Reproductive dynamics of the sympatric topshells Phorcus lineatus and Phorcus sauciatus (Gastropoda: Trochidae) on a rocky intertidal coast in southern Portugal

The present study aimed to describe the reproductive cycle of the topshells Phorcus lineatus and Phorcus sauciatus from an intertidal rocky shore in the Algarve, southern Portugal. Based on two years of monthly sampling performed between January 2017 and December 2018, the study involved analyses of gonad histology and estimation of the mean gonadal index, aiming to investigate their relationship with environmental variables. Both topshell species are dioecious and showed synchrony in the reproductive dynamics between sexes. Indeed, the reproductive cycle of P. lineatus and P. sauciatus exhibited similar patterns, with scarcity of inactive gonads and prolonged periods with mature and spawning individuals. The gonadal index closely reflected the species’ reproductive cycle, which is apparently influenced by seasonal fluctuations in air and seawater temperatures. Additionally, some atmospheric and oceanographic conditions, such as daylight duration and wave height, were also correlated with gonad ripening and spawning of P. lineatus, and wind speed with gonad ripening and spawning of P. sauciatus. Overall, the present study provided valuable information to propose harvesting management measures aiming to ensure the sustainable exploitation of this shellfish resource, namely a closed season in the harvesting of both topshell species in September–October, covering their spawning peak in southern Portugal.