Comparative sperm morphology of the pulmonate limpets Trimusculus costatus, T. Reticulatus (Trimusculidae) and Burnupia stenochorias and Ancylus fluviatilis (Ancylidae)

Abstract

Sperm ultrastructure is described for the first time in representatives of the pulmonate 'limpet' families Trimusculidae (Trimusculus costatus, T. reticulatus: marine) and Ancylidae (Burnupia stenochorias, Ancylus fluviatilis: freshwater). All show characteristic heterobranch sperm features (a spheroidal acrosomal vesicle supported by an acrosomal pedestal; a helically keeled nucleus and a complex, very elongate midpiece featuring paracrystalline and matrix layers sheathing the axoneme, coarse fibers and one or more glycogen helices). Posterior to the midpiece, a glycogen piece (axoneme sheathed by glycogen granules) and annulus are also present in all species. Taxonomically useful differences in the shape and dimensions of the acrosome, nucleus and midpiece occur between the species. Results support the decision of recent workers to transfer the Trimusculidae from the Siphonarioidea to a separate superfamily Trimusculoidea (characteristic sperm features including: narrow acrosomal pedestal overlapping with nuclear apex; heavily keeled nucleus; midpiece with strongly projecting secondary and glycogen helices). The relationship of the Trimusculoidea to other pulmonates, as indicated by sperm ultrastructure, remains uncertain largely because comparative data for several important groups are unavailable. Spermatozoa of the two ancylids most closely resemble those of other investigated planorboideans and to a lesser extent, those of the Lymnaeoidea. However, differences between Burnupia stenochorias (unique(?) accessory structure on the acrosomal pedestal; glycogen wedges within the nuclear fossa; other features similar to planorbids) and Ancylus fluviatilis (all sperm features very similar to planorbids) suggests that these patelliform ancylids are not closely related.