PARASITES OF BUCCINUM-UNDATUM (MOLLUSCA, PROSOBRANCHIA) AS BIOLOGICALINDICATORS OF SEWAGE-SLUDGE DISPERSAL

The common whelk, Buccinum undatum L., was selected as a model host-pa rasite system in a seasonal study at an 'accumulating' sewage-sludge d ump-site in the Firth of Clyde, Scotland, where marked spatial concent ration gradients exist for a variety of trace contaminants in the sedi ments. A total of 2808 whelks was examined from seven sites over 18 mo nths. The prevalence of larval digenean parasites increased significan tly with distance from the dump site from approximately 2% on its peri phery to 15.1% at a reference site 3 km from the centre. At a second r eference site, 13 km from the dumping area, 19.7% of Buccinum were par asitized. The effects of site, season, sex and parasitism on growth an d condition of Buccinum and exposure to sewage sludge on the age-preva lence relationship and host response to infection were examined. Age-f requency distributions of Buccinum were used to compare various source s of mortalities (natural, fishing, pollution and parasite-induced). P atterns in parasite prevalence recorded at the dump site were not corr elated with any natural environmental or host-related factors that wer e examined. The gradient in parasitism in Buccinum is considered to re sult principally from the toxic effects of trace metals on the miracid ium, reducing parasite transmission to the molluscan host. The Buccinu m-parasite system may therefore provide a sensitive and valuable biolo gically-based index of the dispersal of these contaminants around the sewage-sludge dump-site, and may have wider applications for marine en vironmental monitoring.