Spermiogenesis and sperm ultrastructure in Troglocephalus rhinobatidis, Neoheterocotyle rhinobatidis and Merizocotyle australensis (Platyhelminthes, Monogenea, Monopisthocotylea, Monocotylidae)

Abstract

The ultrastructural events of spermiogenesis and the ultrastructure of mature sperm of 3 species of monocotylid monogeneans are described. Two of these species, Troglocephalus rhinobatidis and Neoheterocotyle rhinobatidis are extremely similar in all the aspects studied, and their placement in separate subfamilies is questioned. Evidence is presented in both species for extensive distalward movement during spermiogenesis of an ornamented region associated with cortical microtubules, originally located in the zone of differentiation. Spermatids of Merizocotyle australis lack this ornamentation and the mature sperm also lacks cortical microtubules. Troglocephalus rhinobatidis exhibits the highest number (128) of spermatids in an isogenic group recorded for a flatworm to date. Sperm of all 3 species have 2 normal axonemes, shifted slightly relative to one another. Comparative data are presented on sperm and spermiogenesis of all monocotylids examined to date and the phylogenetic implications are discussed.