The dry weight condition index and water content of two groups of mussels, one with heavy infestations and the other with mild infestations of Polydora ciliata, were determined over a period of 15 consecutive months. The results were seasonally variable, but for most months the heavily infested mussels had a condition index significantly lower, and a water content significantly higher, than the mildly infested mussels. This suggests that heavy infestations of P. ciliata are associated with reduced flesh content in mussels.
Further analyses of the body components of the two groups of mussels, showed that the mantle tissue of heavily infested mussels was reduced much more than the nonmantle tissue. Since the mantle is the main repository of gametes, this suggests that heavy infestations of P. ciliata might lower the fecundity of mussels.