-Research on the biology and ecology of cold-water corals (CWCs) is still in its infancy. The growth patterns of CWCs in their natural environments are poorly known. Growth rate investigations on these deep-sea reef builder species are needed to predict recovery times following damage to their ecosystems. This study investigates a new approach for analysing CWC growth rate, suitable for in situ application. Lophelia pertusa and Madrepora oculata (Scleractinian) were collected from the Lacaze-Duthiers canyon in the northwestern Mediterranean Sea (520 m depth), marked and then either redeployed in situ for 6 months, or maintained in aquaria for growth rate comparison at a constant temperature of 13• C, corresponding to their habitat conditions. Two different types of staining (calcein and manganese) and three different exposure times (30, 60 and 240 min) were tested. The results show that calcein offers rapid incorporation and easy detection, making it particularly suitable for skeletal growth rate investigations compared with other chemical staining. In situ linear polyp growth rates of 7.5 ± 1.2 mm y −1 and 3.5 ± 2.1 mm y −1 were measured in new polyps of L. pertusa and M. oculata, respectively. Those values were significantly higher in young polyps than in older ones, where they decreased to 1.3 ± 1.5 mm y −1 and 1.2 ± 1.2 mm y −1 . Beyond the study of coral reef growth processes, this approach offers a methodological basis for habitat quality assessment which could be used in the management of deep-sea marine protected areas (MPA).

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