James R. Guest

Contrasting Patterns of Coral Bleaching Susceptibility in 2010 Suggest an Adaptive Response to Thermal Stress

Background Coral bleaching events vary in severity, however, to date, the hierarchy of susceptibility to bleaching among coral taxa has been consistent over a broad geographic range and among bleaching episodes. Here we examine the extent of spatial and temporal variation in thermal tolerance among scleractinian coral taxa and between locations during the 2010 thermally induced, large-scale bleaching event in South East Asia. Methodology/Principal Findings Surveys to estimate the bleaching and mortality indices of coral genera were carried out at three locations with contrasting thermal and bleaching histories. Despite the magnitude of thermal stress being similar among locations in 2010, there was a remarkable contrast in the patterns of bleaching susceptibility. Comparisons of bleaching susceptibility within coral taxa and among locations revealed no significant differences between locations with similar thermal histories, but significant differences between locations with contrasting thermal histories (Friedman = 34.97; p<0.001). Bleaching was much less severe at locations that bleached during 1998, that had greater historical temperature variability and lower rates of warming. Remarkably, Acropora and Pocillopora, taxa that are typically highly susceptible, although among the most susceptible in Pulau Weh (Sumatra, Indonesia) where respectively, 94% and 87% of colonies died, were among the least susceptible in Singapore, where only 5% and 12% of colonies died. Conclusions/Significance The pattern of susceptibility among coral genera documented here is unprecedented. A parsimonious explanation for these results is that coral populations that bleached during the last major warming event in 1998 have adapted and/or acclimatised to thermal stress. These data also lend support to the hypothesis that corals in regions subject to more variable temperature regimes are more resistant to thermal stress than those in less variable environments.

Seasonal reproduction in equatorial reef corals

Summary Populations of broadcasting reef corals often exhibit marked reproductive seasonality and spawning synchrony. Within speciose coral assemblages there is often considerable overlap of spawning periods among species, resulting in multi-species spawning events (or “mass coral spawning”). Earlier geographical comparisons of reproductive synchrony suggested a reduction in the extent of mass spawning with proximity to the equator. In contrast, recent studies have revealed that reproductive seasonality and spawning synchrony within and among species are features of coral assemblages on equatorial reefs. Here we review the proposed causes of synchronous spawning among reef corals and discuss how recent findings about reproduction of corals from Singapores equatorial reefs shed light on these various theories. Sexual reproduction in broadcasting corals requires external fertilization, so reproductive seasonality (leading to spawning synchrony) within populations is probably highly adaptive because synchrony increases the chances of gametes meeting, enhances the possibility of outbreeding and may swamp opportunistic predators. No coastal location is truly aseasonal, with even equatorial reefs experiencing marked (albeit less pronounced) rhythmic changes in sea surface temperature. Consequently, if species respond similarly but independently to timing cues to synchronize reproduction within populations, mass spawning is just as likely to occur in equatorial coral assemblages as it is at higher latitudes.

Reproductive seasonality in an equatorial assemblage of scleractinian corals

[Extract] Multi-specific, synchronous spawning of scleractinian corals was first documented on Australia’s Great Barrier Reef (GBR) in the early 1980s (Harrison et al. 1984). There, over a period of eight nights in late spring, at least 133 coral species released their gametes for external fertilisation and more than 30 species spawned on the same night on one reef (Willis et al. 1985; Babcock et al. 1986). However, the causal factors responsible for this remarkable phenomenon are still not clearly understood (see review in Harrison and Wallace 1990). Comparisons of reproductive patterns—from sites at a variety of latitudes, with contrasting seasonal and environmental conditions—can help to elucidate the ‘‘ultimate’’ causes of reproductive seasonality and synchrony (Oliver et al. 1988). Early examples of such comparisons showed that multi-species reproductive synchrony is not a characteristic of all coral communities (Richmond and Hunter 1990). In particular, studies in parts of the Red Sea and the Caribbean found that corals at those sites tended to spawn asynchronously (Shlesinger and Loya 1984; Szmant 1986). This lack of synchrony was attributed to a reduction in environmental seasonality and a narrowing in the ranges of certain environmental parameters, in particular annual sea surface temperatures (Richmond and Hunter 1990) and tidal amplitudes (the difference between mean low water springs and mean high water springs) (Oliver et al. 1988).

Histopathological methods for the investigation of microbial communities associated with disease lesions in reef corals

Aims: To determine the spatial structure of microbial communities associated with disease lesions of reef corals (Scleractinia).

Resilience of Coral-Associated Bacterial Communities Exposed to Fish Farm Effluent

Background The coral holobiont includes the coral animal, algal symbionts, and associated microbial community. These microbes help maintain the holobiont homeostasis; thus, sustaining robust mutualistic microbial communities is a fundamental part of long-term coral reef survival. Coastal pollution is one major threat to reefs, and intensive fish farming is a rapidly growing source of this pollution. Methodology & Principal Findings We investigated the susceptibility and resilience of the bacterial communities associated with a common reef-building coral, Porites cylindrica, to coastal pollution by performing a clonally replicated transplantation experiment in Bolinao, Philippines adjacent to intensive fish farming. Ten fragments from each of four colonies (total of 40 fragments) were followed for 22 days across five sites: a well-flushed reference site (the original fragment source); two sites with low exposure to milkfish (Chanos chanos) aquaculture effluent; and two sites with high exposure. Elevated levels of dissolved organic carbon (DOC), chlorophyll a, total heterotrophic and autotrophic bacteria abundance, virus like particle (VLP) abundances, and culturable Vibrio abundance characterized the high effluent sites. Based on 16S rRNA clone libraries and denaturing gradient gel electrophoresis (DGGE) analysis, we observed rapid, dramatic changes in the coral-associated bacterial communities within five days of high effluent exposure. The community composition on fragments at these high effluent sites shifted towards known human and coral pathogens (i.e. Arcobacter, Fusobacterium, and Desulfovibrio) without the host corals showing signs of disease. The communities shifted back towards their original composition by day 22 without reduction in effluent levels. Significance This study reveals fish farms as a likely source of pathogens with the potential to proliferate on corals and an unexpected short-term resilience of coral-associated bacterial communities to eutrophication pressure. These data highlight a need for improved aquaculture practices that can achieve both sustainable industry goals and long-term coral reef survival.

Multi-species spawning of corals in north-western Philippines

The Philippines has more than 30,000 km of reef area and hosts some of the world’s most diverse and endangered coral communities, however there is little information on patterns of coral reproduction (Bermas et al. 1992) and to date there are no published accounts of direct spawning observations. Sampling to determine the reproductive state of Acropora species and in situ observations of coral spawning was conducted in 2006 and 2007 at sites close to the Bolinao Marine Laboratory (BML) in northwestern Luzon, (16 22¢N 119 54¢E). Prior to the full moon in March 2006, 22 sampled Acropora species (67% of colonies, n = 208) contained white or pigmented oocytes large enough to be visible in branches that were fractured artificially underwater (for methods see Baird et al. 2002) suggesting a seasonal peak in reproduction from March to May. Night dives were carried out during the week following the full moons of 15 March and 14 April 2006; and 2 April, 2 May and 1 June 2007. Multi-species coral spawning was observed during all of the months of observation except in April 2007. Across all months of observation, a total of at least 36 scleractinian species belonging to 14 genera and 7 families (Acroporidae, Mussidae, Agariciidae, Faviidae, Oculinidae, Merulinidae and Poritidae) broadcast spawned (Fig. 1), with a maximum of 13 species observed on the fifth night after full moon in May 2007. Further studies are required to establish the extent of spawning at other times of the year and the reproductive patterns of corals elsewhere in the Philippines.

Sexual systems in scleractinian corals: an unusual pattern in the reef-building species Diploastrea heliopora

The sexual system in corals refers to the spatial and temporal pattern of sexual function within an individual coral polyp, colony or population. Although information on sexual systems now exists for over 400 scleractinian species, data are still lacking for some important reef-building taxa. The vast majority of scleractinians are either simultaneous hermaphrodites or gonochoric with other sexual systems rarely occurring. Diploastrea heliopora is one of the most ubiquitous and easily recognised reef-building species in the Indo-West Pacific; however, surprisingly little is known about its reproductive biology. The aim of the present study was to examine the reproductive biology of D. heliopora colonies on chronically impacted, equatorial reefs south of Singapore. Here we show that in Singapore, D. heliopora is a broadcast spawner with predominantly gonochoric polyps. Colonies, however, contained male, female and a low proportion of cosexual polyps during the 14-month sampling period. The most plausible explanation for this is that polyps switch sexes with oogenic and spermatogenic cycles occasionally overlapping. This leads to colony level alternation of sex function within and between breeding seasons. While this sexual system is atypical for scleractinians, it supports molecular evidence that D. heliopora is phylogenetically distinct from species formerly in the family Faviidae.

Coral community response to bleaching on a highly disturbed reef

While many studies of coral bleaching report on broad, regional scale responses, fewer examine variation in susceptibility among coral taxa and changes in community structure, before, during and after bleaching on individual reefs. Here we report in detail on the response to bleaching by a coral community on a highly disturbed reef site south of mainland Singapore before, during and after a major thermal anomaly in 2010. To estimate the capacity for resistance to thermal stress, we report on: a) overall bleaching severity during and after the event, b) differences in bleaching susceptibility among taxa during the event, and c) changes in coral community structure one year before and after bleaching. Approximately two thirds of colonies bleached, however, post-bleaching recovery was quite rapid and, importantly, coral taxa that are usually highly susceptible were relatively unaffected. Although total coral cover declined, there was no significant change in coral taxonomic community structure before and after bleaching. Several factors may have contributed to the overall high resistance of corals at this site including Symbiodinium affiliation, turbidity and heterotrophy. Our results suggest that, despite experiencing chronic anthropogenic disturbances, turbid shallow reef communities may be remarkably resilient to acute thermal stress.

How Reefs Respond to Mass Coral Spawning

A mass coral-spawning event at the Great Barrier Reef provided a natural experiment for studying energy and nutrient dynamics of the coral reef.

From molecules to moonbeams: Spawning synchrony in coral reef organisms

Summary A mini-symposium at the 11th International Coral Reef Symposium highlighted significant advances towards understanding the factors controlling reproductive timing and spawning synchrony in coral reef organisms. Studies of the phenology of reef organisms are finally moving beyond the purely descriptive and researchers are starting to explore the molecular mechanisms underpinning spawning synchrony. An increasing geographical focus of research, in particular much novel work from the centre of coral reef diversity, is enabling rigorous examination of latitudinal gradients of spawning synchrony and the role of environmental factors such as sea temperature and insolation in regulating spawning timing.