Inter‐ and intra‐colonial genotypic diversity in hermatypic hydrozoans of the family Milleporidae

Genetic diversity is the basis for adaptation and therefore of primary scientific interest, especially in species that are threatened by anthropogenic challenges, e.g. climate change and/or pollution. Coral reefs are among the most threatened but also the most diverse ecosystems and have therefore been studied quite extensively. So far, most investigations have focused on scleractinian corals while the equally important reef builders, the hydrozoans, have been less considered. Here we provide the first study of genotypic variability as well as intra‐colonial genetic variability, the co‐occurrence of more than one genotype within a single colony, in Milleporidae based on microsatellites. We analysed two geographically distinct populations from the Millepora dichotoma complex, one from the Red Sea and one from the Great Barrier Reef. Additionally, a population of Millepora platyphylla was analysed from French Polynesia. We compared microsatellite multilocus genotypes and cytochrome c oxidase subunit I haplotypes for each of the three field sites to detect levels of genotypic diversity at the intra‐ and inter‐specific levels. Furthermore, we examined all species for the occurrence of intra‐colonial genetic variability, a recently described mechanism in scleractinian corals that might enhance the adaptive potential of sessile organisms. We found both species and all field sites to be genotypically variable. Twelve mitochondrial haplotypes and 27 multilocus microsatellite genotypes were identified. In addition, intra‐colonial genetic variability was detected in the M. dichotoma complex from the Great Barrier Reef as well as in M. platyphylla from French Polynesia. All of the intra‐colonial genetically variable colonies consisted of one main genotype and a second divergent genotype caused by somatic mutations (mosaicism). Our study proves that Milleporidae are genetically variable and that the phenomenon of intra‐colonial genetic variability also occurs in this important reef‐building family.