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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.

In corals, different biological processes are associated with the daily variation of temperature and light, which in shallow reefs can vary widely and regularly reach extreme levels. The expression of cellular defense mechanisms such as the heat shock proteins Hsp70 and Hsp60 was analyzed in coral colonies adapted to live in a shallow lagoon, during three consecutive days characterized by the same temperature/light trend. Each of the three species investigated showed a significant diel modulation of both Hsps, which also displayed a coordinated expression. However, the diel modulations of both Hsps were species specific and only that of the branching Acropora tenuis appeared correlated with the temperature. All the species displayed high Hsp levels at 13.00 and 16.00, probably as protection for the cellular stress produced by the elevated temperature and light. Taken together, the results may provide useful information about the cellular mechanisms involved in the corals adaptation to extreme conditions.

To date, four nominal species and several other unidentified species of Zanclea hydrozoans are known to live symbiotically with scleractinians, and recent surveys reported this association also in the Red Sea. Previous molecular studies showed that each coral genus involved in this association hosts only one species or molecular clade of Zanclea, with the only exception being the genus Acropora, which hosts at least two Zanclea species. Moreover, some of the detected genetic lineages were morphologically undistinguishable in the polyp stage, suggesting the presence of cryptic species. In this study, we investigated the morphology and genetic diversity of Acropora-associated Zanclea specimens collected in previous studies in Egypt and Israel, as well as new samples collected in Saudi Arabia. Based on the current data, all the analysed samples were morphologically identical to Zanclea gallii, a species associated with Acropora corals from the Maldives. However, molecular analyses separated the samples collected in the Red Sea from all other coral-associated hydroids. Therefore, phylogenetic reconstructions, haplotype networks, genetic distance analyses and distribution data allowed us to identify a previously unknown cryptic species of Acropora-associated hydroid, here named Zanclea gallii IIa, following a recently proposed molecular nomenclature.

Symbiotic relationships on coral reefs involving benthic hosts other than scleractinian corals have been poorly investigated. The hydroid Pteroclava krempfi is a widespread species known to be mainly associated with alcyonacean octocorals in the Indo-Pacific. In the present study, P. krempfi was discovered in association with octocorals of the genus Antillogorgia (Gorgoniidae) at two localities in the Caribbean Sea (St. Eustatius in the eastern Caribbean and Bocas del Toro in the western part), updating its host range with an additional genus and family. The Caribbean specimens showed no morphological differences and the shape of their polyps was consistent with the original P. krempfi description. A multi-locus phylogeny reconstruction of the P. krempfi species complex based on both mitochondrial and nuclear loci revealed three separate molecular clades. Two of them were composed of P. krempfi associated with the families Plexauridae and Alcyoniidae from the Maldives, whereas a new highly supported molecular lineage included all Caribbean specimens of P. krempfi associated with the family Gorgoniidae. These three divergent molecular clades represent distinct cryptic taxa within the P. krempfi species complex, in which the main interspecific difference consists of their host families.

Scleractinian reef corals have been acknowledged as the most numerous host group for associated hydroids belonging to the genus Zanclea. To date, their geographical distribution is known to include several Indo-Pacific regions. During the Statia Marine Biodiversity Expedition to St. Eustatius (Lesser Antilles, Dutch Caribbean), the Zanclea-coral association was observed for the first time for the Caribbean Sea as well as for the Atlantic Ocean. Our findings confirm that the biodiversity associated with coral reefs remains insufficiently explored worldwide.

Some hydroids are known to form strict relationships with anthozoans. In this study we report the first evidence of the association between Pteroclava krempfi (Hydrozoa: Capitata: Cladocorynidae) and octocorals of the alcyonacean genera Sarcophyton, Lobophytum and Sinularia observed in the Republic of Maldives and in the Red Sea. Our observations contribute to an expansion of both the recorded host-range and geographical distribution of this symbiosis, indicating that the association between hydroids and alcyonaceans is more widespread than previously known.

Coral diseases have been little investigated in the Indian Ocean and especially in the Republic of Maldives, where they were firstly observed in 2010. Through repeated monitoring, we observed a peculiar slow progression of Black Band Disease in several large colonies of the reef-building coral Goniopora cf. columna in the Maldivian reefs of Magoodhoo Island (Faafu Atoll). We argue that this may ensure the local persistence of the disease, possibly promoting long-term mortality of other, more susceptible coral species.

Although corals are known to host a variety of marine organisms, to date few benthic hydrozoans have been observed to be associated with anthozoans. However, three species belonging to the genus Zanclea have recently been discovered that inhabit reef-building scleractinians. Here we report the first evidence of such Zanclea-coral associations from the Red Sea, extending their known geographical distributions and host range.