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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 Indo-Pacific 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.

Coral bleaching represents the most serious threat to contemporary coral reefs. In response, focus is being laid on understanding the cellular processes involved in the response of corals to the environmental stresses and the molecular mechanisms that determine the bleaching patterns. In the present study, a component of the cellular stress response such as the expression of the heat shock proteins (Hsps) was analyzed following the coral bleaching event which occurred in the central Red Sea (Saudi Arabia) in 2015. During this event, corals of different species, growth forms and sites showed variable bleaching susceptibility. In particular, we investigated the expression of Hsp70, Hsp60 and Hsp32 in both healthy and bleached colonies belonging to four different coral species (Goniopora lobata, Porites lobata, Seriatopora hystrix and Stylophora pistillata), in order to explore the intra- and inter-specific modulation of these biomarkers as well as the existence of spatial patterns of Hsp expression. In healthy colonies, the level of all the biomarkers was significantly different among the different species, although within each species it remained similar regardless of the distance from the shore. All the coral species showed a significant modulation of the Hsp expression in response to bleaching, whose typology and amplitude were species-specific. In all the species, Hsp70 and Hsp60 showed a coordinated dual expression, which, in response to bleaching resulted in an up-regulation in G. lobata and P. lobata and in a down-regulation in S. hystrix and S. pistillata. Hsp32 was up-regulated in all four species following bleaching, indicative of elevated oxidative stress. Overall, the protein expression profiles of each species contribute to assess the role of Hsps in regulating the susceptibility to thermal stresses of the various coral taxa of the Red Sea.

Fire corals of the hydrocoral genus Millepora provide an important ecological role as framework builders of coral reefs in the Indo-Pacific and the Atlantic. Recent works have demonstrated the incongruence between molecular data and the traditional taxonomy of Millepora spp. based on overall skeleton growth form and pores. In an attempt to establish a reliable and standardized approach for defining species boundaries in Millepora, we focused on those from the Red Sea. In this region, three species are currently recognized: the fan-shaped branching M. dichotoma, the blade-like M. platyphylla, and the massive/encrusting M. exaesa. A total of 412 colonies were collected from six localities. Two mitochondrial marker genes (COI and 16S rDNA) were sequenced to obtain phylogeny reconstructions and haplotype networks. Eight morphological traits of pores and the nematocysts of both polyp and eumedusoid stages were measured to determine whether significant morphological differences occur among the three species. Both markers clearly resolved M. dichotoma, M. platyphylla, and M. exaesa as distinct, monophyletic lineages in the Red Sea. Nevertheless, they also revealed deep genetic breaks with Southwestern Indian Ocean populations of the three species. In the Red Sea, the three species were further distinguished based on their pore and nematocyst features. A discriminant analysis revealed dactylopore density, number of dactylopores per gastropore, dactylopore distance, and gastropore diameter as the most informative discriminative characters. The heteronemes, the large and small stenoteles of polyps, and the distribution of mastigophores of eumedusoids also showed significant interspecific differences. An integrated morpho-molecular approach proved to be decisive in defining species boundaries of Millepora supported by a combination of pore and nematocyst characters, which may be phylogenetically informative.