Reef-building corals possess a range of acclimatisation and adaptation mechanisms to respond to seawater temperature increases. In some corals, thermal tolerance increases through community composition changes of their dinoflagellate endosymbionts (Symbiodinium spp.), but this mechanism is believed to be limited to the Symbiodinium types already present in the coral tissue acquired during early life stages. Compelling evidence for symbiont switching, that is, the acquisition of novel Symbiodinium types from the environment, by adult coral colonies, is currently lacking. Using deep sequencing analysis of Symbiodinium rDNA internal transcribed spacer 2 (ITS2) PCR amplicons from two pocilloporid coral species, we show evidence consistent with de novo acquisition of Symbiodinium types from the environment by adult corals following two consecutive bleaching events. Most of these newly detected symbionts remained in the rare biosphere (background types occurring below 1% relative abundance), but one novel type reached a relative abundance of ~33%. Two de novo acquired Symbiodinium types belong to the thermally resistant clade D, suggesting that this switching may have been driven by consecutive thermal bleaching events. Our results are particularly important given the maternal mode of Symbiodinium transmission in the study species, which generally results in high symbiont specificity. These findings will cause a paradigm shift in our understanding of coral-Symbiodinium symbiosis flexibility and mechanisms of environmental acclimatisation in corals.

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Marine seismic surveys produce high intensity, low-frequency impulsive sounds at regular intervals, with most sound produced between 10 and 300Hz. Offshore seismic surveys have long been considered to be disruptive to fisheries, but there are few ecological studies that target commercially important species, particularly invertebrates. This review aims to summarise scientific studies investigating the impacts of low-frequency sound on marine fish and invertebrates, as well as to critically evaluate how such studies may apply to field populations exposed to seismic operations. We focus on marine seismic surveys due to their associated unique sound properties (i.e. acute, low-frequency, mobile source locations), as well as fish and invertebrates due to the commercial value of many species in these groups. The main challenges of seismic impact research are the translation of laboratory results to field populations over a range of sound exposure scenarios and the lack of sound exposure standardisation which hinders the identification of response thresholds. An integrated multidisciplinary approach to manipulative and in situ studies is the most effective way to establish impact thresholds in the context of realistic exposure levels, but if that is not practical the limitations of each approach must be carefully considered.

Extensive coral bleaching on the world's southernmost coral reef at Lord Howe Island, Australia The world's southernmost fringing coral reef and extensive high-latitude coral and reef assemblages occur at Lord Howe Island (LHI) (31°33¢S, 159°05¢E) (Harriott et al. 1995). More than 80 scleractinian species have been recorded from LHI reefs, and these corals dominate much of the reef benthos (Harriott et al. 1995; Harrison 2008). The first widespread coral bleaching event recorded at LHI occurred during the 1998 austral summer season when sea temperatures increased above 27°C (P. Harrison pers. obs.), but the bleaching had limited detectable impact on coral cover. During the 2010 summer season, sea temperatures around LHI were abnormally high and exceeded 28°C (~2-3°C above normal summer maximum), with an accumulated thermal stress of more than 19 degree heating weeks (http://coralreefwatch.noaa.gov). This thermal stress coincided with calm seas and high light penetration, resulting in the most extensive and severe coral bleaching event recorded at LHI to date (Fig. 1). Bleached and partially bleached coral cover exceeded 90% at Sylph's Hole and Comet's Hole in the lagoon during March 2010, with less extensive and patchy bleaching at other reef sites around LHI. Pocilloporid corals (Stylophora, Pocillopora and Seriatopora) and Montipora spp. bleached more extensively than other corals, with some Porites, Isopora and other acroporid and faviid colonies, and host sea anemones, observed with substantial or partial pigmentation loss at some sites. Some bleaching-related coral mortality was evident during March 2010, with up to 25% of corals at Comet's Hole having partial or complete bleaching-induced mortality. Rising sea temperatures are predicted to induce more frequent coral bleaching events in future, leading to range shifts in reef corals to higher-latitude regions (Greenstein and Pandolfi 2008). However, this severe coral bleaching event at LHI demonstrates that even the highest latitude coral reef assemblages are also susceptible to bleaching stressors, which could limit future reef development and predicted range shifts to higher latitudes. Isolated reefs such as those at LHI, which lie more than 1,000 km south of the Great Barrier Reef, are likely to be slower to recover from severe disturbances due to their geographic and genetic isolation from other reefs that could potentially supply allochthonous coral larvae for recruitment (Harrison 2008).

Mortality of newly settled individuals is a key factor in shaping adult population size and distribution of many marine invertebrates. Despite this recognized importance, few studies have addressed early post-settlement mortality rates and causes in reef-building corals. To investigate the effects of exclusion of macro-predators and grazers on mortality of Acropora striata (Verrill) recruits, a caging experiment was completed at Moorea, French Polynesia. More than half of the recruits died within 7 days in the field, and although the exclusion of macro-predators and grazers did not significantly reduce nor enhance mortality, it certainly altered their cause. In the presence of macro-predators and grazers, coral recruits faced significant predation-induced mortality (50.0% of dead recruits presented a heavily damaged or missing skeleton). Conversely, in the absence of macro-predators and grazers, dead recruits were mainly intact (91.7%), and most likely suffered from competition with turf algae and associated sediment trapping. These results underline complex interacting effects of predation, competition with turf algae, and sedimentation on coral early postsettlement mortality.

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