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Coral reefs are declining worldwide due to increased incidence of climate-induced coral bleaching, which will have widespread biodiversity and economic impacts. A simple method to measure the sub-bleaching level of heat-light stress experienced by corals would greatly inform reef management practices by making it possible to assess the distribution of bleaching risks among individual reef sites. Gene expression analysis based on quantitative PCR (qPCR) can be used as a diagnostic tool to determine coral condition in situ. We evaluated the expression of 13 candidate genes during heat-light stress in a common Caribbean coral Porites astreoides, and observed strong and consistent changes in gene expression in two independent experiments. Furthermore, we found that the apparent return to baseline expression levels during a recovery phase was rapid, despite visible signs of colony bleaching. We show that the response to acute heat-light stress in P. astreoides can be monitored by measuring the difference in expression of only two genes: Hsp16 and actin. We demonstrate that this assay discriminates between corals sampled from two field sites experiencing different temperatures. We also show that the assay is applicable to an Indo-Pacific congener, P. lobata, and therefore could potentially be used to diagnose acute heat-light stress on coral reefs worldwide.

Stylophora pistillata is a widely used coral “lab-rat” species with highly variable morphology and a broad biogeographic range (Red Sea to western central Pacific). Here we show, by analysing Cytochorme Oxidase I sequences, from 241 samples across this range, that this taxon in fact comprises four deeply divergent clades corresponding to the Pacific-Western Australia, Chagos-Madagascar-South Africa, Gulf of Aden-Zanzibar-Madagascar, and Red Sea-Persian/Arabian Gulf-Kenya. On the basis of the fossil record of Stylophora, these four clades diverged from one another 51.5-29.6 Mya, i.e., long before the closure of the Tethyan connection between the tropical Indo-West Pacific and Atlantic in the early Miocene (16–24 Mya) and should be recognised as four distinct species. These findings have implications for comparative ecological and/or physiological studies carried out using Stylophora pistillata as a model species, and highlight the fact that phenotypic plasticity, thought to be common in scleractinian corals, can mask significant genetic variation.

During

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BackgroundComplete mitochondrial (mt) genomes have been sequenced for thousands of animals and represent a molecule of choice for many evolutionary studies. Nevertheless, some animal groups have remained under-sampled. Ctenophora (comb jellies) is one such example, with only two complete mt sequences determined hitherto for this phylum, which encompasses ca. 150–200 described species. This lack of data derives from the extremely fast mt evolutionary rate in this lineage, complicating primer design and DNA amplification. Indeed, in the two ctenophore mt genomes sequenced to date, i.e. those of Mnemiopsis leidyi (order Lobata) and Pleurobrachia bachei (order Cydippida), both rRNA and protein coding genes exhibit an extraordinary size reduction and have highly derived sequences. Additionally, all tRNAs, and the atp6 and atp8 genes are absent. In order to determine whether these characteristics are shared by other ctenophores, we obtained the complete mt genomes of three benthic ctenophores belonging to the so far unsampled order of Platyctenida: Coeloplana loyai, Coeloplana yulianicorum and Vallicula multiformis.ResultsThe mt genomes of benthic ctenophores reveal the same peculiarities found in Mnemiopsis and Pleurobrachia, demonstrating that the fast evolutionary rate is a general trait of the ctenophore mt genomes. Our results also indicate that this high evolutionary rate not only affects the nucleotide substitution but also gene rearrangements. Indeed, gene order was highly rearranged among representatives of the different taxonomic orders in which it was close to random, but also quite variable within Platyctenida, in which the genera Coeloplana and Vallicula share only four conserved synteny blocks. However, the two congeneric Coeloplana species display exactly the same gene order. Because of the extreme evolutionary rate, our phylogenetic analyses were unable to resolve the phylogenetic position of ctenophores within metazoans or the relationships among the different Ctenophora orders. Comparative sequence-analyses allowed us to correct the annotation of the Pleurobrachia mt genome, confirming the absence of tRNAs, the presence of both rRNA genes, and the existence of a reassignment of codon TGA from tryptophan to serine for this species.ConclusionsSince Platyctenida is an early diverging lineage among Ctenophora, our findings suggest that the mt traits described above are ancestral characteristics of this phylum.Electronic supplementary materialThe online version of this article (10.1186/s12862-018-1186-1) contains supplementary material, which is available to authorized users.

The Gulf of Aqaba (Red Sea) is characterized by seasonal plankton blooms which are driven by vertical nutrient upwelling during winter (Genin et al. 1995). In March 2009, following the seasonal upwelling, large numbers of the moon jellyfish (Aurelia aurita) were recorded at the local fringing reefs of Eilat, at depths of 2-20 m (Fig. 1a). During this event, several large (ca. 20-25 cm) solitary corals (Fungia scruposa) were observed to feed on these jellyfish (Fig. 1b, c). A. aurita is known to be eaten by a wide variety of relatively large predators, including fish, sea turtles and even sea birds; however, it has never been reported as a coral's prey. Despite the fact that hermatypic corals may feed heterotrophically on a broad variety of sources ranging in size from bacteria to mesozooplankton (up to 1,000 lm) (Houlbrèque and Ferrier-Pagès 2009), this is the first report of solitary corals feeding on large gelatinous plankton (ca. 12 cm in diameter) in their natural habitat. Other cnidarians, such as the sea anemone Entacmaea medusivora in Palau, have also been shown to feed on jellyfish (Mastigias papua). However, as opposed to F. scruposa, these sea anemones are constantly surrounded by their prey and they lack photosynthetic endosymbionts (Fautin and Fitt 1991). Our observations revealed that the large-mouthed solitary fungiids can consume relatively large prey organisms, which are not available to other corals, as an additional source of protein. It remains to be shown, however, how fungiid corals manage to capture these large jellyfish while overcoming their motility. This trophic opportunism and reproductive plasticity exhibited by fungiid corals (Loya and Sakai 2008) are suggested as an important asset in determining their evolutionary success.

The successful recruitment of planktonic larvae to coral reefs is essential for the continued existence of these highly diverse ecosystems. Feeding strategies may affect recruitment success and potentially determine species distribution by controlling the dispersal range of the larvae. Our aim here was to ascertain the feeding strategies of planula larvae of the coral Stylophora pistillata by using stable isotopes. Planula larvae, fragments of parental colonies, and 3 potential food types were analyzed for carbon and nitrogen stable isotope compositions and C/N ratios. We found that planulae were depleted in 13 C when compared to parental tissues, whereas their C/N ratios were 2-fold higher. Following lipid extraction, there were no significant differences in δ 13 C values and C/N ratios between lipid-free planulae and parental colonies. This indicates that the differences in δ 13 C originate in the lipid content of the planulae and not from any isotopic fractionation that may occur during embryological development. Controlled feeding experiments were conducted using phytoplankton, zooplankton, and bacteria. Despite the presence of an oral opening, the planulae did not show any feeding behavior, and the stable isotope data corroborated the observations of no feeding. Moreover, following 2 wk of starvation in the dark, planulae started to utilize their lipid and protein reservoirs. These results stress the importance of the photosynthates translocated from the algal symbionts to these planulae as an energy source.