Uktena riparia

(Figure 5)

In this lumbriculid species, both well-developed spermathecae (filled with sperm) and spermatophores are present. Spermathophores in U. riparia are long, thin tubes (Fig. 5A,B); they develop in the atrial duct (Fig. 5F), apparently using secretions from a section of the atrial ampulla that has a specialized, highly folded epithelium and unsually elongate prostatic glands (Fig. 5E; Fend et al. 2015: Fig. 4J–L). Therefore, the spermatophores appear wholly formed within the male bursa, before being transferred to the spermathecal bursa of the partner. As in other microdrile species producing spermatophores, several glandular organs are associated with both male and spermathecal bursae (Fig. 5C,D). A muscular copulatory organ within the spermathecal bursa (protruded in Fig. 5A) is probably related to the transfer and attachment of the spermatophores to the concopulant, using a secretion from a large basal gland (Fig. 5C; Fend et al. 2015: Fig. 3H,L,M). When transferred, spermatophores are attached at the base of the partner's copulatory organ, and positioned inside the spermathecal bursa with one end close to the entrance to the spermathecal duct (Fig. 5C). Then, the sperm leave the spermatophore and directly pass to the spermatheca (Fend et al. 2015: Figs. 2A–C, 3 I,L).

Sperm transfer by spermatophores is otherwise unknown in microdrile oligochaetes with well-developed spermathecae, although it has been described in some megadriles (Omodeo & Rota 1989). The use of spermatophores to transfer sperm from the male duct to the spermatheca, instead of a more typical mating behavior involving a direct sperm transfer by opposition of male and spermathecal pores, suggests an unusual mating behavior and a specific function of the spermatophores (see discussion).

The rare occurrence of sperm transfer through spermatophores in microdriles contrasts with its common occurrence in rhynchobdellid leeches. Evolutionary derivation of leeches from a lumbriculid-like ancestor had been suggested by several phylogenetic studies based on morphological characters (Brinkhurst & Nemec 1987; Jamieson et al. 1987) and most recently, by molecular analyses (Erséus 2005; Martin 2000; Siddall et al. 2001). The formation of spermatophores in the lining atrial epithelium of the lumbriculid U. riparia suggests an additional approach to investigating the relationships of leeches and lumbriculids.