Montastraea cavernosa corallite structure demonstrates distinct morphotypes across shallow and mesophotic depth zones in the Gulf of Mexico

Abstract

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.