Characterization of fatty acid composition in healthy and bleached corals from Okinawa, Japan

Under bleaching conditions, corals lose their symbiotic zooxanthellae, and thus, the ability to synthesize fatty acids (FAs) from photosynthetically derived carbon. This study investigated the lipid content and FA composition in healthy and bleached corals from the Odo reef flat in Okinawa, southern Japan, following a bleaching event. It was hypothesized that the FA composition and abundance would change as algae are lost or die, and possibly microbial abundance would increase in corals as a consequence of bleaching. The lipid content and FA composition of three healthy coral species (Pavona frondifera, Acropora pulchra, and Goniastrea aspera) and of partially bleached and completely bleached colonies of P. frondifera were examined. The FA composition did not differ among healthy corals, but differed significantly among healthy, partially bleached, and completely bleached specimens of P. frondifera. Completely bleached corals contained significantly lower lipid and total FA content, as well as lower relative amounts of polyunsaturated FAs and higher relative amounts of saturated FAs, than healthy and partially bleached corals. Furthermore, there was a significantly higher relative concentration of monounsaturated FAs and odd-numbered branched FAs in completely bleached corals, indicating an increase in bacterial colonization in the bleached corals.     

Separation of highly fluorescent proteins by SDS-PAGE in Acroporidae corals

This study characterized the spectral properties of Acropora tenuis, A. nasuta, A. secale, and A. aspera, all of which showed strong colorful fluorescence under ultraviolet light-A (black light). The emission maxima of fluorescence from the intact corals were 517, 482, 484, and 514 nm in A. tenuis, A. nasuta, A. secale, and A. aspera, respectively. Using a soluble fraction of cell-free extract of the corals, we applied a method of sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) to separate each fluorescent protein component contained in the corals. Green fluorescent bands were detected in all Acropora examined, although their apparent molecular mass and relative content were different. A. aspera had two orange bands in addition to the green one. The major excitation and emission peaks of the orange fluorescence bands were almost identical (476 and 478 nm), however, they were discernible by their spectral profiles and molecular masses. Some biochemical properties of the highly fluorescent proteins of Acropora are described and implications of the results are discussed.     

Temporal settlement patterns of larvae of the broadcast spawning reef coral Favites chinensis and the broadcast spawning and brooding reef coral Goniastrea aspera from Okinawa, Japan

The faviid corals, Favites chinensis and Goniastrea aspera are widely distributed in the Indo-Pacific region. Both corals are hermaphroditic broadcast spawners, but G. aspera is also known to brood planula larvae in Okinawa. This study investigated the temporal settlement patterns of planula larvae of the scleractinian corals F. chinensis and G. aspera that developed from spawned gametes, and planula release and settlement of brooded larvae of G. aspera from Okinawa, Japan. Some of the broadcast-spawned larvae of F. chinensis and G. aspera had very short pre-competency periods of 1–2 and 2–3 days after spawning, and relatively long maximum settlement-competency periods of 56–63 and 63–70 days after spawning, respectively. These pre-competency periods are among the shortest reported for larvae of broadcast spawning coral species, and appear to be negatively correlated with seawater temperature. F. chinensis larvae tended to settle rapidly with 34–39% of larvae settling in the first week after spawning, while broadcast-spawned G. aspera larvae had a slower settlement pattern with 11–15% of larvae settling in the first week after spawning. Brooded larvae of G. aspera settled more rapidly, with settlement rates of 27–31% within the first 24 h and 45–65% within the first week after the start of the experiment. The production of planula larvae with rapid settlement capabilities may enable F. chinensis and G. aspera to establish and maintain populations in shallow reef sites at Okinawa. The release of the brooded planulae for up to 2 months may explain why G. aspera is locally more dominant on shallow reefs in Okinawa than F. chinensis. On a broader scale, the longer settlement competency periods of some of the broadcast-spawned larvae of these species increase their potential for longer-distance dispersal and may partly explain the wide biogeographic distribution of these species in the Indo-Pacific region.     

Survival dynamics of scleractinian coral larvae and implications for dispersal

Survival of pelagic marine larvae is an important determinant of dispersal potential. Despite this, few estimates of larval survival are available. For scleractinian corals, few studies of larval survival are long enough to provide accurate estimates of longevity. Moreover, changes in mortality rates during larval life, expected on theoretical grounds, have implications for the degree of connectivity among reefs and have not been quantified for any coral species. This study quantified the survival of larvae from five broadcast-spawning scleractinian corals (Acropora latistella, Favia pallida, Pectinia paeonia, Goniastrea aspera, and Montastraea magnistellata) to estimate larval longevity, and to test for changes in mortality rates as larvae age. Maximum lifespans ranged from 195 to 244 d. These longevities substantially exceed those documented previously for coral larvae that lack zooxanthellae, and they exceed predictions based on metabolic rates prevailing early in larval life. In addition, larval mortality rates exhibited strong patterns of variation throughout the larval stage. Three periods were identified in four species: high initial rates of mortality; followed by a low, approximately constant rate of mortality; and finally, progressively increasing mortality after approximately 100 d. The lifetimes observed in this study suggest that the potential for long-distance dispersal may be substantially greater than previously thought. Indeed, detection of increasing mortality rates late in life suggests that energy reserves do not reach critically low levels until approximately 100 d after spawning. Conversely, increased mortality rates early in life decrease the likelihood that larvae transported away from their natal reef will survive to reach nearby reefs, and thus decrease connectivity at regional scales. These results show how variation in larval survivorship with age may help to explain the seeming paradox of high genetic structure at metapopulation scales, coupled with the maintenance of extensive geographic ranges observed in many coral species. Communicated by Environment Editor Prof. van Woesik.     

Projected Near-Future Levels of Temperature and pCO2 Reduce Coral Fertilization Success

Increases in atmospheric carbon dioxide (pCO₂) are projected to contribute to a 1.1–6.4°C rise in global average surface temperatures and a 0.14–0.35 reduction in the average pH of the global surface ocean by 2100. If realized, these changes are expected to have negative consequences for reef-building corals including increased frequency and severity of coral bleaching and reduced rates of calcification and reef accretion. Much less is known regarding the independent and combined effects of temperature and pCO₂ on critical early life history processes such as fertilization. Here we show that increases in temperature (+3°C) and pCO₂ (+400 µatm) projected for this century negatively impact fertilization success of a common Indo-Pacific coral species, Acropora tenuis. While maximum fertilization did not differ among treatments, the sperm concentration required to obtain 50% of maximum fertilization increased 6- to 8- fold with the addition of a single factor (temperature or CO₂) and nearly 50- fold when both factors interact. Our results indicate that near-future changes in temperature and pCO₂ narrow the range of sperm concentrations that are capable of yielding high fertilization success in A. tenuis. Increased sperm limitation, in conjunction with adult population decline, may have severe consequences for coral reproductive success. Impaired sexual reproduction will further challenge corals by inhibiting population recovery and adaptation potential.     

Intergenerational Transfer of Specific Bacteria in Corals and Possible Implications for Offspring Fitness

Diverse and abundant bacterial populations play important functional roles in the multi-partite association of the coral holobiont. The specificity of coral-associated assemblages remains unclear, and little is known about the inheritance of specific bacteria from the parent colony to their offspring. This study investigated if broadcast spawning and brooding corals release specific and potentially beneficial bacteria with their offspring to secure maintenance across generations. Two coral species, Acropora tenuis and Pocillopora damicornis, were maintained in 0.2 μm filtered seawater during the release of their gametes and planulae, respectively. Water samples, excluding gametes and planulae, were subsequently collected, and bacterial diversity was assessed through a pyrosequencing approach amplifying a 470-bp region of the 16S rRNA gene including the variable regions 1–3. Compared to the high bacterial diversity harboured by corals, only a few taxa of bacteria were released by adult corals. Both A. tenuis and P. damicornis released similar bacteria, and the genera Alteromonas and Roseobacter were abundant in large proportions in the seawater of both species after reproduction. This study suggests that adult corals may release bacteria with their offspring to benefit the fitness in early coral life stages.     

The roles of age,parentage and environment on bacterial and algal endosymbiont communities in Acropora corals

The bacterial and microalgal endosymbiont (Symbiodiniaceae spp.) communities associated with corals have important roles in their health and resilience, yet little is known about the factors driving their succession during early coral life stages. Using 16S rRNA gene and ITS2 metabarcoding, we compared these communities in four Acropora coral species and their hybrids obtained from two laboratory crosses (Acropora tenuis × Acropora loripes and Acropora sarmentosa × Acropora florida) across the parental, recruit (7 months old) and juvenile (2 years old) life stages. We tested whether microbiomes differed between (a) life stages, (b) hybrids and purebreds, and (c) treatment conditions (ambient/elevated temperature and pCO₂). Microbial communities of early life stage corals were highly diverse, lacked host specificity and were primarily determined by treatment conditions. Over time, a winnowing process occurred, and distinct microbial communities developed between the two species pair crosses by 2 years of age, irrespective of hybrid or purebred status. These findings suggest that the microbial communities of corals have a period of flexibility prior to adulthood, which can be valuable to future research aimed at the manipulation of coral microbial communities.     

Coral microbiome diversity reflects mass coral bleaching susceptibility during the 2016 El Niño heat wave

Repeat marine heat wave‐induced mass coral bleaching has decimated reefs in Seychelles for 35 years, but how coral‐associated microbial diversity (microalgal endosymbionts of the family Symbiodiniaceae and bacterial communities) potentially underpins broad‐scale bleaching dynamics remains unknown. We assessed microbiome composition during the 2016 heat wave peak at two contrasting reef sites (clear vs. turbid) in Seychelles, for key coral species considered bleaching sensitive (Acropora muricata, Acropora gemmifera) or tolerant (Porites lutea, Coelastrea aspera). For all species and sites, we sampled bleached versus unbleached colonies to examine how microbiomes align with heat stress susceptibility. Over 30% of all corals bleached in 2016, half of which were from Acropora sp. and Pocillopora sp. mass bleaching that largely transitioned to mortality by 2017. Symbiodiniaceae ITS2‐sequencing revealed that the two Acropora sp. and P. lutea generally associated with C3z/C3 and C15 types, respectively, whereas C. aspera exhibited a plastic association with multiple D types and two C3z types. 16S rRNA gene sequencing revealed that bacterial communities were coral host‐specific, largely through differences in the most abundant families, Hahellaceae (comprising Endozoicomonas), Rhodospirillaceae, and Rhodobacteraceae. Both Acropora sp. exhibited lower bacterial diversity, species richness, and community evenness compared to more bleaching‐resistant P. lutea and C. aspera. Different bleaching susceptibility among coral species was thus consistent with distinct microbiome community profiles. These profiles were conserved across bleached and unbleached colonies of all coral species. As this pattern could also reflect a parallel response of the microbiome to environmental changes, the detailed functional associations will need to be determined in future studies. Further understanding such microbiome‐environmental interactions is likely critical to target more effective management within oceanically isolated reefs of Seychelles.     

Reproduction and distribution of two species of Goniastrea (Scleractinia) from the Great Barrier Reef province

The reproductive biology of Goniastrea aspera at Magnetic Island was compared with that of a very similar sympatric species, G. favulus as reported by Kojis and Quinn at Heron Island. The development of gametes was similar in both species, but there was no evidence for an adolescent protandrous period of development in G. aspera such as that recorded for G. favulus at Heron Island. Other reproductive differences between the two species were found in egg size and the mode of spawning. The eggs of G. aspera are smaller and more numerous than those of G. favulus. Goniastrea aspera expelled buoyant packets of eggs and sperm, while G. favulus had sticky sinking eggs which were released separately from the sperm. The spatial pattern of the two species was examined on the reef flat at Magnetic Island to determine whether the observed differences in spawning behaviour and egg buoyancy might have an effect on egg retention and the distribution of adult colonies. The results of this comparison failed to detect any difference in the degree of aggregation of the two species. This is not the result which would be expected if sticky sinking eggs helped retain developing larvae in the vicinity of the adult. These results, together with evidence from a wide range of coelenterates and observations on the larvae of G. aspera point to post spawning larval behaviour as the most likely factor in determining where these species will settle.     

Enhancing coral larval supply and seedling production using a special bundle collection system “coral larval cradle” for large‐scale coral restoration

Larval recruitment is essential for sustaining coral communities and a fundamental tool in some interventions for reef restoration. To improve larval supply and post‐settlement survival in sexually assisted coral restoration efforts, an integrated in situ collector system, the larval cradle, was designed to collect spawned gametes then culture the resulting larvae until settled on artificial substrates. The final design of the larval cradle was cylindrical, a nylon mesh structure with a volume of 9 m³, suspended in the sea and extending vertically toward the seabed. We found three key design features that improved the efficiency of the apparatus: (1) an open area of sea surface and mesh size of less than 100 μm produced high fertilization and optimal survival (>90%), (2) a special skirt‐shaped net (3 m in diameter) with a connection hose for attaching the cradle to collect bundles from many adult colonies over a wide area and at various depths, and (3) adding short square tube pieces, called square hollow sections, as a substrate for enhancing larval settlement and survival, to a larval cradle at 4 days after spawning was optimal for uniform settlement. This system allowed not only the collection of several million eggs, but also subsequent production of several thousand settled juvenile corals, without land facilities. Our design achieved several hundred times higher survival for early life stages of Acropora tenuis compared to nature.     

High rates of nitrogen fixation (acetylene reduction) on coral skeletons following bleaching mortality

Nubbins of the coral Acropora aspera were artificially bleached and nitrogen fixation (acetylene reduction) rates were measured on the developing epilithic communities. Seasonal comparisons were made between corals that died in summer of heat stress and corals that died in winter from natural cold stress. Rates of acetylene reduction from artificially bleached corals peaked at 26.66 nmol cm⁻² h⁻¹ 2 weeks after summer mortality, while rates from natural winter mortality peaked at 18.07 nmol cm⁻² h⁻¹ 12 days after coral death. Comparative rates of acetylene reduction taken from live corals and coral rubble ranged between 0.56 and 1.16 nmol cm⁻² h⁻¹, and 0.15 and 12.77 nmol cm⁻² h⁻¹, respectively. N₂-fixation rates from dead corals were up to 30 times greater than those measured on live corals. The observed increase in N₂-fixation from dead corals may increase the availability of nitrogen for use in trophic processes within the reef for an extended period following the initial mortality event. If the spatial scale over which coral mortality has occurred in past thermal bleaching events is considered the ramifications of such an increase may be substantial.     

Unexpectedly complex gradation of coral population structure in the Nansei Islands,Japan

To establish effective locations and sizes of potential protected areas for reef ecosystems, detailed information about source and sink relationships between populations is critical, especially in archipelagic regions. Therefore, we assessed population structure and genetic diversity of Acropora tenuis, one of the dominant stony coral species in the Pacific, using 13 microsatellite markers to investigate 298 colonies from 15 locations across the Nansei Islands in southwestern Japan. Genetic diversity was not significant among sampling locations, even in possibly peripheral locations. In addition, our results showed that there are at least two populations of A. tenuis in the study area. The level of genetic differentiation between these populations was relatively low, but significant between many pairs of sampling locations. Directions of gene flow, which were estimated using a coalescence‐based approach, suggest that gene flow not only occurs from south to north, but also from north to south in various locations. Consequently, the Yaeyama Islands and the Amami Islands are potential northern and southern sources of corals. On the other hand, the Miyako Islands and west central Okinawa Island are potential sink populations. The Kerama Islands and the vicinity of Taketomi Island are potential contact points of genetic subdivision of coral populations in the Nansei Islands. We found that genetic population structure of A. tenuis in the Nansei Islands is more complex than previously thought. These cryptic populations are very important for preserving genetic diversity and should be maintained.     

Water flow controls distribution and feeding behavior of two co-occurring coral reef fishes: II. Laboratory experiments

The chaenopsid blenny Acanthemblemaria spinosa occupies topographically high locations on coral reefs where flow speeds and turbulence are frequently greater than those experienced by its congener, A. aspera, which occupies locations close to the reef surface. To investigate the adaptive mechanisms resulting in this microhabitat differentiation, the foraging effort and success of these fishes were determined in laboratory flumes that produced flow conditions approximating those experienced in the field. Individual fish were subjected to unidirectional (smooth and turbulent) and oscillatory flows while they fed on calanoid copepods, Acartia tonsa, whose vulnerability to predation varies with water flow. In unidirectional flow both blenny species had their greatest foraging success at intermediate flow speeds (ca. 10 cm s⁻¹) and under turbulent flow. Under all conditions, Acanthemblemaria spinosa exhibited greater foraging effort and attacked at greater distances, greater mean water speeds, and in oscillatory flow, over a greater proportion of the wave cycle than did A. aspera. A. spinosa also exhibited greater foraging success under turbulent flow conditions. These differences in feeding patterns allow A. spinosa, with its higher metabolic rate, to occupy the more energetic higher locations in corals where planktonic food is more abundant. A. aspera occupies the poorer quality habitat in terms of planktonic food availability but its lower metabolic rate allows it to thrive there. Consequently, these species divide the resource in short supply, i.e., shelter holes, based on their differing abilities to capture prey in energetic water conditions in conjunction with their differing food energy requirements. Communicated by Biology Editor Dr. Mark McCormick     

Natural inducers for coral larval metamorphosis

 Coral gametes from Acropora millepora (Ehrenberg, 1834) and from multi-species spawning slicks provided larvae for use in metamorphosis assays with a selection of naturally occurring inducer chemicals. Four species of crustose coralline algae, one non-coralline crustose alga and two branching coralline algae induced larval metamorphosis. However, one additional species of branching coralline algae did not produce a larval response. Metamorphosis was also observed when larvae were exposed to skeleton from the massive coral Goniastrea retiformis (Lamarck, 1816) and to calcified reef rubble, demonstrating metamorphosis is possible in the absence of encrusting algae. Chemical extracts from these algae and the coral skeleton, obtained using either decalcification or simple methanol extraction procedures, also contained active inducers. These results extend the number of crustose algal species known to induce coral metamorphosis, suggest that some inducers may not necessarily be strongly associated with the calcified algal cell walls, and indicate that inducer sources in reef habitats may be more diverse than previously reported. Accepted: 21 May 1999     

Long-term laboratory culture of symbiotic coral juveniles and their use in eco-toxicological study

Reef-building (or hermatypic) corals live in mutualistic symbiosis with the dinoflagellates Symbiodinium spp. (Alveolata, Dinophyceae, Gymnodiniales), and contribute to the accretion of coral reefs. Due to the difficulty in culturing them in laboratories, these ecologically important cnidarians have not been characterized extensively in physiological, biochemical, molecular and toxicological experiments. The present study was conducted to develop a model symbiosis system for long-term experimental analyses of a symbiotic coral. Aposymbiotic (symbiont-free) juveniles of the hermatypic coral Acropora tenuis were infected with three Symbiodinium strains, and the resulting symbiotic corals were examined for growth and maintenance of the symbiosis for approx. three months. Of the tested Symbiodinium cell lines, CCMP2467 (clade A1) inhabited the host the most densely, and the population in hospite did not decline over the period of three months in laboratory culture. The CCMP2467-inhabited juveniles outgrew the populations infected with the other two strains and aposymbiotic specimens. The A. tenuis juveniles in symbiosis with CCMP2467 cells were used in eco-toxicological tests to study long-term effects of two commonly used biocides (tributyltin-chloride and diuron). Delay in growth was observed after exposing the symbiotic juveniles to the two chemicals for approx. 50 days at the nominal concentrations of 0.4 and 1 μg/L, respectively.     

Thermal history influences lesion recovery of the threatened Caribbean staghorn coral Acropora cervicornis under heat stress

Anthropogenic climate change is the biggest threat to coral reefs, but reef restoration efforts are buying time for these ecosystems. Lesion recovery, which can be a determinant of colony survival, is particularly important for restored species. Here, we evaluate lesion recovery of 18 genets of Acropora cervicornis from Florida reefs with different thermal regimes in a temperature challenge experiment. Genets demonstrated significant variability in healing, which greatly slowed under heat stress. Only 35% of fragments healed at 31.5 °C compared to 99% at 28 °C. Donor reef thermal regime significantly influenced lesion recovery under heat stress with corals from warmer reefs demonstrating greater healing than corals from cooler reefs, but did not influence recovery under ambient conditions. These findings should encourage practitioners to utilize rapidly healing genets, avoid fragmentation in high temperatures, and incorporate assisted relocation by moving corals from warmer to cooler reefs, where they might succeed under future climate conditions.

     

Feasibility of early outplanting of sexually propagated Acropora verweyi for coral reef restoration demonstrated in the Philippines

Over the last 20 years, coral sexual propagation techniques for reef restoration have been steadily developed and improved. However, these techniques involve considerable time and costs to grow coral propagules. There is a need to examine the optimal size of juvenile corals for outplantation. Here, we outplanted sexually propagated small (3–5 mm diameter) and large (10–15 mm diameter) Acropora verweyi corals at 4 months after fertilization at two sites in northwestern Philippines, and compared their survival and radial growth rate after a year. A. verweyi coral juveniles (n = 240) exhibited an overall mean survival of 29.5% and growth rate of 11.12 ± 6.2 mm/year (mean ± SD). Large colonies had a significantly higher growth rate than smaller colonies. Although survivorship of large juveniles was significantly better than that of the smaller ones at one site, it did not differ significantly at the other. Each 4‐month‐old coral cost US$1.52 to produce, while the cost of each of the outplanted juveniles (n = 240) was about US$2.67, whereas the cost of each survivor about a year after outplantation was US$11.47. Results suggest that A. verweyi reared in ex situ nurseries for only 4 months can survive reasonably well when outplanted onto coral reefs.     

Inducing broadcast coral spawning ex situ: Closed system mesocosm design and husbandry protocol

For many corals, the timing of broadcast spawning correlates strongly with a number of environmental signals (seasonal temperature, lunar, and diel cycles). Robust experimental studies examining the role of these putative cues in triggering spawning have been lacking until recently because it has not been possible to predictably induce spawning in fully closed artificial mesocosms. Here, we present a closed system mesocosm aquarium design that utilizes microprocessor technology to accurately replicate environmental conditions, including photoperiod, seasonal insolation, lunar cycles, and seasonal temperature from Singapore and the Great Barrier Reef (GBR), Australia. Coupled with appropriate coral husbandry, these mesocosms were successful in inducing, for the first time, broadcast coral spawning in a fully closed artificial ex situ environment. Four Acropora species (A. hyacinthus, A. tenuis, A. millepora, and A. microclados) from two geographical locations, kept for over 1 year, completed full gametogenic cycles ex situ. The percentage of colonies developing oocytes varied from ~29% for A. hyacinthus to 100% for A. millepora and A. microclados. Within the Singapore mesocosm, A. hyacinthus exhibited the closest synchronization to wild spawning, with all four gravid colonies releasing gametes in the same lunar month as wild predicted dates. Spawning within the GBR mesocosm commenced at the predicted wild spawn date but extended over a period of 3 months. Gamete release in relation to the time postsunset for A. hyacinthus, A. millepora, and A. tenuis was consistent with time windows previously described in the wild. Spawn date in relation to full moon, however, was delayed in all species, possibly as a result of external light pollution. The system described here could broaden the number of institutions on a global scale, that can access material for broadcast coral spawning research, providing opportunities for institutions distant from coral reefs to produce large numbers of coral larvae and juveniles for research purposes and reef restoration efforts.     

More than one genotype: how common is intracolonial genetic variability in scleractinian corals?

In recent years, a few colonial marine invertebrates have shown intracolonial genetic variability, a previously unreported phenomenon. Intracolonial genetic variability describes the occurrence of more than a single genotype within an individual colony. This variability can be traced back to two underlying processes: chimerism and mosaicism. Chimerism is the fusion of two or more individuals, whereas mosaicism mostly derives from somatic cell mutations. Until now, it remained unclear to what degree the ecologically important group of hermatypic (reef building) corals might be affected. We investigate the occurrence of intracolonial genetic variability in five scleractinian corals: Acropora florida, Acropora hyacinthus, Acropora sarmentosa, Pocillopora species complex and Porites australiensis. The main focus was to test different genera for the phenomenon via microsatellite markers and to distinguish which underlying process caused the genetic heterogeneity. Our results show that intracolonial genetic variability was common (between 46.6% for A. sarmentosa and 23.8% for P. species complex) in all tested corals. The main process was mosaicism (69 cases of 222 tested colonies), but at least one chimera existed in every species. This suggests that intracolonial genetic variability is widespread in scleractinian corals and could challenge the view of a coral colony as an individual and therefore a unit of selection. However, it might also hold potential for colony survival under rapidly changing environmental conditions.