Imposex in Cronia margariticola (mollusca, prosobranchia) as a potential marine pollution biomonitor around Mactan Island, central Philippines

Individuals (n=30) of the common prosobranch mollusk Cronia margariticola from three populations around Mactan Island were examined for the occurrence of imposex. These populations were obtained from habitats exhibiting three presumed different levels of pollution, namely a ship repair and oil depot facility (Caltex Depot), a popular tourist area (Maribago) and a relatively pristine and sparsely populated beach (Punta Engano). Imposex in female mollusks was scored using Relative Penis Size Index (RPSI) and Vas Deferens Sequence Index (VDSI). Imposex registered 100% occurrence in the Caltex Depot sampling site, while in Maribago imposex was recorded in 70% of the samples. In Punta Engano, only 9% imposex was documented. Highest RPSI of 38.57% was recorded in Calex Depot while the lowest RPSI of only 0.07% was documented in Punta Engano. VDSI, however, ranged from 0 to 3 in the three study sites indicating low risk for sterility among the molluscan populations sampled. In all three sites, there was significant correlation between imposex stage (expressed as VDSI) and female pseudopenis length. A lower level of correlation was observed between VDSI and shell and aperture height. The results obtained suggest that RPSI and VDSI data (i.e., imposex) may have some predictive functions in assessing the severity of marine pollution in countries where expensive and sophisticated instrumentation may not be available.