The ecology of Polydora, epp was reviewed and the infestation of molluscs by the boring, spionid polychaete, Polydora ciliata, was studied in detail. X-radiography showed that, P. ciailt a infested a wide variety of molluscan substrates, but it . occurred in greatest abundance on large, rough-surfaced, epibenthic shells, which were collected from the-sublittoral zone and the level of extreme low water spring tides, of shores sheltered from wave action. P. ciliata was contagiously dispersed on mussel--shells. Monthly radiographs of mussels indicated that maximum settlement of P. ciliata larvae in the river Fowey, 1 occurred in May. The effect of heavy infestations of P. ciliata on limpets P. vul ata , mussels (Eytilus edulis), and oysters (Crassostrea is9 was investigated. At certain times of the year, heavy infestations of P. ciliata reduced the dry weight condition and fecundity of limpets and mussels, but did not seem'to adversely affect C. i ae. Compression tests showed that P. ciliate weakened mussel shells. Predation experiments, with Cancer a s, indicated that this might increase the mussels' vulnerability to predation. Heavy P. ciliata infestation also reduced the mussels' resistance to loads which tended to force the mussele' valves apart. This may decrease the mussels' phyeico-chemical tolerance, and increase their vulnerability to starfish predation. Limpet adhesion, measured on the shore with a simple dynamometer, was impaired by P. ciliata infestation. This may reduce the limpets' resistance to wave action, and increase their vulnerability to predation. Reasons for the adverse effects of P. ciliata infestation on limpets and mussels are suggested, and the ecological and commercial implications of the infestation are discussed.

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