Sex and site-specific trends in veined rapa whelk (Rapana venosa) tributyltin bioaccumulation: considerations for biomonitoring

The imposition of male sexual characteristics onto the female (imposex) is present in wild populations of the non-native veined rapa whelk (Rapana venosa) in Chesapeake Bay, USA but does not appear to compromise reproductive function. Cultured whelks were used to test two hypotheses: (1) Observed imposex metrics will be similar to tributyltin (TBT) water concentrations at each of three sites; (2) Male and imposex/female whelks from the same site will have similar TBT body burdens. Cultured 2-year-old whelks were transplanted to three field sites in the York River, USA at the onset of their second reproductive season. Transplant site mean TBT water concentrations ranged from 1.4 ± 0.77 to 64.2 ± 57.8 ng l−1. Imposex incidence was 100% after 28 weeks with an observed M:F:IF ratio of 81:0:92 across all sites. Imposex stages (median vas deferens scale index = 4) and reproductive output were similar across sites. The imposex severity (IS = penis length/shell length) increased with increasing TBT concentrations. The relative penis length (RPLI) and relative penis size (RPSI) indices were positively related to site-specific TBT levels. Male whelks accumulated significantly higher TBT concentrations than female whelks at the site with the highest TBT concentration. Mean TBT concentrations in whelk egg capsules were significantly higher than concentrations in male or female whelk tissue. Egg capsule deposition provides a depuration mechanism for female whelks to reduce body burden of lipophilic TBT. Sex, season and reproductive status should be considered when using gastropod bioaccumulation to monitor TBT effects.