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In situ growth patterns of two-species of reef-building cold-water corals were investigated for the first time at different temporal scales, based on the redeployment of coral nubbins in their natural environment. Lophelia pertusa and Madrepora oculata were collected in November 2010 and May 2011 from the Lacaze-Duthiers canyon in the northwestern Mediterranean Sea (520 m depth). Three in situ growth experiments were performed from November 2010 to May 2011, May to September 2011 and November 2010 to September 2011. For comparison, aquaria experiments over comparable lengths of time were conducted with coral colonies collected in November 2010. In the canyon, new polyps from M. oculata and L. pertusa displayed similar growth rates in summer (5.8±3.8 and 7.3±1.7 mm yr−1 respectively), but M. oculata growed significantly slower during winter/spring than L. pertusa (4.1±1.8 and 8.4±2.7 mm yr−1 respectively). Budding rates however (the rate of new polyp addition per mother polyp per year) were similar between both species in winter/spring (45±40% for M. oculata and 48±72% for L. pertusa), but were significantly lower in summer for M. oculata (14±19%) compared to L. pertusa (58±94%). This seasonal difference in the growth between L. pertusa and M. oculata might reflect differences in species-specific physiology (such as reproduction) or feeding strategy, or a higher sensitivity of M. oculata to the variability of food supply in the Lacaze-Duthiers canyon resulting from periodic cascading events. The comparison of in situ and aquaria growth experiments showed no significant differences for budding and new polyp growth rates, which supports the validity of aquaria experiments for these kinds of investigations. However the budding rates observed were consistently lower in coral maintained in aquaria than those in in situ conditions, a finding which is to be considered when extrapolating laboratory based investigation results to the naturally occurring coral ecosystem.

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).