Imposex in a local marine gastropod

Abstract

Imposex, the imposition of male sexual characteristics on female marine gastropods, has been used as a bioindicator of TBT pollution in Maltese coastal waters (Axiak et. al., 1995; Muscat, 1995). A wider survey of imposex was however required for an improved understanding of the condition and of the implication that these effects might have on the species under study. Imposex was studied in 14 populations of Hexaplex trunculus collected during the period 1993-1995. The degree of imposex was measured using the indices described by Gibbs et al. (1987) and Stroben et al. (1992) with slight modifications for the Vas Deferens Sequence Index (VDS) as described in Axiak et al. (1995). Imposex was ubiquitous in populations of Hexaplex trunculus studied during the period 1993-1995. The highest levels of the condition were always found in areas close to marinas and harbours known to be highly contaminated by TBT (Vella, in press). Over the years, the degree of imposex did not decline in the marina and harbour areas which receive a continuous input of the contaminant, but levels decreased drastically at White Tower Bay and Anchor Bay. Such observations are concordant with the decreased inputs of TBT in the local environment due to decreased importation of TBT-based paints (Mr. Mercieca, CEF Trading; personal communication). Seven stages of imposex development were distinguished (Axiak et al., 1995) however two different types corresponding to Stroben et al. (1992)'s -a and -b pathways could be distinguished in Stage 1 and Stage 3. The stages of imposex development are documented with scanning electronmicrographs for the first time. There was no indication that imposex in its earliest stages leads to reduced reproductive potential. Histological examination of the reproductive system revealed a purely female structure and gametogenesis was proceeding normally. Moreover several egg capsules were observed in late June and July 1996 at White Tower Bay and Mellieha Bay (S. Micallef, J. A. Borg, D. Pefia Romero; personal communication). In areas close to marinas, however, imposex leads to sterility. All females had a grossly malformed oviduct, split along two thirds of its length. Additional histological analysis revealed masculinisation of the pallial genital tract. The ovaries were in a severe state of degeneration. Ultrastructural studies of the gonads showed histopathological alterations commonly attributed to TBT (Brick & Deutsch, 1993). There was no indication of preferential female mortalities in these areas, although the oldest individuals were usually females; this is indicative of aging populations (Feare, 1970). Female Penis Lengths, the Relative Penis Size Index (RPS) and the Vas Deferens Sequence Index (VDS) were assessed with regard to their quality as indices for TBT biomonitoring. Based on the pattern of development of imposex in Hexaplex trunculus, the VDS is regarded as the more valid index. Additionally it is the more informative criterion in evaluating the reproductive competency of afflicted populations. An important finding of this study was the presence of imposex (albeit mild) in individuals collected from Filfla, inspite of the lack of TBT (as measured by chemical analysis) in the area. Such an observation has far-reaching implications. It casts doubts on the claim that imposex is a specific response to TBT. It is suggested that pseudohermaphroditism may be a normal phenomenon in the species under investigation as a normal response to some environmental factor (s) other than TBT. This leads to the occurrence of low background levels of imposex expression in populations unexposed to TBT. Subsequent TBT exposure will then aggravate such syndrome beyond its background level of expression. The validity of imposex as a biomonitoring tool is discussed.