Light is one of the most important abiotic factors influencing the (skeletal) growth of scleractinian corals. Light stimulates coral growth by the process of light-enhanced calcification, which is mediated by zooxanthellar photosynthesis. However, the quantity of light that is available for daily coral growth is not only determined by light intensity (i.e. irradiance), but also by photoperiod (i.e. the light duration time). Understanding and optimizing conditions for coral growth is essential for sustainable coral aquaculture. Therefore, in this study, the question was explored whether more light (i.e. more photons), presented either as irradiance or as light duration, would result in more growth. A series of nine genetically identical coral colonies of Galaxea fascicularis L. were cultured for a period of 18 weeks at different light duration times (8 hours 150 μE m−2 s−1:16 hours dark, 12 hours 150 μE m−2 s−1:12 hours dark, 16 hours 150 μE m−2 s−1:8 hours dark, 24 hours 150 μE m−2 s−1:0 hours dark) and different irradiance levels (8 hours 150 μE m−2 s−1:16 hours dark, 8 hours 225 μE m−2 s−1:16 hours dark and 8 hours 300 μE m−2 s−1:16 hours dark). Growth was determined every two weeks by measuring buoyant weight. Temperature, salinity and feeding levels were kept constant during the experiment. To detect possible acclimation of the corals to an increased light duration, rates of net photosynthesis and dark respiration were measured, hereby comparing coral colonies grown under an 8:16 hours light (150 μE m−2 s−1):dark cycle with corals grown under a 16:8 hours light (150 μE m−2 s−1):dark cycle. No increase in growth was detected with either increasing photoperiod or irradiance. Continuous lighting (24 hours 150 μE m−2 s−1:0 hours dark) resulted in immediate bleaching and the corals died after 14 weeks. Hourly photosynthetic rates were significantly reduced in the 16 hour light treatment compared to the 8 hour light treatment. As a result, daily net photosynthetic rates were not significantly different, which may explain the observed specific growth rates. Acclimation to photoperiod duration appeared neither to be mediated by changes in chlorophyll-a concentration nor zooxanthellae density. Based on the results of this study, we can conclude that the enhancing effect of light on coral growth is not only a matter of photons. Obviously, the availability of light was not limiting growth in these experiments and was probably in excess (i.e. stressful amounts). Other factors are discussed that play a role in determining growth rates and might explain our results.

Lophelia pertusa, a cosmopolitan cold-water coral, offers potential for new palaeoclimate proxies. This study investigated its skeletal growth patterns to aid in this development. Corallite characteristics (calyx diameter, height, thecal width and banding) of L. pertusa sampled from oil platforms in the northern North Sea were examined. The mean distance between daughter polyps along a growth axis (27.4 ± 5 mm, SD) was equivalent to the estimated annual growth rate; hence, the polyps bud once a year. The majority of growth occurred in the first year when the characteristic trumpet shape of a corallite was formed, while the thecal wall thickened more consistently. Further examination of two polyps showed a dark growth band and centres of calcification along the full length of the inner theca, which represents early skeletal growth. Skeletal sampling adjacent to this area along sequential polyps shows promise as an annual chronology in these North Sea corals.

Halimeda opuntia and Diplosoma virens are common marine organisms in the Indo-Pacific. Halimeda grows to dense populations, and the ability of didemnids to overgrow coral has been well documented. In the light of their abundance in the Hikkaduwa Nature Park in Sri Lanka, this study investigated the effect of these two associates on the growth and survival of the commonly occurring staghorn coral, Acropora formosa. The results showed that both affected the growth rates of the host coral colonies significantly (P = 0.11 at 0.15 error level). The two associates also displayed a strong negative correlation between the spreading rate and the host growth rate (−0.71 for H. opuntia and −0.55 for D. virens). The results show that D. virens could have serious implications on coral survival with a population bloom.