Control of modiolid mussels in cooling water systems by continuous chlorination

Modiolid mussels such as Modiolus philippinarum and Modiolus metcalfei constitute a numerically significant group in fouling communities, especially in tropical and subtropical industrial cooling water systems. Nevertheless, there are hardly any published reports on the tolerance of these species to chlorination or biofouling control measures, This article provides data on the mortality pattern (LT(50) and LT(100)) and physiological activities (oxygen consumption, filtration rate, foot activity, and byssus thread production) of different size groups of M. philippinarum (7- to 30-mm shell lengths) and M. metcalfei (6- to 26-mm shell lengths) exposed to different concentrations of residual chlorine (0.25, 0.50, 0.75, and 1.00 mg/L for sublethal responses; 1, 2, 3, and 5 mg/L for mortality). It is shown that exposure time for 100% mortality of M. philippinarum and M. metcalfei significantly decreased with increasing residual chlorine concentration. For example, 30-mm size group M. philippinarum exposed to 1 mg/L chlorine residual took 402 h to reach 100% mortality, whereas those exposed to 5 mg/L chlorine took 108 h. Among the two mussels, M. philippinarum showed slightly higher tolerance (9-20%) to chlorine when compared to M. metcalfei. The effect of mussel size on mortality of M. philippinarum was significant, with the larger size group mussels showing greater resistance than smaller ones. However, in M. metcalfei, size of the mussel does not seem to be a determinant of its chlorine tolerance. All size groups of M. philippinarum and M. metcalfei showed progressive reduction in physiological activities (oxygen consumption, filtration rate, foot activity index, and byssus thread production) when chlorine residuals gradually increased from 0 to 1 mg/L. Reduction in physiological activities was strongly correlated with chlorine concentration. A comparison of present data with data available for other coexisting mussel species suggests that M. philippinarum and M. metcalfei are relatively less tolerant to chlorine than Perna viridis, Perna perna, and Brachidontes striatulus, which also cause fouling problems in tropical coastal waters.