Increasing frequency and severity of disturbances is causing global degradation of coral reef ecosystems. This study examined temporal changes in live coral cover and coral composition in the central Maldives from 1997 to 2016, encompassing two bleaching events, a tsunami, and an outbreak of Acanthaster planci. We also examined the contemporary size structure for five dominant coral taxa (tabular Acropora, Acropora muricata, Acropora humilis, Pocillopora spp, and massive Porites). Total coral cover increased throughout the study period, with marked increases following the 1998 mass-bleaching. The relative abundance of key genera has changed through time, where Acropora and Pocillopora (which are highly susceptible to bleaching) were under-represented following 1998 mass-bleaching but increased until outbreaks of A. planci in 2015. The contemporary size-structure for all coral taxa was dominated by larger colonies with peaked distributions suggesting that recent disturbances had a disproportionate impact on smaller colonies, or that recruitment is currently limited. This may suggest that coral resilience has been compromised by recent disturbances, and further bleaching (expected in 2016) could lead to highly protracted recovery times. We showed that Maldivian reefs recovered following the 1998 mass-bleaching event, but it took up to a decade, and ongoing disturbances may be eroding reef resilience.

The number and prevalence of coral diseases/syndromes are increasing worldwide. Dark Spot Syndrome (DSS) afflicts numerous coral species and is widespread throughout the Caribbean, yet there are no known causal agents. In this study we aimed to characterise the microbial communities (bacteria and fungi) associated with DSS lesions affecting the coral Stephanocoenia intersepta using nonculture molecular techniques. Bacterial diversity of healthy tissues (H), those in advance of the lesion interface (apparently healthy AH), and three sizes of disease lesions (small, medium, and large) varied significantly (ANOSIM R  = 0.052 p<0.001), apart from the medium and large lesions, which were similar in their community profile. Four bacteria fitted into the pattern expected from potential pathogens; namely absent from H, increasing in abundance within AH, and dominant in the lesions themselves. These included ribotypes related to Corynebacterium (KC190237), Acinetobacter (KC190251), Parvularculaceae (KC19027), and Oscillatoria (KC190271). Furthermore, two Vibrio species, a genus including many proposed coral pathogens, dominated the disease lesion and were absent from H and AH tissues, making them candidates as potential pathogens for DSS. In contrast, other members of bacteria from the same genus, such as V. harveyii were present throughout all sample types, supporting previous studies where potential coral pathogens exist in healthy tissues. Fungal diversity varied significantly as well, however the main difference between diseased and healthy tissues was the dominance of one ribotype, closely related to the plant pathogen, Rhytisma acerinum, a known causal agent of tar spot on tree leaves. As the corals’ symbiotic algae have been shown to turn to a darker pigmented state in DSS (giving rise to the syndromes name), the two most likely pathogens are R. acerinum and the bacterium Oscillatoria, which has been identified as the causal agent of the colouration in Black Band Disease, another widespread coral disease.

Climate change is the greatest threat to coral reef ecosystems. In particular, increasing ocean temperatures are causing severe and widespread coral bleaching, contributing to extensive coral loss and degradation of coral reef habitats globally. Effects of coral bleaching are not however, equally apportioned among different corals, leading to shifts in population and community structure. This study explored variation in bleaching susceptibility and mortality associated with the 2016 severe mass bleaching in the Central Maldives Archipelago. Five dominant coral taxa (tabular Acropora , Acropora humilis , Acropora muricata , Pocillopora and massive Porites ) were surveyed in February 2016 and October 2017 to test for changes in abundance and size structure. Substantial taxonomic differences in rates of mortality were observed; the most severely affected taxa, Acropora , were virtually extirpated during the course of this study, whereas some other taxa (most notably, massive Porites ) were relatively unaffected. However, even the least affected corals exhibited marked changes in population structure. In February 2016 (prior to recent mass-bleaching), size-frequency distributions of all coral taxa were dominated by larger size classes with over-centralized, peaked distributions (negatively skewed with positive kurtosis) reflecting a mature population structure. In October 2017, after the bleaching, coral populations were dominated by smaller and medium size classes, reflecting high levels of mortality and injury among larger coral colonies. Pronounced changes in coral populations and communities in the Maldives, caused by coral bleaching and other disturbances (outbreaks of crown-of-thorns starfish and sedimentation), will constrain recovery capacity, further compounding upon recent coral loss.

No abstract

Pronounced differences exist in the biodiversity and structure of coral reef assemblages with increasing distance from shore, which may be expected given marked cross-shelf gradients in environmental conditions. Cross-shelf variation in the abundance of coral reef organisms is likely to be caused, at least in part, by differences in demography (e.g., growth and survival), though this has rarely been tested. This study quantified growth of three distinct branching coral taxa (Acropora nasuta, Pocillopora spp. and Stylophora pistillata) at six locations on Australia’s Great Barrier Reef (GBR), encompassing inshore, mid-shelf and outer-shelf reefs. Replicate colonies (0–15 colonies per species, per reef) were stained using Alizarin Red in December 2015 and retrieved one year later to quantify linear extension on replicate branches for each colony. Annual linear extension varied within and among coral taxa, with pronounced differences among reefs. For A. nasuta. and S. pistillata, growth rates were highest at one of the inshore reefs, Orpheus Island. However, inter-reef differences in linear extension were not explained by shelf position. Based on differences in skeletal density, which did vary according to shelf position, branching corals at the inshore sites may actually have higher rates of calcification compared to conspecifics on mid-and outer-shelf reefs. This study shows that growth of branching corals is not lower at inshore sites (and perhaps even higher) compared to sites at mid-shelf and outer reefs, despite generally higher levels of sedimentation and turbidity.

Crown-of-thorns starfish (CoTS; Acanthaster spp.) are among the most extensively studied coral reef taxa, largely owing to their devastating impacts on live coral cover during population outbreaks. Much of this research has however, been conducted in the western Pacific, although it is now apparent that there are several distinct species of Acanthaster spp. across the Indo-Pacific. The purpose of this study was to test for biogeographical variation in behaviour, comparing between Acanthaster planci at Lankanfushi Island in the Maldives and Acanthaster cf. solaris at Rib Reef on Australia's Great Barrier Reef. The extent to which CoTS were exposed (cf. concealed within or beneath coral substrates) was substantially higher (63.14%) for A. planci at Lankanfushi Island, compared to 28.55% for A. cf. solaris at Rib Reef, regardless of time of day. More importantly, only 52% of individuals were exposed at night at Rib Reef compared to >97% at reefs around Lankanfushi Island. Biogeographic variation in the behaviour of Acanthaster spp. was independent of differences in the size structure of starfish and coral cover at specific study sites, but may be attributable to other environmental factors such as habitat complexity or prey availability. This is the first study to explicitly test for biogeographical differences in the biology and behaviour of Acanthaster spp., potentially linked to species-specific differences in the causes and explanations of population outbreaks. However, we did not find evidence at this stage of differences in behavior among regions, rather behavioural differences observed were most likely products of different environments.

No abstract

Obligate coral feeders such as many members of the Chaetodontidae family (also known as butterflyfish) often show strong preferences for particular coral species. This is thought to have evolved through natural selection as an energy-maximising strategy. Although some species remain as highly specialised feeders throughout their lifetime, many corallivores show a degree of dietary versatility when food abundance is limited; a strategy described by the optimal foraging theory. This study aimed to examine if, within-reef differences in the feeding regime and territory size of the Triangle Butterflyfish Chaetodon triangulum occurred, as a function of resource availability. Results showed that the dietary specialisation of C. triangulum was significant in both areas of low and high coral cover (χL22 = 2.52 x 102, P<0.001 and χL22 = 3.78 x 102, P<0.001 respectively). Resource selection functions (RSFs), calculated for the two main sites of contrasting coral assemblage, showed that in the resource-rich environments, only two Genera (Acropora and Pocillopora) were preferentially selected for, with the majority of other corals being actively ‘avoided’. Conversely, in territories of lower coral coverage, C. triangulum was being less selective in its prey choice and consuming corals in a more even distribution with respect to their availability. Interestingly, coral cover appeared to show no significant effect on feeding rate, however it was a primary determinant of territory size. The findings of the study agree with the predictions of the optimal foraging theory, in that where food supply is scarce, dietary specialisation is minimised and territory size increased. This results in maximising energy intake. This study represents the first scientific evidence that C. triangulum is an obligate corallivore and, as with many other butterflyfish, is therefore dependent on healthy scleractinian corals for survival.