During a period when there was considerable capital dredging in progress in the Conwy Estuary, the shell boring parasite Polydora ciliata became unusually frequent resulting in both loss of condition and higher vulnerability to predation by crabs (Ambariyanto & Seed, 1991). Mass mortalities of farmed mussels have been attributed to the parasite Mytilicola intestinalis but this was considered by Bower (1992) to be unsubstantiated (see chapter IV).

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