Saki Harii

Early molecular responses of coral larvae to hyperthermal stress.

Most of the work on the impact of elevated temperature and light on Symbiodinium‐invertebrate symbioses have focused primarily on how the photosynthetic (algal) partner is impacted. Understanding how the same stresses affect the invertebrate host, however, is in its infancy. In this study, we re‐examined the direct effect of elevated temperatures on the invertebrate host exploring the early transcriptional response of aposymbiotic (without algal symbionts) coral larvae. The temperatures tested in the experimental design were 24 °C (ambient seawater temperature), 28 °C and 31 °C; and the sampling points were 3 and 10 h after temperature exposure. We explored relative changes in transcription using a cDNA microarray constructed for the scleractinian coral, Acropora millepora, and containing 18 142 expressed sequence tag (EST) clones/8386 unigenes. Our study identified 29 genes that were significantly up‐ and down‐regulated when A. millepora coral larvae were exposed to elevated temperatures. Down‐regulation of several key components of DNA/RNA metabolism was detected implying inhibition of general cellular processes. The down‐regulation of protein synthesis, however, was not simple and random, which suggested that the stress response was a more complicated adjustment of cellular metabolism. We identified four significant outcomes during the very early hours of the transcriptional response to hyperthermal stress in coral larvae. First, the expression of heat‐shock proteins increased rapidly (within 3 h) in response to hyperthermal stress. Second, a fluorescent protein homologue, DsRed‐type FP, decreased its expression in response to elevated temperature reinforcing a potential role as a molecular marker for monitoring hyperthermal stress in nature. Third, the down‐regulation of a coral mannose‐binding C‐type lectin under elevated temperature suggests that heat stress might compromise some components of the coral immune defence and therefore might bring about susceptibility to pathogenic diseases. And last, genes involved in protecting cells against oxidative stress showed little response at the early hours to heat stress, supporting the proposal that up‐regulation of cnidarian host oxidative stress genes may require reactive oxygen species generated by stressed algal symbionts.

The impact of benthic algae on the settlement of a reef-building coral

The capacity of corals to re-establish in degraded and algal-dominated habitats will depend on the effects of algae on coral settlement and growth. We tested the effect of 11 macroalgal species, of widely different functional-forms, on swimming and settlement by larvae of the coral Platygyradaedalea from the Great Barrier Reef. Algal turfs and the crustose calcareous algae groups had minor effects on coral settlement, while upright calcareous and fleshy macroalgae inhibited settlement. However, the extent of inhibition of larval settlement differed amongst upright macroalgal species, variations that were not well explained by physical differences and probably reflect chemical differences not explained by functional-form. Thus, while algal functional-form is useful in identifying general competition patterns, more detailed taxonomic and chemical approaches may be required to fully understand algal effects on corals. Different macroalgal communities on degraded reefs may have different effects on coral settlement, and hence on coral population resilience.

Grazing by a small fish affects the early stages of a post-settlement stony coral

Short-term experiments were used to isolate the detrimental effects of grazer disturbance on young corals, and determine the stage of development at which recruits are no longer susceptible to this disturbance. Artificial substrata containing an algal matrix and coral recruits of different life stages were exposed to grazing by epilithic algal matrix (EAM) feeding combtoothed blennies, Salarias fasciatus. Single polyp recruits were vulnerable to grazer disturbance, while multi-polyp recruits (ca. 6–8 polyps) survived with evidence of minor damage in the form of tissue and polyp loss. The result indicates that blennies, although small and possessing weak dentition, can negatively influence the survival of young coral recruits. The protruding structure of micro-nubbins, representing juvenile corals were not damaged, suggesting that coral achieving that size and form can escape such damage.

Increased local retention of reef coral larvae as a result of ocean warming

The impact of ocean warming on coral larvae survival and dispersal is investigated using a dynamic model. The authors find that globally most reefs will experience large increases in the local retention of larvae, which make populations more responsive to local conservation efforts. However, increased larvae retention will also weaken connectivity between populations, which may affect recovery if a local population is severely disturbed.

Worldwide Analysis of Sedimentary DNA Reveals Major Gaps in Taxonomic Knowledge of Deep-Sea Benthos

Deep-sea sediments represent the largest but least known ecosystem on earth. With increasing anthropogenic pressure, it is now a matter of urgency to improve our understanding of deep-sea biodiversity. Traditional morpho-taxonomic studies suggest that the ocean floor hosts extraordinarily diverse benthic communities. However, due to both its remoteness and a lack of expert taxonomists, assessing deep-sea diversity is a very challenging task. Environmental DNA (eDNA) metabarcoding offers a powerful tool to complement morpho-taxonomic studies. Here we use eDNA to assess benthic metazoan diversity in 39 deep-sea sediment samples from bathyal and abyssal depths worldwide. The eDNA dataset was dominated by meiobenthic taxa and we identified all animal phyla commonly found in the deep-sea benthos; yet, the diversity within these phyla remains largely unknown. The large numbers of taxonomically unassigned molecular operational taxonomic units (OTUs) were not equally distributed among phyla, with nematodes and platyhelminthes being the most poorly characterized from a taxonomic perspective. While the data obtained here reveal pronounced heterogeneity and vast amounts of unknown biodiversity in the deep sea, they also expose the difficulties in exploiting metabarcoding datasets resulting from the lack of taxonomic knowledge and appropriate reference databases. Overall, our study demonstrates the promising potential of eDNA metabarcoding to accelerate the assessment of deep-sea biodiversity for pure and applied deep-sea environmental research but also emphasises the necessity to integrate such new approaches with traditional morphology-based examination of deep-sea organisms.

Algal symbiont type affects gene expression in juveniles of the coral Acropora tenuis exposed to thermal stress.

Reef-building corals harbor symbiotic dinoflagellates, Symbiodinium spp., which are currently divided into several clades. The responses of corals associated with different Symbiodinium clades to thermal stress are not well understood, especially at a gene expression level. Juveniles of the coral Acropora tenuis inoculated with different algal types (clade A or D) were exposed to thermal stress and the expression levels of four putative stress-responsive genes, including genes coding green and red fluorescent proteins, an oxidative stress-responsive protein, and an ascorbic acid transporter, were analyzed by quantitative real-time PCR. The expression levels of the four genes decreased at high temperatures if juveniles were associated with clade A symbionts but increased if the symbionts were in clade D. The intensity of green fluorescence increased with temperature in clade D symbionts harboring juveniles, but not in juveniles associated with clade A symbionts. The present results suggest that genotypes of endosymbiotic algae affect the thermal stress responses of the coral juveniles.

The transcriptomic response of the coral Acropora digitifera to a competent Symbiodinium strain: the symbiosome as an arrested early phagosome

Despite the ecological significance of the relationship between reef‐building corals and intracellular photosynthetic dinoflagellates of the genus Symbiodinium, very little is known about the molecular mechanisms involved in its establishment. Indeed, microarray‐based analyses point to the conclusion that host gene expression is largely or completely unresponsive during the establishment of symbiosis with a competent strain of Symbiodinium. In this study, the use of Illumina RNA‐Seq technology allowed detection of a transient period of differential expression involving a small number of genes (1073 transcripts; <3% of the transcriptome) 4 h after the exposure of Acropora digitifera planulae to a competent strain of Symbiodinium (a clade B strain). This phenomenon has not previously been detected as a consequence of both the lower sensitivity of the microarray approaches used and the sampling times used. The results indicate that complex changes occur, including transient suppression of mitochondrial metabolism and protein synthesis, but are also consistent with the hypothesis that the symbiosome is a phagosome that has undergone early arrest, raising the possibility of common mechanisms in the symbiotic interactions of corals and symbiotic sea anemones with their endosymbionts.

“Locally extinct” coral species Seriatopora hystrix found at upper mesophotic depths in Okinawa

Following the bleaching events of 1998 and 2001, Seriatopora hystrix disappeared from shallow reefs around Okinawa Island, Japan (van Woesik et al. 2011). Here we report finding S. hystrix in a mesophotic coral ecosystem near Motobu Peninsula on Okinawa Island. This diverse coral community occurs on the reef downslope from ~35 m and extends to at least 47 m depth, with the lower boundary yet to be defined. Along with Acropora tenella (Fig. 1) and Pachyseris speciosa (Fig. 2), S. hystrix (Fig. 3) is one of the most abundant species in this community. Following the ‘‘extinction’’ of shallow S. hystrix, no new recruitment events were observed in nearby shallow reefs; thus, the presence of a deep population of this species likely does not originate in recent recruitment but would demonstrate that S. hystrix has not gone extinct in this location. Potentially, this deep population could contribute to the recolonization of S. hystrix at shallower depths, although that is strongly dependent on the level of genetic structuring over depth (Bongaerts et al. 2011; van Oppen et al. 2011). The importance of other ‘‘shallow’’ species in community composition is being further investigated to determine the possible role of this reef as refugium.

Spectral Diversity and Regulation of Coral Fluorescence in a Mesophotic Reef Habitat in the Red Sea

The phenomenon of coral fluorescence in mesophotic reefs, although well described for shallow waters, remains largely unstudied. We found that representatives of many scleractinian species are brightly fluorescent at depths of 50–60 m at the Interuniversity Institute for Marine Sciences (IUI) reef in Eilat, Israel. Some of these fluorescent species have distribution maxima at mesophotic depths (40–100 m). Several individuals from these depths displayed yellow or orange-red fluorescence, the latter being essentially absent in corals from the shallowest parts of this reef. We demonstrate experimentally that in some cases the production of fluorescent pigments is independent of the exposure to light; while in others, the fluorescence signature is altered or lost when the animals are kept in darkness. Furthermore, we show that green-to-red photoconversion of fluorescent pigments mediated by short-wavelength light can occur also at depths where ultraviolet wavelengths are absent from the underwater light field. Intraspecific colour polymorphisms regarding the colour of the tissue fluorescence, common among shallow water corals, were also observed for mesophotic species. Our results suggest that fluorescent pigments in mesophotic reefs fulfil a distinct biological function and offer promising application potential for coral-reef monitoring and biomedical imaging.

A pre-zygotic barrier to hybridization in two con-generic species of scleractinian corals

Hybridization is often cited as a potential source of evolutionary novelty in the order Scleractinia. While hybrid embryos can be produced in vitro, it has been difficult to identify adult hybrids in the wild. Here, we tested the potential for hybridization between two closely related species in the family Fungiidae. We mixed approximately 5000 eggs of Ctenactis echinata with sperm from C. crassa. No hybrid embryos were produced. This observation adds to a growing body of evidence for pre-zygotic barriers to hybridization in corals and challenges the claim that hybridization is a major source of evolutionary novelty in the order.