Integrating cryptic diversity into coral evolution, symbiosis and conservation.

Understanding how diversity evolves and is maintained is critical to predicting the future trajectories of ecosystems under climate change; however, our understanding of these processes is limited in marine systems. Corals, which engineer reef ecosystems, are critically threatened by climate change, and global efforts are underway to conserve and restore populations as attempts to mitigate ocean warming continue. Recently, sequencing efforts have uncovered widespread undescribed coral diversity, including 'cryptic lineages'-genetically distinct but morphologically similar coral taxa. Such cryptic lineages have been identified in at least 24 coral genera spanning the anthozoan phylogeny and across ocean basins. These cryptic lineages co-occur in many reef systems, but their distributions often differ among habitats. Research suggests that cryptic lineages are ecologically specialized and several examples demonstrate differences in thermal tolerance, highlighting the critical implications of this diversity for predicting coral responses to future warming. Here, we draw attention to recent discoveries, discuss how cryptic diversity affects the study of coral adaptation and acclimation to future environments, explore how it shapes symbiotic partnerships, and highlight challenges and opportunities for conservation and restoration efforts.

Cryptic lineages respond differently to coral bleaching.

Coral cover is decreasing worldwide largely as a result of a rise in seawater temperatures that triggers coral bleaching and induces coral mortality. How coral reefs will respond to climate change will be a function of genetic variation and how it is partitioned within and among species. A critical initial step is to accurately delineate species and quantify their physiological potential to cope with heat stress. Cryptic species are morphologically similar but genetically distinct and may respond physiologically differently to climate change. A dominant Caribbean reef builder severely affected by climate change is the mountainous star coral, Orbicella faveolata. Recently in this journal, Dziedzic et al. reported quantitative genetic variation in the physiological response to thermal stress in a single population of this species, suggesting that variation within populations will allow these corals to adapt to rising ocean temperatures. We reanalysed their data and found multiple cryptic lineages rather than a single panmictic population, with one of the lineages being heat-intolerant. While different cryptic lineages co-occur in certain locations, there is at least one lineage that occurs only in a single location. Our finding of hidden lineages within a threatened species highlights the varying extinction risks faced by these independently evolving groups, especially when the prospects of survival under warmer oceans seem favourable for only some of them.