Widespread hermaphroditism in freshwater gastrotrichs

Freshwater members of the phylum Gastrotricha were long thought to lack mal e gametes and to exist exclusively as parthenogenetic females. The surprisi ng 1978 discovery of sperm in the common species Lepidodermella squamata ra ised the question of how many other freshwater gastrotrichs might likewise be hermaphroditic. In a comparative study of species from across both major families, sperm have been found in every species examined intensively. The y were detected in 19 species of Chaetonotidae (from Aspidiophorus, Chaeton otus, Heterolepidoderma, Ichthydium, Lepidodermella, and Polymerurus) and 3 species of Dasydytidae (from Haltidytes. Setopus, and Stylochaeta), charac teristically occurring ventrally in single unilateral or (more often) 2-12 bilateral packets. Their shape ranges from filiform (length in Chaetonotus bisacer, similar to 40 mum) to rodlike, spindlelike. oval, and possibly sph erical (some in Stylochaeta scirtetica measure only 1 mum). With light micr oscopy, a dense nucleus appears to fill the entire volume of these aflagell ate cells. Spermatogenesis within cysts (maximally, 16 sperm/clone) is evid ently characteristic of both families. each cyst generating one large resid ual body. Sperm-bearers display oocytes with sometimes distinctive cytoplas mic elements and a posterior X-organ whose organization can be complex. Evi dence supports an unusual life cycle in which parthenogenesis is followed b y simultaneous hermaphroditism. These findings may illuminate the reproduct ive characters as well as ancestry of marine and brackish-water taxa of Cha etonotida.