Studivan, M. S. | bioRxiv Science

bioRxiv2023

Performance of Orbicella faveolata larval cohorts does not align with previously observed thermal tolerance of adult source populations

Orbicella faveolata, commonly known as the mountainous star coral, is a dominant reef-building species in the Caribbean, but populations have suffered sharp declines since the 1980s due to repeated bleaching and disease-driven mortality. Prior research has shown that inshore adult O. faveolata populations in the Florida Keys are able to maintain high coral cover and recover from bleaching faster than their offshore counterparts. However, whether this origin-specific variation in thermal resistance is heritable remains unclear. To address this knowledge gap, we produced purebred and hybrid larval crosses from O. faveolata gametes collected at two distinct reefs in the Upper Florida Keys, a nearshore site (Cheeca Rocks, CR) and an offshore site (Horseshoe Reef, HR), in two different years (2019, 2021). We then subjected these aposymbiotic larvae to severe (36 {degrees}C) and moderate (32 {degrees}C) heat challenges to quantify their thermal tolerance. Contrary to our expectation based on patterns of adult thermal tolerance, HR purebred larvae survived better and exhibited gene expression profiles that were less driven by stress response under elevated temperature compared to purebred CR and hybrid larvae. One potential explanation could be compromised reproductive output of CR adult colonies due to repeated summer bleaching events in 2018 and 2019, as gametes originating from CR in 2019 contained less storage lipids than those from HR. These findings provide an important counter-example to the current selective breeding paradigm, that more tolerant parents will yield more tolerant offspring, and highlight the importance of adopting a holistic approach when evaluating larval quality for conservation and restoration purposes.

physiology↗

bioRxiv2018

Montastraea cavernosa corallite structure demonstrates ‘shallow-only’ and ‘depth-generalist’ morphotypes across shallow and mesophotic depth zones in the Gulf of Mexico

This study assessed morphological variation in corallite structure of the depth-generalist coral Montastraea cavernosa across shallow and mesophotic coral ecosystems in the Gulf of Mexico (GOM). Among eight corallite metrics examined, corallite diameters were smaller and spacing was greater in mesophotic corals as compared to shallow corals. Additional corallite variation, including greater corallite height of mesophotic samples, were hypothesized to be photoadaptive responses to low light environments. Although comparison of shallow and mesophotic M. cavernosa was the initial objective of the study, multivariate analyses revealed two distinct morphotypes able to be identified by the significant variation in corallite spacing with >90% accuracy. A shallow-only morphotype was characterized by larger, more closely-spaced corallites, while a depth-generalist type exhibited smaller, further-spaced corallites. The depth-generalist morphotype comprised the majority of mesophotic M. cavernosa colonies sampled from sites in the northwest GOM including the Flower Garden Banks (FGB), Bright Bank, and McGrail Bank. A combination of both morphotypes were found at shallow depths in the FGB. Conversely, in the southeast GOM, the shallow-only morphotype was observed in shallow Dry Tortugas. Mesophotic M. cavernosa at Pulley Ridge were comparable to the shallow-only morphotype in terms of corallite spacing but shared morphological similarities with mesophotic corals for other measured characters, likely representing a hybrid morphology. The variable presence of the morphotypes across sites likely indicates a genotypic influence on corallite morphology, as there was a slight, but significant, impact of morphotype on genetic differentiation within the FGB. Patterns of increased algal symbiont (Symbiodiniaceae) density and chlorophyll concentration were retained in the depth-generalist morphotype even at shallow depths, suggesting multiple photoadaptive strategies between morphotypes. Despite the observed polymorphism in shallow and mesophotic M. cavernosa, the implications for these morphological differences in coral physiology and fitness are not fully known, although some evidence suggests skeletal optical properties may influence bleaching resilience.

ecology↗