Ontogenetic characterization of the secondary sexual characters of Penaeus schmitti Burkenroad, 1936 and Penaeus vannamei Boone, 1931 (Decapoda: Penaeidae)

Secondary sexual characters are commonly used in taxonomic studies of penaeid shrimp, as these structures are the main distinguishing features between species. However, there are few studies on the ontogenetic development of these structures, and most of them describe only a single developmental stage. The ontogenetic comparison of these structures may be particularly useful for accurately assessing the biological invasion process of the exotic species Penaeus vannamei in the central-western South Atlantic, given the difficulty of distinguishing it from its natural congener Penaeus schmitti at juvenile stages. To fill this gap, here we describe the morphology of secondary sexual characters in P. vannamei and P. schmitti along the ontogeny. We confirmed the efficiency of these structures to discriminate them at different ontogenetic stages. Our results show that the structures of secondary sexual characters in juvenile shrimp are different from those in adults, but nevertheless, sets of characters in both the thelycum and the petasma allow a successful distinction between the studied species at different ontogenetic stages. Our results also support the hypothesis that endopodite fusion may not be the final step in male sexual morphological development. Our research may facilitate the assessment of the establishment process of P. vannamei along the American Atlantic coastline, a critical step in monitoring and controlling the spread of alien species. In addition, these results may serve as a baseline for future studies on the reproductive biology and development of penaeid shrimp.