Abstract Summary
Intervention strategies to create “climate smart” corals for restoration include manipulating the association between corals and their algal symbionts. In adult corals, “stress-hardening” through controlled bleaching and recovery can shift symbiont assemblages in favor of thermally-tolerant Durusdinium (formerly Symbiodinium clade D) and increase thermal tolerance by 1-2°C. We tested methods for manipulating symbiosis in coral recruits using elevated temperature and proximity to adult colonies containing thermally-tolerant Durusdinium to increase symbiont acquisition rates or abundance of Durusdinium. Aposymbiotic Diploria labyrinthiformis recruits from Curaçao were exposed to four experimental treatments: (1) ambient temperature (27°C) and proximity to adult corals predominantly hosting Durusdinium; (2) ambient temperature and proximity to adult corals predominantly hosting Cladocopium (formerly Symbiodinium clade C); (3) elevated temperature (30°C) and proximity to Durusdinium-dominated adults and (4) elevated temperature and proximity to Cladocopium-dominated adults. On average, recruits reared at 30°C experienced a >30% reduction in survivorship compared with those at 27°C. Of the surviving recruits, those exposed to 30°C hosted a greater proportion of Durusdinium (20% of their symbionts) but had fewer symbionts (0.13 per host cell) than those exposed to 27°C, which hosted an average of 11% Durusdinium and 0.27 symbionts per host cell. In addition, at 27°C recruits reared in proximity to Durusdinium-dominated adults hosted three times as many Durusdinium (~17.5%) as those reared with Cladocopium-dominated adults (~5.5%), but there was no significant difference in proportion of Durusdinium based on adult symbiont type at 30°C. This is the first study to investigate how temperature and symbiont availability impacts symbiont acquisition in Caribbean scleractinian coral recruits. Future studies will test the applicability of these findings for different scleractinian species from different source locations. If elevated temperatures and/or proximity to Durusdinium-dominated adults increases the proportion of Durusdinium in recruits, restoration practitioners may choose to rear recruits under these conditions in order to boost their thermal tolerance prior to releasing them onto reefs.
