Gonad characterization and reproductive seasonality in Siphonaria lessonii (Gastropoda: Heterobranchia) from the southwestern Atlantic Ocean

Histological characterization of the hermaphroditic gonad (HG) and seasonality of gametogenesis were investigated in a population of Siphonaria lessonii from the coast of Buenos Aires Province (37°16′S, 56°58′W). Monthly analysis of the frequency of gametogenic stages, as well as the number and mean size of oocytes, were used to determine reproductive events over a 2‐year period (June 2012–May 2014). Female and male gametes were observed simultaneously within acini of adult individuals and continuously throughout the period studied. Oogenesis commenced in the beginning of austral autumn, with gonads characterized mainly by proliferation of female cells. From this moment, oocytes gradually increased in number and area until spring, when a large number of individuals were found in the evacuation stage. The same trend was observed from early gonad maturation to advanced stages, indicating that gonad development was closely related to the frequency of oocyte stages and to the area (size) of oocytes. Spermatogenesis was also observed as a continuous process throughout the year, although spent acini were more frequent from November until February. Reproductive seasonality and gametogenesis were associated with changes in temperature and day length.