Intraspecific variation in coral reproductive output and symbiont community structure following a thermal bleaching event

Abstract

Marine heatwaves and associated bleaching events are projected to increase in frequency and severity in the future, imperiling charismatic ecosystems such as coral reefs. Although mass bleaching events are often accompanied by widespread coral mortality, there is intraspecific variation in bleaching susceptibility, leading to a mosaic of responses where corals may bleach and die, bleach and recover, or resist bleaching altogether. Because the ecological persistence of coral reefs hinges on the survivors of environmental stress, understanding the sublethal effects of bleaching and the mechanisms that promote resilience or resistance are crucial for both projecting coral populations into the future and developing effective management or intervention tools. This thesis examines natural intraspecific variation in bleaching susceptibility in the Indo-Pacific reef-building coral Acropora hyacinthus during and after the 2019 thermal anomaly in Mo’orea, French Polynesia, with particular emphasis on coral reproductive output and microalgal endosymbiont community structure. In Chapter 1, I employed reproductive histology and energetic assays to relate coral host energy reserves to gamete quantity and quality in resistant and recovered colonies. I found that, despite healthy appearances in all individuals five months after the bleaching event, recovered colonies harbored diminished energy reserves compared to resistant conspecifics and exhibited compound effects of stress on reproduction: they displayed not only a lower probability of containing gametes, but also lower fecundity per polyp. These results illustrate energy allocation strategies among physiological processes and indicate that bleaching imposes a constraint on concurrent stress recovery and gamete production, with the decreased reproductive capacity of bleaching survivors possibly restricting overall reef resilience. In Chapter 2, I utilized ITS2 amplicon sequencing to assess spatial and temporal flexibility in Symbiodiniaceae community structure and their relationship to the host heat stress response during and after the thermal anomaly. I found that there was substantial flexibility in coral-microalgal associations across the reefscape, but that symbiont community composition was strongly linked to both reef zone and holobiont affinity for resistance or recovery, signifying a role of local environmental conditions in structuring symbiont communities and thus heat stress response. Although no temporal shifts in symbiont assemblages were observed, comparisons with previous studies suggest thermal stress may have induced a switch to novel coral-microalgal combinations. Further, I identified symbiont types that are potentially diagnostic of bleaching susceptibility and could be developed into biomarkers or used to enhance coral thermal tolerance. My thesis provides fundamental yet critical insight into the complex dynamics of coral recovery after thermal stress. Together, the results highlight the presence of intraspecific responses to bleaching and underscore the importance of accounting for multiple trajectories of individual species when projecting population recovery after disturbance.