RESEARCH ARTICLE A morphological study of the male reproductive tract, post-testicular acrosome maturation and spermatophore formation in the black tiger prawn (Penaeus monodon) Tianyi Feng 1 | Brian Paterson 2 | Stephen Johnston 1 1 School of Agriculture and Food Sciences, University of Queensland, Gatton, Queensland, Australia 2 Queensland Department of Agriculture and Fisheries, Bribie Island Research Centre, Woorim, Queensland, Australia Correspondence School of Agriculture and Food Sciences, University of Queensland, Gatton, Queensland 4343, Australia. Email: tianyi.feng@uqconnect.edu.au Funding information University of Queensland Abstract Inferior larval production rates of domesticated Penaeus monodon broodstock has been a major hurdle to the expansion of its aquaculture, so that a better understanding of basic male repro- ductive biology is critical to improve the reproductive performance of this commercially impor- tant penaeid species. Following our previous study of spermatogenesis in the species, this study explored the mechanism of spermatophore formation with regards to the contribution of the reproductive tract epithelium by light and transmission electron microscopy. Four types of epi- thelial secretions (S1–4) were observed contributing to the three layers of the spermatophore. The primary layer of spermatophore was composed of S1 and S2, which were released from the secretory epithelial cells of the proximal vas deferens (PVD) and the secretory epithelial cells of the sperm-bearing lumen of the mid vas deferens, respectively. The secondary layer of the sper- matophore was composed of S3, the secretory product of epithelial cells in the accessory tubule lumen of the mid vas deferens. The outer layer of the spermatophore was formed from S4 which was secreted by the epithelial cells in the posterior mid vas deferens and the terminal ampulla. Unique folds of the vas deferens epithelium appeared to play an important role in the formation of the spermatophore, particularly in the formation of the laminated structure of the spermatophore appendage. With respect to acrosome maturation along the reproductive tract, most spermatozoa did not have a fully developed anterior spike and a subacrosomal region when in the PVD, whereas both structures were fully formed by the time the spermatozoa reached the mid vas deferens and increased electron density when in the terminal ampulla; this observation represents the first morphological evidence of post-testicular acrosome maturation in this taxon. KEYWORDS decapod crustacean, shrimp, TEM, terminal ampulla, vas deferens 1 | INTRODUCTION Poor reproductive potential of domesticated Penaeus monodon brood- stock has been a major hurdle to the expansion of aquacultural produc- tion in this species (Kumar & Engle, 2016). Reproductive issues for the male P. monodon broodstock in captivity include abnormal spermatogen- esis (Chong et al., 2015), a lack of mating behavior (Marsden, Richardson, Mather, & Knibb, 2013) and pathological changes in the reproductive tract (Chamberlain, Johnson, & Lewis, 1983); consequently, a better understanding of basic male reproductive biology including spermatogen- esis, post-testicular sperm maturation and spermatophore formation is fundamental and critical to addressing these reproductive problems in aquaculture. Although, the male reproductive tract has been studied in several penaeid species such as Litopenaeus setiferus (King, 1948; Ro, Talbot, Leung-Trujillo, & Lawrence, 1990), Litopenaeus vannamei (Chow, Dougherty, Dougherty, & Sandifer, 1991; Chow, Dougherty, & Sandifer, 1991), Litopenaeus schmitti (Fransozo, Fernandes, López-Greco, Zara, & Received: 10 April 2018 Revised: 3 June 2018 Accepted: 21 June 2018 DOI: 10.1002/jmor.20869 Journal of Morphology. 2018;1–11. wileyonlinelibrary.com/journal/jmor © 2018 Wiley Periodicals, Inc. 1