Understanding corallite demography to comprehend potential bias in sclerochronology: Analysis of coral modular growth by micro-computed tomography

Abstract

The discovery of alternating growth bands in the skeletons of massive corals led to the possibility of using them as environmental indicators. However, skeleton formation is the result of the growth of thousands of polyps depositing millimeter-sized CaCO3 structures, called corallites. Nevertheless, the orientation of the corallite trajectories and their position with respect to the colony could be altering the information obtained from the skeleton. In this sense, to obtain reliable information from coral skeletons, it is necessary to understand how polyp population growth influences coral growth rates. For this reason, we present a study that combines computed tomography image segmentation, optical densitometry, and demographic equations to follow the trajectory of corallites in order to model coral growth from the replication of its corallites and understand the responses in terms of their growth. We observed that both corallite replication and skeletal growth characteristics vary significantly according to the position they occupy within the colony. The central zone of the colony promotes corallite replication, and it is in this zone that we measured the highest values of extension and calcification. These variations in vegetative and skeletal growth are possibly in accordance with the variability of available resources and environmental stimuli in different zones of the skeleton. This approach will allow us to explore future lines of research associated with the size limits of different coral species and to observe how different drivers modulate polyp budding.