Chemistry and Ecology Vol. 21, No. 2, April 2005, 119–132 Response of mussel Brachidontes variabilis to chlorination S. RAJAGOPAL*†, V. P. VENUGOPALAN‡, G. VAN DER VELDE† and H. A. JENNER§ †Department ofAnimal Ecology and Ecophysiology, Institute for Water and Wetland Research, Radboud University Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands ‡Water and Steam Chemistry Laboratory, BARC Facilities, Kalpakkam-603 102, India §KEMA Power Generation and Sustainables, PO Box 9035, 6800 ET Arnhem, The Netherlands (Received 10 November 2004; in final form 17 January 2005) Brachidontes variabilis is a common fouling mussel species in cooling water systems of tropical coastal power stations. However, there are hardly any data available on the response of B. variabilis to chlorine, a commonly used antifouling biocide. Therefore, lethal and sublethal responses of this mussel to chlorine are of considerable interest to the industry. The response of mussels in terms of mortality pattern (LT 50 and LT 100 ) and physiological activities (oxygen consumption, filtration rate, foot activity and byssus thread production) in different size groups (with shell lengths of 7–24 mm) of B. variabilis was studied in the laboratory under different chlorine concentrations (0.25, 0.50, 0.75 and 1.00 mg l −1 for sublethal responses and 1, 2, 3 and 5 mg l −1 for mortality). The results showed that the exposure time for 100% mortality of mussels decreased significantly with increasing chlorine concentration. However, mussel size was not a determinant of its chlorine tolerance: all size groups tested (with shell lengths of 7–24 mm) took comparable exposure times to reach 100% mortality at a given chlorine concentration (1–5 mg l −1 ). All size groups of B. variabilis showed a progressive reduction in physiological activities such as oxygen consumption, filtration rate, foot activity and byssus thread production, when chlorine residuals were increased from 0 to 1 mg l −1 . The data generated in the present work are compared with similar data available for other tropical fouling mussel species to see how far relative chlorine toxicity could have influenced the relative distribution of the mussels inside the seawater intake tunnel of a power station at Kalpakkam in India. It is shown that in this insufficiently chlorinated system, the relative distribution of Brachidontes striatulus, B. variabilis and Modiolus philippinarum reflects the relative tolerance of the species to chlorine. Keywords: Mussel fouling; Brachidontes variabilis; Chlorine; Mortality; Physiological activities 1. Introduction Cooling water systems of coastal power stations, if not properly treated, invariably have problems arising from biofouling. Chemical control techniques involving injectable biocides are widely used to control biofouling in such systems. For example, chlorination has been the most commonly used fouling control method in industrial cooling water systems since more than five decades [1–4]. Chlorine as a biocide is effective against a variety of fouling organisms including bacteria, algae, fungi and invertebrates [3,5]. Its advantages include relatively low *Corresponding author. Email: s.rajagopal@science.ru.nl Chemistry and Ecology ISSN 0275-7540 print/ISSN 1029-0370 online © 2005 Taylor & Francis Group Ltd http://www.tandf.co.uk/journals DOI: 10.1080/02757540500071788