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IntroductionClimate change has resulted in elevated sea surface temperature as well as increased frequency of extreme weather events, e.g. cyclones and rainstorms, which could lead to reduced seawater salinity. While temperature effects on corals have been widely examined, the combined effects of both temperature and salinity on corals, especially their early stages, remain poorly known. This study aimed to examine how the larvae of Acropora pruinosa in a marginal coral habitat, Hong Kong, respond to high temperature (+5°C ambient, HT), low salinity (26 psu, LS), and the combined effects of both stressors (HTLS).MethodsWe recorded larval survival and settlement success under different experimental treatments, and used RNA-Seq technique to compare the gene expression patterns of these larvae to understand the underlying molecular mechanism of stress responses.ResultsOur results showed that the survivorship of coral larvae was not affected in all experimental treatments, with all larvae surviving through the 72-hour period of the experiment. Yet, larval settlement was compromised under all stress treatments. The settlement rates were 39.3%, 12%, and 0% for the elevated temperature, reduced salinity, and the combined treatment, respectively, which were all significantly lower than that under the control treatment (78%). We demonstrated that low salinity (LS) triggered responsive gene sets with functions in ATP production, protein translation, and receptor for neuroactive ligands. In addition, high temperature (HT) treatment also triggered MAPK and NF-kB signaling and apoptosis in these coral larvae. The combined stressor treatment (HTLS) acted synergistically, resulting in the up-regulation of intracellular transducers that could trigger the intrinsic apoptosis pathway. This may explain the total failure in larval settlement under HTLS that could further increase larval vulnerability in the natural environment.DiscussionOur results provide new insights into the molecular responses of coral larvae and represent an essential first step in expanding ourunderstanding of the mechanisms of tolerance that may be exhibited by coral larvae exposed to multiple stressors.

Interpreting the coral-Symbiodiniaceae symbiosis in light of the day-night cycle may provide missing links in understanding the function of endosymbiosis. In this study, we found the photo-physiology, Symbiodinaiceae cell density and gene transcription of two coral holobionts (Acropora pruinosa-Cladocopium sp. and Pocillopora damicornis-Durusdinium sp.) have clear day-night oscillations. These two coral holobionts showed lower maximum quantum yield of photosystem II but higher Symbiodiniaceae cell density at day-time, as compared to at night-time. At day-time, the genes related to circadian rhythm and symbiosis in both hosts were up-regulated, while those related to immunity were down-regulated. Simultaneously, both symbionts had lower abundances of genes involved in the light reaction, Calvin cycle and glycolysis, but higher abundances of genes involved in the NH4+ assimilation. These results indicated the high density of Symbiodiniaceae at day-time might be attributed to up-regulating of genes involved in symbiosis and nitrogen metabolism but down-regulating of genes involved in immunity. Moreover, the A. pruinosa-Cladocopium sp. holobiont had larger day-night oscillations than P. damicornis-Durusdinium sp. holobiont, in terms of photo-physiology, Symbiodinaiceae cell density and gene transcription, revealing species-specific day-night oscillation. This study provides valuable insights into the cooperation strategies of coral holobionts to adapt to the day-night environmental changes.

Coral recruitment plays a vital role in mediating adult coral population structure as well as in promoting coral reef recovery after disturbances. As one of the most delicate life stages of coral, coral recruits were reported to be sensitive to several environmental, physical and biological stresses. This study aimed to examine the effects of Drupella corallivory on coral recruits of the massive Platygyra acuta and branching Acropora pruinosa in Hong Kong. Coral recruits were induced to settle on settlement tiles (R tiles). These tiles, alongside with tiles without coral recruits (NR tiles) and brand new tiles (NE tiles), were used in the prey-choice experiment to test the attractiveness of coral recruits to Drupella . The overall attractiveness of different types of tiles to the snails was low, and more Drupella snails were found on NR tiles instead of the R tiles. Effect of Drupella corallivory was also evaluated by counting the number of recruits lost after a movement experiment. The results revealed that only <10% of coral recruits were removed by Drupella , and similar outcomes were also obtained when herbivorous gastropods Tectus pyramis and Euplica scripta were used. This suggested that the loss of coral recruits due to corallivorous gastropods was not any greater than that due to the herbivores. All these results indicate that coral recruits were not attractive to Drupella , and the effect posed by this corallivore gastropod on the survival of coral recruits was limited. [ABSTRACT FROM AUTHOR]

[Display omitted] • Day-night cycle had notable effect on the photo-physiology and Symbiodiniaceae cell density of coral holobionts. • The Symbiosis, immune system and NH 4 + assimilation related genes were affected by day-night oscillation. • The A. pruinosa holobiont had larger day-night oscillations than the P. damicornis holobiont. • The cooperation strategies of coral holobionts were affected by day-night cycle. Interpreting the coral-Symbiodiniaceae symbiosis in light of the day-night cycle may provide missing links in understanding the function of endosymbiosis. In this study, we found the photo-physiology, Symbiodinaiceae cell density and gene transcription of two coral holobionts (Acropora pruinosa-Cladocopium sp. and Pocillopora damicorni s- Durusdinium sp.) have clear day-night oscillations. These two coral holobionts showed lower maximum quantum yield of photosystem II but higher Symbiodiniaceae cell density at day-time, as compared to at night-time. At day-time, the genes related to circadian rhythm and symbiosis in both hosts were up-regulated, while those related to immunity were down-regulated. Simultaneously, both symbionts had lower abundances of genes involved in the light reaction, Calvin cycle and glycolysis, but higher abundances of genes involved in the NH 4 + assimilation. These results indicated the high density of Symbiodiniaceae at day-time might be attributed to up-regulating of genes involved in symbiosis and nitrogen metabolism but down-regulating of genes involved in immunity. Moreover, the A. pruinosa - Cladocopium sp. holobiont had larger day-night oscillations than P. damicornis - Durusdinium sp. holobiont, in terms of photo-physiology, Symbiodinaiceae cell density and gene transcription, revealing species-specific day-night oscillation. This study provides valuable insights into the cooperation strategies of coral holobionts to adapt to the day-night environmental changes. [ABSTRACT FROM AUTHOR]