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Aim The aim of this study is to determine the genetic diversity, population structure and connectivity of the broadcast-spawning coral Acropora tenuis (Cnidaria; Scleractinia; Acroporidae). Based on the long pelagic larval duration (PLD) of the species, long-distance dispersal resulting in high connectivity among populations is hypothesized.Location East Africa (Kenya and Tanzania; 2.5°S~10°S) Methods A total of 269 samples were collected from 11 sample sites in Kenya and Tanzania spanning a distance of 900 km. The coral fragments were genotyped using seven microsatellite markers. Analyses included population genetic estimations of diversity and population differentiation, principal coordinate analysis (PCoA), Bayesian clustering approaches and testing for isolation by distance (IBD).Results Moderate, but significant, genetic structure was found when comparing all sample sites, but IBD could not be detected. Based on Bayesian cluster analyses three groups of samples sites could be identified: (1) Kenya and northern Tanzania, (2) southern Tanzania and (3) sample sites located in the Zanzibar and Pemba channels.Main conclusions High connectivity can be explained by the long-distance dispersal capacity of A. tenuis and by the influence of the northbound East African Coastal Current facilitating dispersal by effectively spreading larvae along the coast. Oceanographic characteristics rather than distance are factors that determine connectivity among populations of A. tenuis in Kenya and Tanzania. No clear genetic break was identified. However, variable connectivity between sample sites does occur, with limited connectivity of the sample sites Misali and Stonetown.

The coral reefs of the Western Indian Ocean (WIO) and the Red Sea offer a unique environment to investigate large scale connectivity of corals along a north-south gradient with variable environmental conditions. However, these highly biodiverse coral reefs remain understudied and few studies have addressed connectivity patterns.In this study, the genetic diversity and connectivity of the brooding scleractinian coral Seriatopora hystrix were assessed on two scales: first, by comparing the Red Sea and the WIO, and second, by comparing sites within the WIO. Ten microsatellites were used to genotype 356 individual colonies from 14 locations. Additionally, a haploweb analysis of an ITS2 marker was used for species delimitation in the genus Seriatopora. Seriatopora hystrix showed limited dispersal capacity and high differentiation both at a large scale between the Red Sea and the WIO, and smaller scale for sites along the coast of East Africa. Red Sea populations appear to belong to a different species than the populations in the WIO. Within the WIO three groups can be distinguished, (I) north Madagascar, (II) south-west Madagascar together with one site in Mozambique (Nacala), and (III) all other sites along the coast of Mozambique, Tanzania and Kenya. These patterns of contemporary dispersal barriers can be explained by the short pelagic larval duration of S. hystrix and by oceanographic conditions, such as eddies in the Mozambique channel causing larval retention in northern Madagascar. This study can guide coral reef management and supports the conservation priority status of the Northern Mozambique Channel region.

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 :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.