Sperm packaging in the seminal vesicles of Hygrophila (Gastropoda: Pulmonata)

Spermatozoa of freshwater pulmonates (Hygrophila) are morphologically complex cells that may reach significant lengths (up to 1 mm). This raises a question as to whether these cells are arranged or compacted in any specialized fashion as they move through the reproductive passages. The use of chemically unprocessed, rather than fixative-treated, material shows that the spermatozoa of Hygrophila found in the seminal vesicles, unlike other spermatozoa, are arranged in a compacted fashion implying that they undergo complex folding and unfolding processes on the way toward and from these organs. This study examined sperm packaging in the seminal vesicles of six hygrophilan families. The packaged spermatozoon forms a ring-shaped structure consisting of three domains. The first domain (the anterior portion of the spermatozoon) participates only in the formation of the outer coils of the ring. The second domain (a major portion of the sperm midpiece) is folded and twisted on itself and coiled into a ring. The third domain (the posterior portion of the spermatozoon) is coiled in a smaller ring that lies inside the first ring. The folding process is reconstructed as the inverse of the unfolding sequence observed in dry mounts and in the spermatozoa mounted in Berlese solution. The possible mechanism of the sperm-packaging process and phylogenetic implications are discussed.