Tradeoffs to thermal acclimation : energetics and reproduction of a reef coral with heat tolerant Symbiodinium type-D

The photo-physiological characteristics of thermo-tolerant Symbiodinium types have been postulated to have negative effects on the energetics of the reef corals by reducing fitness. To investigate this, two key and inextricably coupled indicators of fitness, lipids and reproduction, were monitored in colonies of the broadcast-spawning coral Acropora millepora over a two-year period that included a natural bleaching event. In the absence of bleaching ITS1-type clade D predominant colonies had 26% lower stored lipids compared to C2 colonies. At spawning time, this correlated with 28% smaller eggs in type-D colonies. This energetic disparity is expected to have reduced larval duration and settlement-competency periods in type-D compared to type-C2 colonies. More importantly, irrespective of the effect of genotype, the fitness of all corals was adversely affected by the stress of the bleaching event which reduced prespawning lipids by 60% and halved the number of eggs compared to the previous year. Our results extend work that has shown that direct temperature stress and symbiont change are likely to work in concert on corals by demonstrating that the lipids and reproduction of the reef building corals on tropical reefs are likely to be impaired by these processes as our climate warms.