Imposer in three species of Thais from Singapore, with additional observations on T. clavigera (Kuster) from Japan

Abstract

Observations on imposer are provided for Thais bitubercularis, T. clavigera and T. jubilaea (Mollusca: Neogastropoda: Muricidae) from Singapore. The results suggest that the three species are affected by tributyltin. Thais bitubercularis appear to be least affected, while T. jubilaea was the most affected, in terms of the size of the pseudopenis. The sex ratios in most cases were in favour of the males, suggesting a higher mortality amongst females. Relative penial size (RPS) values were less than 25% but aborted capsules were present in the lumen of the capsule glands of females examined. In contrast, additional observations of T. clavigera from Japan had RPS values up to 88%. These differences probably reflect seasonal variation in male penial size, but the possibility that females respond to high concentrations of TBT by growing a larger pseudopenis despite blockage of the oviduct, should not be discounted.