in two species of Choanomphalus (Hygrophila: Lymnaeoidea: Planorbidae).

Sperm ultrastructure has been successfully used in gastropod taxonomy and here we attempted to identify potential synapomorphies within the Hygrophila, a group of freshwater lung-bearing gastropods, by examining sperm morphology and ultrastructure in two hygrophilan species of the genus Choanomphalus Gerstfeldt, 1859 (Lymnaeoidea: Planorbidae): Ch. riparius (Westerlund, 1865) and Ch. rossmaessleri (A. Schmidt, 1851). The spermatozoa of both species, like those of other Hygrophila, are divided into a head with an acrosome and nucleus, a midpiece containing a mitochondrial derivative, a glycogen piece, and an endpiece. The acrosome consists of an apical vesicle and a lucent-cored pedestal, plugged basally by compact electron-dense material (acrosomal plug) and separated from the nucleus by a membrane-bound vesicle (basal cap). The midpiece bears apically four surface ridges and contains four glycogen-filled helices inside the mitochondrial derivative. The midpiece is separated from the glycogen piece by an annulus composed of electron-dense material at the tip of the mitochondrial derivative and a hollow cylinder, but the latter appears to consist of two fused structures, making the annulus essentially tripartite. Spermatozoa of the two species show interspecific differences: in Ch. rossmaessleri, the acrosomal plug lacks a stem and the midpiece ridges differ in size, while Ch. riparius has spermatozoa with a stemmed plug and equal-sized ridges. The results of this and earlier studies suggest that sperm ultrastructure can provide synapomorphies for hygrophilan groups at different taxonomic levels. The Lymnaeoidea, one of the two superfamilies of the Hygrophila, appears to be characterised by a lucent-cored acrosomal pedestal, multiple glycogen helices and a tripartite annulus. The presence of a basal plug in the acrosomal pedestal is a possible synapomorphy for the clade uniting Planorbini Rafinesque, 1815 and Segmentinini F.C. Baker, 1945. In Planorbini including the two species of Choanomphalus, the pedestal is separated from the nucleus by a membrane-bound basal cap, a feature readily distinguishing this tribe from Segmentinini and other Lymnaeoidea. The spermatozoa of the two Choanomphalus species differ from those of other Planorbini in having two nuclear keels instead of one, but this character may not be taxonomically reliable.