Synergistic effects of warming and lower salinity on the asexual reproduction of reef-forming corals

Tropical storms produce competent self-regenerating coral fragments capable of originating new coral colonies. These climatic events are predicted to increase in the future due to ocean warming, the main cause of coral bleaching. Estimated increases in precipitation will affect salinity at shallow reef areas which combined with ocean warming will aggravate the effects of coral bleaching. In this study the long-term effects of increased temperature (30 °C), low salinity (20 psu) and the combination of these stressors were investigated and compared to the control treatment (26 °C, 33 psu). Mortality, condition, growth, and regeneration ability of nine Indo-Pacific reef-forming coral species were assessed, for 60 days. The coral colonies used were kept in captivity for several years, providing information on their thermal and salinity history. Twenty replicate fragments were cut from each coral species’ colony, weighed and glued to the top of a numbered expansion anchor and then acclimated to the experimental aquarium. Half of these fragments was inflicted with one circular injury in order to test its effects on mortality, condition, and growth rate and also to estimate tissue regeneration rates. The results showed that mortality rate was highest in the high temperature + low salinity treatment, reaching 100% for seven out of the nine coral species, in the first six days of experiment, 100% for Psammocora contigua on the 40th day and 75% for Galaxea fascicularis on the 60th day of experiment. Partial mortality was lowest for P. contigua and G. fascicularis in the low salinity treatment. These two coral species also presented the best coral condition. Coral fragments growth rates decreased as temperature increased and salinity decreased except for G. fascicularis fragments that showed similar growth rates at 26 °C and 30 °C for both salinities. Regeneration rates increased with temperature being highest in the high temperature treatment and lowest at 20 psu. It was concluded that G. fascicularis and P. contigua are the most resilient to low salinity. G. fascicularis fragments were the only ones that withstood the synergistic effects of high temperature and low salinity and are likely to become dominant through asexual reproduction in the Indo-Pacific coral reefs in a warmer ocean, especially in coastal areas prone to low salinity episodes.