Climate change and the Great Barrier Reef: a vulnerability assessment

Reef-building corals (Order Scleractinia Class Anthozoa) form extensive skeletons of calcium carbonate (limestone), depositing enough material over time to form vast reef structures that may be easily seen from space. The majority of reef-building corals are hard (stony) scleractinian corals. Many octocorals (especially soft corals in the family Alcyoniidae and the blue coral Heliopora) and some hydrozoan corals (such as Millepora) also contribute to reef-building. Corals form the framework of reef structures, while other organisms such as calcareous algae (especially red coralline algae) play a key role in cementing and consolidating the reef framework. This chapter focuses on the vulnerability of reef-building corals to climate change. The implications of climate change for macroalgae are covered in chapter 7 and a broader treatment of reef processes is provided in chapter 17.This is Chapter 10 of Climate change and the Great Barrier Reef: a vulnerability assessment. The entire book can be found at http://hdl.handle.net/11017/13

High genetic differentiation and cross-shelf patterns of genetic diversity among Great Barrier Reef populations of Symbiodinium

The resilience of Symbiodinium harboured by corals is dependent on the genetic diversity and extent of connectivity among reef populations. This study presents\ud genetic analyses of Great Barrier Reef (GBR) populations\ud of clade C Symbiodinium hosted by the alcyonacean coral,\ud Sinularia Xexibilis. Allelic variation at four newly developed microsatellite loci demonstrated that Symbiodinium\ud populations are genetically diVerentiated at all spatial\ud scales from 16 to 1,360 km (pairwise ST = 0.01–0.47,\ud mean = 0.22); the only exception being two neighbouring\ud populations in the Cairns region separated by 17 km. This\ud indicates that gene Xow is restricted for Symbiodinium C\ud hosted by S. Xexibilis on the GBR. Patterns of population\ud structure reXect longshore circulation patterns and limited cross-shelf mixing, suggesting that passive transport by currents is the primary mechanism of dispersal in Symbiodinium types that are acquired horizontally. There\ud was no correlation between the genetic structure of Symbiodinium populations and their host S. Xexibilis, most likely because diVerent factors aVect the dispersal and recruitment of each partner in the symbiosis. The genetic diversity of these Symbiodinium reef populations is on average 1.5 times lower on inshore reefs than on oVshore reefs. Lower inshore diversity may reXect the impact of recent bleaching events on Sinularia assemblages, which have been more widespread and severe on inshore reefs, but may also have been shaped by historical sea level Xuctuations or recent migration patterns

Microsatellite allele sizes alone are insufficient to delineate species boundaries in Symbiodinium

Symbiodinium are a diverse group of unicellular dinoflagellates that are important nutritional symbionts of reef-building corals. Symbiodinium putative species ('types') are commonly identified with genetic markers, mostly nuclear and chloroplast encoded ribosomal DNA regions. Population genetic analyses using microsatellite loci have provided insights into Symbiodinium biogeography, connectivity and phenotypic plasticity, but are complicated by: (i) a lack of consensus criteria used to delineate inter- vs. intragenomic variation within species; and (ii) the high density of Symbiodinium in host tissues, which results in single samples comprising thousands of individuals. To address this problem, Wham & LaJeunesse (2016) present a method for identifying cryptic Symbiodinium species from microsatellite data based on correlations between allele size distributions and nongeographic genetic structure. Multilocus genotypes that potentially do not recombine in sympatry are interpreted as secondary 'species' to be discarded from downstream population genetic analyses. However, Symbiodinium species delineations should ideally incorporate multiple physiological, ecological and molecular criteria. This is because recombination tests may be a poor indicator of species boundaries in Symbiodinium due to their predominantly asexual mode of reproduction. Furthermore, discontinuous microsatellite allele sizes in sympatry may be explained by secondary contact between previously isolated populations and by mutations that occur in a nonstepwise manner. Limitations of using microsatellites alone to delineate species are highlighted in earlier studies that demonstrate occasional bimodal distributions of allele sizes within Symbiodinium species and considerable allele size sharing among Symbiodinium species. We outline these issues and discuss the validity of reinterpretations of our previously published microsatellite data from Symbiodinium populations on the Great Barrier Reef (Howells et al. 2013)

Examination of species boundaries in the Acropora cervicornis group (Scleractinia, Cnidaria) using nuclear DNA sequence analyses

Although Acropora is the most species-rich genus of the scleractinian (stony) corals, only three species occur in the Caribbean: A. cervicornis, A. palmata and A. prolifera. Based on overall coral morphology, abundance and distribution patterns, it has been suggested that A. prolifera may be a hybrid between A. cervicornis and A. palmata. The species boundaries among these three morphospecies were examined using DNA sequence analyses of the nuclear Pax-C 46/47 intron and the ribosomal DNA Internal Transcribed Spacer (ITS1 and ITS2) and 5.8S regions. Moderate levels of sequence variability were observed in the ITS and 5.8S sequences (up to 5.2% overall sequence difference), but variability within species was as large as between species and all three species carried similar sequences. Since this is unlikely to represent a shared ancestral polymorphism, the data suggest that introgressive hybridization occurs among the three species. For the Pax-C intron, A. cervicornis and A. palmata had very distinct allele frequencies and A. cervicornis carried a unique allele at a frequency of 0.769 (although sequence differences between alleles were small). All A. prolifera colonies examined were heterozygous for the Pax-C intron, whereas heterozygosity was only 0.286 and 0.333 for A. cervicornis and A. palmata, respectively. These data support the hypothesis that A. prolifera is the product of hybridization between two species that have a different allelic composition for the Pax-C intron, i.e. A. cervicornis and A. palmata. We therefore suggest that A. prolifera is a hybrid between A. cervicornis and A. palmata, which backcrosses with the parental species at low frequency

Location and disturbance affect population genetic structure in four coral species of the genus Acropora on the Great Barrier Reef

The impact of a mass bleaching event on temporal and spatial population genetic structure in 4 scleractinian coral species in the Acropora aspera group was studied around the Palm Islands in the central Great Barrier Reef. Species status of sympatric populations of 2 of the 4 species, A. millepora and A. spathulata, was confirmed by the population genetic data; these species have recently been separated based on morphological and breeding characters. Spatial analyses of population samples from 2004 detected differences in the level of gene flow among locations. No significant genetic differentiation was inferred between conspecific populations at Orpheus and Pelorus Islands, which are both located in the northern part of the island group and separated by ~1000 m. In contrast, all populations at Fantome Island were genetically differentiated, despite this island being located only 11 km south. Sampling of A. millepora and A. pulchra in the year prior to the 1998 mass bleaching event enabled a temporal comparison across this event. The genetic composition of these populations changed between 1997 and 2004, but patterns of genetic differentiation among locations were similar in 1997 and 2004. Extensive mortality of these species following the 1998 bleaching event did not cause an apparent reduction in genetic diversity and identical multi-locus genotypes were encountered in both temporal samples, suggesting that re-growth of surviving genotypes contributed to the recovery of these populations. Comparisons among the 4 study species revealed lower genetic diversity in A. papillare, consistent with its low abundance throughout its distributional range

Allorecognition maturation in the broadcast-spawning coral Acropora millepora

Many sessile marine invertebrates discriminate self from non-self with great precision, but maturation of allorecognition generally takes months to develop in juveniles. Here, we compare the development of allorecognition in full-sibling, half-sibling and non-sibling contact reactions between newly settled juveniles of the broadcast-spawning coral Acropora millepora on the Great Barrier Reef (Australia). Absence of a rejection response showed that A. millepora lacks a mature allorecognition system in the first 2 months post-settlement. From thereon, incompatibilities were observed between juveniles, their level of relatedness (i.e. full-, half- and non-sibling status) governing the rate of allorecognition maturation. All contact reactions between non-siblings resulted in rejections by 3 months post-settlement, whereas the expression of allorecognition took at least 5 months between half-siblings and longer than 13 months for some full-siblings. Approximately 74 % of fused full-siblings (n = 19) persisted as chimeras at 11 months, thus maturation of allorecognition in this spawning coral appeared to be slower (>13 months) than in brooding corals (∼4 months). We hypothesize that late maturation of allorecognition may contribute to flexibility in Symbiodinium uptake in corals with horizontal transmission, and could allow fusions and chimera formation in early ontogeny, which potentially enable rapid size increase through fusion

Larval retention and connectivity among populations of corals and reef fishes: history, advances and challenges

The extent of larval dispersal on coral reefs has\ud important implications for the persistence of coral reef\ud metapopulations, their resilience and recovery from an\ud increasing array of threats, and the success of protective\ud measures. This article highlights a recent dramatic increase\ud in research effort and a growing diversity of approaches to\ud the study of larval retention within (self-recruitment) and\ud dispersal among (connectivity) isolated coral reef populations. Historically, researchers were motivated by\ud alternative hypotheses concerning the processes limiting\ud populations and structuring coral reef assemblages,\ud whereas the recent impetus has come largely from the need\ud to incorporate dispersal information into the design of no-take marine protected area (MPA) networks. Although the\ud majority of studies continue to rely on population genetic\ud approaches to make inferences about dispersal, a wide\ud range of techniques are now being employed, from smallscale\ud larval tagging and paternity analyses, to large-scale\ud biophysical circulation models. Multiple approaches are\ud increasingly being applied to cross-validate and provide\ud more realistic estimates of larval dispersal. The vast\ud majority of empirical studies have focused on corals and\ud fishes, where evidence for both extremely local scale patterns of self-recruitment and ecologically significant\ud connectivity among reefs at scales of tens of kilometers\ud (and in some cases hundreds of kilometers) is accumulating.\ud Levels of larval retention and the spatial extent of\ud connectivity in both corals and fishes appear to be largely\ud independent of larval duration or reef size, but may be\ud strongly influenced by geographic setting. It is argued that\ud high levels of both self-recruitment and larval import can\ud contribute to the resilience of reef populations and MPA\ud networks, but these benefits will erode in degrading reef\ud environments