Imposex in Thais gradata and Chicoreus capucinus (Mollusca, Neogastropoda, Muricidae) from the Straits of Johor: A Case Study using Penis Length, Area and Weight as Measures of Imposex Severity

Imposex, or the presence of a pseudopenis in female snails, is quantified by comparing the length of the female pseudopenis with the male penis. However, many muricid gastropods in the Malayan archipelago possess penes that are flagellate and coiled, making linear measurement difficult under the microscope. In addition, it is unclear if penis length is dependent on shell size. Accurate assessment of penis size is a necessary step towards determining the severity of imposex (and implicitly, the amount of organotins from antifouling paints that is associated with this phenomenon). Two mangrove muricid gastropod species, Thais gradata and Chicoreus capucinus, were collected from six sites along the Johor Straits. Penis/pseudopenis length and area measurements based on digitalized camera lucida drawings were generally well correlated with penis/pseudopenis dry weight, although penis area appeared to be a marginally better variable than penis length for T. gradata. Penis dry weight was also well correlated with shell height in male T. gradata, while no significant correlation was detected for female T. gradata, suggesting pseudopenis size alone may be a better measure of imposex than the relative penis size index (RPSI) for this species. There was no obvious relationship between penis size and shell height for either sex in C. capucinus.