Shallow water zooxanthellate zoanthids are a common component of the coral reef ecosystems of the Caribbean. Despite this, their species diversity remains poorly understood. In this study, collected Palythoa, Zoanthus, Isaurus, and Terrazoanthus specimens from the waters of Florida were phylogenetically examined to obtain a better understanding of zoanthid species diversity in the Caribbean. Surprisingly, the results from analyses utilizing three DNA markers (mitochondrial 16S ribosomal DNA, cytochrome oxidase subunit I, and the internal transcribed spacer of ribosomal DNA) showed the presence of at least eleven species, of which up to four appear undescribed. Additionally, the presence of the genus Terrazoanthus in the Caribbean was confirmed for the first time. Attempts to match phylogenetic species or clades with original literature were hampered by vague and short original descriptions, and it is clear that for Atlantic Palythoa and Zoanthus species an in-depth and multidisciplinary investigation is needed to reconcile recent phylogenetic results such as in this study with traditional taxonomy. Furthermore, most shallow water zoanthid species from Florida were observed to have close, sister-species relationships with previously investigated species in the Pacific Ocean. These results indicate that many brachycnemic zoanthid species likely had a Caribbean-Pacific distribution until the formation of the Isthmus of Panama. However, due to inadvertent redescriptions, overall species diversity in these two common genera is likely much lower than literature indicates.

Healthy coral communities can be found on artificial structures (concrete walls and riprap) within the Port of Miami (PoM), Florida. These communities feature an unusually high abundance of brain corals, which have almost entirely vanished from nearby offshore reefs. These corals appear to be thriving in very low-quality waters influenced by dense ship and boat traffic, dredging, and numerous residential and industrial developments. The PoM basin is part of Biscayne Bay, an estuarine environment that experiences frequent freshwater input, high nutrient loading, hypoxia, and acidification. To investigate if there is a molecular basis behind the ability of these corals to persist within these highly “urbanized” waters, we compared whole transcriptome expression profiles from 25 PoM Pseudodiploria strigosa colonies against six conspecifics from a nearby offshore reef. We found that the urban corals exhibited higher expression of (1) transcripts encoding pattern-recognition receptors which may allow these corals to better sense and detect food particles and pathogenic invaders; (2) digestive and degradation-associated enzymes, which may suggest an elevated capacity for heterotrophy and pathogen digestion; and (3) transcripts related to innate immunity, defense, and cellular detoxification, which may collectively protect against pathogenic organisms and water pollution impacts. Large ribosomal subunit rRNA gene mapping revealed that P. strigosa colonies from the PoM sites predominantly hosted heat-tolerant endosymbionts from the genus Durusdinium while offshore conspecifics’ communities were dominated by symbionts in the genus Breviolum. These findings reveal transcriptomic plasticity and molecular mechanisms contributing to the persistence of these corals within a highly urbanized habitat.

Coral cover has declined worldwide due to anthropogenic stressors that manifest on both global and local scales. Coral communities that exist in extreme conditions can provide information on how these stressors influence ecosystem structure, with implications for their persistence under future conditions. The Port of Miami is located within an urbanized environment, with active coastal development, as well as commercial shipping and recreational boating activity. Monitoring of sites throughout the Port since 2018 has revealed periodic extremes in temperature, seawater pH, and salinity, far in excess of what have been measured in most coral reef environments. Despite conditions that would kill many reef species, we have documented diverse coral communities growing on artificial substrates at these sites—reflecting remarkable tolerance to environmental stressors. Furthermore, many of the more prevalent species within these communities are now conspicuously absent or in low abundance on nearby reefs, owing to their susceptibility and exposure to stony coral tissue loss disease. Natural reef frameworks, however, are largely absent at the urban sites and while diverse fish communities are documented, it is unlikely that these communities provide the same goods and services as natural reef habitats. Regardless, the existence of these communities indicates unlikely persistence and highlights the potential for coexistence of threatened species in anthropogenic environments, provided that suitable stewardship strategies are in place.

Anemone-like animals belonging to the order Zoantharia are common anthozoans widely distributed from shallow to deep tropical and subtropical waters. Some species are well-known because of their high toxicity due to the presence of palytoxin (PLTX) in their tissues. PLTX is a large polyhydroxylated compound and one of the most potent toxins known. Currently, the PLTX biosynthetic pathway in zoantharians and the role of the host or the putative symbiotic organism(s) involved in this pathway are entirely unknown. To better understand the presence of PLTX in some Zoantharia, twenty-nine zoantharian colonies were analysed in this study. All zoantharian samples and their endosymbiotic dinoflagellates (Symbiodiniaceae = Zooxanthellae) were identified using DNA barcoding and phylogenetic reconstructions. Quantification of PLTX and its analogues showed that the yields contained in Palythoa heliodiscus, Palythoa aff. clavata and one potentially undescribed species of Palythoa are among the highest ever found (up to > 2 mg/g of wet zoantharian). Mass spectrometry imaging was used for the first time on Palythoa samples and revealed that in situ distribution of PLTX is mainly located in ectodermal tissues such as the epidermis of the body wall and the pharynx. Moreover, high levels of PLTX have been detected in histological regions where few or no Symbiodiniaceae cells could be observed. Finally, issues such as host-specificity and environmental variables driving biogeographical patterns of hosted Symbiodiniaceae in zoantharian lineages were discussed in light of our phylogenetic results as well as the patterns of PLTX distribution. It was concluded that (1) the variability of Symbiodiniaceae diversity may be related to ecological divergence in Zoantharia, (2) All Palythoa species hosted Cladocopium Symbiodiniaceae (formerly clade C), (3) the sole presence of Cladocopium is not sufficient to explain the presence of high concentrations of PLTX and/or its analogues and (4) the ability to produce high levels of PLTX and/or its analogues highlighted in some Palythoa species could be a plesiomorphic character inherited from their last common ancestor and subsequently lost in several lineages.