Increasing comparability among coral bleaching experiments

Abstract

Coral bleaching is the single largest global threat to coral reefs worldwide. Integrating the diverse body of work on coral bleaching is critical to understanding and combating this global problem. Yet investigating the drivers, patterns, and processes of coral bleaching poses a major challenge. A recent review of published experiments revealed a wide range of experimental variables used across studies. Such a wide range of approaches enhances discovery, but without full transparency in the experimental and analytical methods used, can also make comparisons among studies challenging. To increase comparability but not stifle innovation, we propose a common framework for coral bleaching experiments that includes consideration of coral provenance, experimental conditions, and husbandry. For example, reporting the number of genets used, collection site conditions, the experimental temperature offset(s) from the maximum monthly mean (MMM) of the collection site, experimental light conditions, flow, and the feeding regime will greatly facilitate comparability across studies. Similarly, quantifying common response variables of endosymbiont (Symbiodiniaceae) and holobiont phenotypes (i.e., color, chlorophyll, endosymbiont cell density, mortality, and skeletal growth) could further facilitate cross-study comparisons. While no single bleaching experiment can provide the data necessary to determine global coral responses of all corals to current and future ocean warming, linking studies through a common framework as outlined here, would help increase comparability among experiments, facilitate synthetic insights into the causes and underlying mechanisms of coral bleaching, and reveal unique bleaching responses among genets, species, and regions. Such a collaborative framework that fosters transparency in methods used would strengthen comparisons among studies that can help inform coral reef management and facilitate conservation strategies to mitigate coral bleaching worldwide.

Keywords: common framework; coral bleaching; coral heat stress; cross-study comparisons; experimental design methods; feeding; flow; light; phenotype; standardization; temperature.

Fig. 1

Two examples of acute and short‐term coral heat‐stress experimental setups. Photo in panel (a) by S. Palumbi and panel (b) by C. R.Voolstra.

Fig. 2

Temperature profiles of coral (a) heat‐pulse, (b) heat‐pulse with multiple temperatures, and (c) heat‐hold acute and short‐term thermal stress experiments. Number of days will depend on the specific study. Yellow bars indicate light cycles. Line breaks indicate night. MMM, maximum monthly mean temperature.

Fig. 3

Example of an (a) outdoor and (b) indoor moderate‐duration coral heat‐stress experiment setup. Long‐term experimental setups are similar. Photo in panel (a) by D. Kemp and panel (b) by A. Grottoli.