Ultrastructure and Function of the Male Reproductive System in the Rotifer Asplanchnopus cf. multiceps (Rotifera:...

Male rotifers are both rare and inconspicuous members of freshwater environments. Consequently, their anatomy is poorly known, and many details about sexual reproduction remain vague. Here, we explore the reproductive anatomy of the dwarf male of Asplanchnopus cf. multiceps using light microscopy, confocal microscopy, and transmission electron microscopy to describe its fine structure and function and compare it to what is known among other male rotifers. We reveal a male system consisting of a single testis and vas deferens, two prostate glands, and several muscles that function for copulation. The testis is comprised of a thin cellular lining that surrounds spindle-shaped spermatozoa and tapered rods with an ultrastructure similar to those in Asplanchna. Both are released from the testis via a junctional sphincter, which permits their entry into a ciliated vas deferens. The vas deferens consists of three hollow cells, the most distal of which is a copulatory cell that projects into a partly ciliated male canal. During copulation, retractor muscles dilate the canal opening before a semicircular muscle contract to increase hydrostatic pressure at the posterior end. We hypothesize this pressure forces secretions from the prostate glands out the opening and just prior to canal evagination. The hollow copulatory cell at the base of the vas deferens is centered in this evagination and together creates a bulbous copulatory organ that projects from the rotifer's posterior end. This copulatory cell has microvilli that may help to form a strong bond with the prostatic secretions on the female integument. Longitudinal muscles along the vas deferens contract and pressurize the vessel, thereby ejecting rods and spermatozoa through the female integument. Additional muscles function to retract the entire complex back into the body. The organization of the reproductive system is similar to males of Asplanchna and this study expands on what is known from historical light microscopical analyses.