Population genetic structure of the stony coral Acropora tenuis shows high but variable connectivity in East Africa

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.