Thermal stress impairs coral reproduction without visible bleaching

Coral reproduction is sensitive to environmental changes. Under stress, corals allocate energy preferentially towards survival diverting energy away from reproduction. Thermal stress is well-known to cause coral bleaching; however, it is unclear if the deleterious effects of thermal stress on reproduction precede visible bleaching. The objective of this study was to determine the effect of thermal stress at the end of the gametogenic cycle on the reproductive success of corals. Seven colonies of Platygyra yaeyamaensis were each split in half and separated into two temperature treatments (ambient and +2˚C) during the week before spawning. Fluorescence yield was measured daily and fecundity samples were taken before spawning from each colony half (4 days after the full moon) for polyp dissections. In addition, colony halves were observed for spawning. Elevated temperature significantly decreased fluorescence yield after 3 days, but there was no visible paling nor bleaching in any colony halves. A decrease in fluorescence yield was accompanied by a significant decrease in fecundity and no observed spawning. However, there was intraspecific variation in the tolerance to thermal stress. The fluorescence yield, fecundity, and spawning of one colony was not affected by elevated temperature. Overall, exposure to thermal stress prior to spawning had significant deleterious effects on reproduction, even without visible bleaching. These findings suggest that monitoring corals based exclusively on visible bleaching is insufficient and/or may misrepresent coral health.