Aphally and imposex in Nucella lapillus from Galicia (NW Spain):: incidence, geographical distribution and consequences for the biomonitoring of TBT contamination

Males of Nucella lapillus (L.), an intertidal gastropod, can suffer from a curious genetic disorder that causes underdevelopment of the reproductive tract ('Dumpton Syndrome' [DS]). Its extreme form leads to a lack of a penis ('aphally') and an incompletely developed vas deferens (VD). DS-affected females are also recognisable by a lesser degree of masculinisation ('imposex') when exposed to tributyltin (TBT) pollution, i.e. penis development map be lacking (female 'aphally') and VD formation may be weak. Based on the results of an extensive survey (at 56 stations) around Galicia (NW Spain) from 1996 to 1998, it is now known that imposer is widespread and the variations shown in its development indicate that DS also occurs widely. A modified version of the Vas Deferens Sequence (VDS) schemes currently devoted to assess the intensity of masculinization of female N, Lapillus is proposed. It attempts to accommodate the variants in imposer development observed in a DS-affected population whilst remaining both simple and comparable with previously applied schemes. The proposed VDS classification solely employs VD development as a ranking criterion so as to attain a consistent scoring of both aphallic and penis-bearing females. Importantly, the evidence indicates aphally need not preclude a female from being sterilized. Both males and females afflicted by DS were scattered throughout the Galician region, although aphallic females were present at many sites at which DS males went unrecorded. Also, female aphally was commonly more frequent than male aphally on a per site basis, DS-affected males were very uncommon, never exceeding similar to 20% and more frequently constituted <10%. In contrast, aphallic females were common, reaching frequencies of 20 to 50% at some sites, although overall it also must be considered a low-frequency phenomenon. Imposer levels, overwhelmingly dominated by VDS Stage 4, showed that TBT pollution is widespread throughout the region. However, a high degree of inter-individual variability was found at many locations. At those sites, females showing no signs of imposer or only an early stage (i.e. VDS Stages 0 to 2) were found thriving alongside heavily afflicted individuals (VDS Stages 4 to 6). The significant association between this variability in the effects of imposer and the presence of DS strongly reinforces the interpretation of DS as a genetic disorder which lessens the effects of TBT. The consequences those TBT-resistant specimens might have for the impact of TBT on the population level are discussed as well as the impact their presence might pose for the assessment of TBT pollution and its effects based on imposer indices currently in use.