Persistence of phenotypic responses to short-term heat stress in the tabletop coral Acropora hyacinthus

Abstract

Widespread mapping of coral thermal resilience is essential for developing effective management strategies and requires replicable and rapid multi-location assays of heat resistance and recovery. One- or two-day short-term heat stress experiments have been previously employed to assess heat resistance, followed by single assays of bleaching condition. We tested the reliability of short-term heat stress resistance, and linked resistance and recovery assays, by monitoring the phenotypic response of fragments from 101 Acropora hyacinthus colonies located in Palau (Micronesia) to short-term heat stress. Following short-term heat stress, bleaching and mortality were recorded after 16 hours, daily for seven days, and after one and two months of recovery. To follow corals over time, we utilized a qualitative, non-destructive visual bleaching score metric that correlated with standard symbiont retention assays. The bleaching state of coral fragments 16 hours post-heat stress was highly indicative of their state over the next 7 days, suggesting that symbiont population sizes within corals may quickly stabilize post-heat stress. Bleaching 16 hours post-heat stress predicted likelihood of mortality over the subsequent 3–5 days, after which there was little additional mortality. Together, bleaching and mortality suggested that rapid assays of the phenotypic response following short-term heat stress were good metrics of the total heat treatment effect. Additionally, our data confirm geographic patterns of intraspecific variation in Palau and show that bleaching severity among colonies was highly correlated with mortality over the first week post-stress. We found high survival (98%) and visible recovery (100%) two months after heat stress among coral fragments that survived the first week post-stress. These findings help simplify rapid, widespread surveys of heat sensitivity in Acropora hyacinthus by showing that standardized short-term experiments can be confidently assayed after 16 hours, and that bleaching sensitivity may be linked to subsequent survival using experimental assessments.

Coral bleaching resistance variation is linked to differential mortality and skeletal growth during recovery

“…We collected branches from 39 A. hyacinthus colonies located on two sections of Double Reef in the Indo‐Pacific archipelago of Palau's southern lagoon (Patch Reefs 7 and 9 in Cornwell et al, 2021 ; Walker et al, 2022 ). All colonies were within 1.5 km of each other (Figure 1a ), and we confirmed no colonies were identical using single nucleotide polymorphism genotypes called from transcriptome data (BioProject accession number PRJNA872206 in the NCBI BioProject database https://www.ncbi.nlm.nih.gov/bioproject/ ).…”

“…Heated ramets in the heat stress experiment ramped from 30 to 34.5°C then back down to 30°C daily, and controls were maintained at 30°C. Heated ramets remained in the heat stress experiment daily ramp cycle for however many days necessary to reach moderate bleaching (see Section 2 ; described in Walker et al, 2022 ). (b) Violin chart of days to reach target moderate bleaching across all genets, based on average number of days ramets bleached.…”

“…The functional importance of resistance and recovery suggests that their correlation is also fundamental to resilience dynamics. There is high intra‐ and interspecific variation in coral heat stress resistance (e.g., Cornwell et al, 2021 ; Morikawa & Palumbi, 2019 ; Schoepf et al, 2019 ; Thomas et al, 2018 ; Ulstrup et al, 2006 ) and recovery (e.g., Matsuda et al, 2020 ; Rodrigues & Grottoli, 2007 ; Schoepf, Grottoli, et al, 2015 ; Schoepf, Stat, et al, 2015 ; Walker et al, 2022 ), which allows for assays that can harness variation to evaluate heat stress resistance and recovery in tandem. Further, the coral holobiont transcriptome responds rapidly and significantly to heat stress (Savary et al, 2021 ; Seneca & Palumbi, 2015 ; Traylor‐Knowles et al, 2017 ), and correlates of faster recovery differ from transcriptome correlates of higher resistance (Thomas & Palumbi, 2017 ; Thomas et al, 2019 ), suggesting that components of the underlying genetic mechanisms of resistance and recovery differ.…”

“…Few studies to date have explicitly compared resistance and recovery among a set of coral colonies within a species (Leinbach et al, 2021 ; Matsuda et al, 2020 ; Morikawa & Palumbi, 2019 ; Walker et al, 2022 ). A consistent and important limitation of these comparisons is that corals with different levels of heat stress resistance experience the same amount of heat exposure, meaning that recovery is monitored in corals that sustained different levels of heat stress and bleaching.…”

“…A consistent and important limitation of these comparisons is that corals with different levels of heat stress resistance experience the same amount of heat exposure, meaning that recovery is monitored in corals that sustained different levels of heat stress and bleaching. For example, highly heat‐resistant A. hyacinthus corals with little to no bleaching were shown to have significantly less mortality during the first 2 months of recovery compared to severely bleached low‐resistant corals after a standardized short‐term heat stress assay (Walker et al, 2022 ). In these experiments, like many others, there are two variables among colonies—the level of heat resistance and the level of bleaching—making it difficult to assess what is driving differences in recovery traits.…”