Morpho-molecular traits of Indo-Pacific and Caribbean Halofolliculina ciliate infections

Coral diseases are emerging as a major threat to coral reefs worldwide, and although many of them have been described, knowledge on their epizootiology is still limited. This is the case of the Halofolliculina ciliate infections, recognized as the skeletal eroding band (SEB) and Caribbean ciliate infection (CCI), two diseases caused by ciliates belonging to the genus Halofolliculina (Class Heterotrichea). Despite their similar macroscopic appearance, the two diseases are considered different and their pathogens have been hypothesized to belong to different Halofolliculina species. In this work, we analysed the morphology and genetic diversity of Halofolliculina ciliates collected in the Caribbean Sea, Red Sea and IndoPacific Ocean. Our analyses showed a strong macroscopic similarity of the lesions and similar settlement patterns of the halofolliculinids from the collection localities. In particular, the unique erosion patterns typical of the SEB were observed also in the Caribbean corals. Fine-scale morphological and morphometric examinations revealed a common phenotype in all analysed ciliates, unequivocally identified as Halofolliculina corallasia. Phylogenetic analyses based on nuclear and mitochondrial (COI) molecular markers consistently found all samples as monophyletic. However, although the nuclear marker displayed an extremely low intra-specific diversity, consistent with the morphological recognition of a single species, the analyses based on COI showed a certain level of divergence between samples from different localities. Genetic distances between localities fall within the intra-specific range found in other heterotrich ciliates, but they may also suggest the presence of a H. corallasia species complex. In conclusion, the presented morpho-molecular characterization of Halofolliculina reveals strong similarities between the pathogens causing SEB and CCI and call for further detailed studies about the distinction of these two coral diseases.