Spermiogenesis in the Western Siren, Siren nettingi (Caudata: Sirenidae)

I investigated spermiogenesis in the Western Siren, Siren nettingi (formerly Western Lesser Siren, Siren intermedia nettingi) from salamanders collected periodically over a span of 21 yr (1994–2015) in northeastern Arkansas. My primary objective in this study is to present histologically and ultrastructurally, for the first time, the novel sequence of spermatological events of the spermiogenic process in this species. I describe the transformational stages from secondary spermatocytes to mature biflagellated spermatozoa, all of which occur within the lumen of testicular lobules. These new-to-science processes begin with nuclear displacement and expansion within spermatids, which occur after their detachment from Sertoli cells lining the lobular germinal epithelium. Formation of a sinistral twisting, pleated plasma membrane comprised of 8 linear folds extends the length of each developing spermatid and contributes to the eventual structure of 2 undulating membranes in each mature biflagellated spermatozoon. Spermiogenesis in sirens, therefore, reveals an entirely different sequence of events when comparing it to the release of mature spermatozoa from their attachments to Sertoli cells—the spermiation stage found in acystic tubular spermatogenesis in testes of higher vertebrates. Finally, these remarkable cellular stages do not conform to spermiogenesis in any other vertebrate species and are a part of an extraordinary assemblage of maturation stages associated with acystic lobular spermatogenesis found only in members of the family Sirenidae. This species, along with another genus (Pseudobranchus) within the sirenid family, are the only known vertebrates to possess this unique type of spermatogenesis