IMPOSEX AND REPRODUCTIVE FAILURE IN HYDROBIA-ULVAE (GASTROPODA, PROSOBRANCHIA)

Hydrobia ulvae specimens collected from 16 stations along the German N orth Sea and Baltic coasts exhibited imposer (occurrence of male parts in addition to the female genital system). For the purposes of compar ison, a description of both the male and unaffected female genital sys tems is presented. Four different imposer stages with two types of dev elopment were identified and documented with scanning electron microgr aphs for the first time. The percentage of imposex-affected females (a n average over all the localities sampled) was about 44.3%, and 12.9% were definitively sterilized. The phenomenon of sex change was not obs erved. The vas deferens sequence index, imposer incidence, percentage of sterilized females and the average female penis length are recommen ded as biological effect monitoring parameters in response to tributyl tin pollution.