Coral recruits demonstrate thermal resilience

Abstract

Marine heatwaves are becoming more frequent during summer and pose a significant threat to coral reef ecosystems. Restoration efforts have the potential to support native coral populations and guard them against some degree of environmental change, while global action against climate change takes place. Interspecific hybridization is one approach through which resilient coral stock could be generated for restoration. Here we compared the performance of Acropora kenti and A. loripes hybrid and purebred coral recruits under a simulated thermal stress event. A. kenti eggs were successfully fertilized by A. loripes sperm to produce 'KL' hybrids, but no 'LK' hybrids could be produced from A. loripes eggs and A. kenti sperm. Despite corals in the elevated treatment accruing thermal stress (>12 degree heating weeks over 2 months) known to result in mass bleaching, both purebred and hybrid recruits showed no signs of stress under the simulated temperature regime, based on the performance indicators survivorship, size, color (a proxy of bleaching), and photochemical efficiency of photosystem II. Comparisons between the hybrids and purebreds studied here must be interpreted with caution because hybrid sample sizes were small. The hybrids did not outperform both of their purebred counterparts for any metrics studied here, demonstrating that there are limitations to the extent to which interspecific hybridization may boost the performance of coral stock. In general, the purebred A. loripes recruits performed best under both ambient and elevated conditions. The performance of the KL hybrid corals was similar to the maternal parental species, A. kenti, or not significantly different to either parental purebred species. The Symbiodiniaceae communities of the KL hybrids were characteristic of their maternal counterparts and may have underpinned the performance differences between the A. kenti/KL hybrid and A. loripes recruits.

Keywords: Climate change; Coral; Hybrid; Juvenile; Reef; Resilience.

Figure 1. Schematic of the experimental temperature profiles.

Temperature profiles of the elevated (dash-dot black and continuous blue line) and ambient (dotted black line) experimental treatments. The temperature in the elevated treatment tanks was ramped between days 0–9 of the experiment and simulated the temperatures recorded at Davies Reef during the 2020 mass bleaching event between days 9–31 (continuous blue) of the experiment. The cumulative degree heating weeks (DHWs) experienced by the corals in the elevated temperature treatment over the course of the experiment are also shown (dashed red). After 31 days, the elevated treatment tanks were held at the peak of the heat wave until the corals had experienced >12 degree heating weeks (DHWs).

Figure 2. Juvenile survivorship.

Boxplots depicting the distributions of proportions of surviving recruits per tile for each of the offspring groups (A. loripes purebred–orange, KL hybrid–green, and A. kenti purebred–blue), in each of the treatments (ambient and elevated), and at timepoints T30 and T65 of the experiment. The horizontal lines of the boxes represent the lower quartile, median, and upper quartile values, the “whiskers” represent the extreme values and dots represent single outlier datapoints. Sample sizes (number of tiles) are shown. There was no significant effect of temperature treatment on survivorship based on generalized linear mixed effects modelling.

Figure 3. Juvenile size.

Recruit surface area (i.e., size) over time shown for each of the offspring groups (A. loripes purebred–orange, KL hybrid–green, and A. kenti purebred–blue) in each of the treatments (ambient and elevated temperature). The data points represent the mean area (mm²), and the upper and lower limits of each ribbon represent the standard error around the mean. Sample sizes (number of recruits) are included next to the data points. There was no significant effect of temperature treatment on size based on linear mixed effects modelling.

Figure 4. Juvenile color.

Color score over time shown for each of the offspring groups (A. loripes purebred–orange, KL hybrid–green, and A. kenti purebred–blue) in each of the treatments (ambient and elevated temperature). The data points represent the mean color score and the upper and lower limits of each ribbon represent the standard error around the mean. Sample sizes (number of recruits) are included next to the data points. Color score is used here as a proxy for the density of algal symbionts in the coral tissue where a lower number/lighter color indicates a lower algal symbiont density that is indicative of coral bleaching. There was no significant effect of temperature treatment on recruit color based on linear mixed effects modelling.

Figure 5. Juvenile photochemical efficiency.

The change in maximum quantum yield (F_v/F_m) over time (days) for each treatment (ambient and elevated temperature) and offspring group: A. loripes purebreds (orange), KL hybrids (green), and A. kenti purebreds (blue). The data points represent the mean F_v/F_m, and the upper and lower limits of each ribbon represent the standard error around the mean. Sample sizes (number of recruits) are shown next to each data point. There was no significant effect of temperature treatment on F_v/F_m based on linear mixed effects modelling.

Figure 6. Juvenile Symbiodiniaceae communities.

Symbiodiniaceae community information where each aligned row/tree branch represents data from one sample and the columns show the following: (A left) relative abundance of Symbiodiniaceae profiles that are putatively characteristic of unique taxa, where each color represents a profile, (A right) relative abundance of within‐sample ITS2 sequences, where each color represents one unique sequence, and, (B) a tree that visualizes the hierarchical clustering of samples according to their unifrac distances, where the tips (representing samples) are colored by offspring group (A. loripes purebred–orange, A. kenti purebred–blue, and KL hybrid–green).