Size matters: importance of penis length variation on reproduction studies and imposex monitoring in Bolinus brandaris (Gastropoda: Muricidae)

The present study aimed at analysing the monthly variation in penis length (PL) during the reproductive cycle of the purple dye murex (Bolinus brandaris). Two hypotheses were tested: PL variation reflects male maturation and reproductive activity; PL variation bias the calculation of imposex indices based on penis measurement (RPLI and RPSI). Sampling was performed during 1 year in a population from Ria Formosa lagoon (Algarve coast, southern Portugal) with high incidence of imposex. Penis-bearing individuals were measured for shell length (SL) and PL, and two bio-physiological indices were calculated for both sexes, the general condition index (K) and the gonadosomatic index (GSI). PL presented monthly variation in both sexes, but while female PL showed small and random oscillation, male PL exhibited significant variation throughout the reproductive cycle (as evidenced by the similar and synchronous trends between male PL and GSI). These findings have implications both for reproduction studies and imposex monitoring: calculation of standardised PL (PL/SL) allows for gender identification without killing the organism and constitutes a useful penial index for assessing male maturation and reproductive activity; RPLI and RPSI should be applied cautiously in spatial and temporal comparisons of imposex severity in B. brandaris.