Pocillopora spp. species id differentially influences skeletal structure and the response of skeletal growth to flow regimes
H1: Depending of the environmental condition, Pocillopora spp. will don't have the same skeleton structure interspecific and intraspecific.
Known for their wide geographical distribution in the Indo-Pacific region, the genus Pocillopora spp. is a foundational species of coral reefs (Adjeroud et al. 2018), especially in French Polynesia due to its high resilience (Pérez-Rosales et al. 2021a). Like many coral species, Pocillopora spp. exhibits high growth rates (Tortolero-Langarica et al. 2017), high recruitment rates (Trapon et al. 2013; Leonard et al. 2022; Edmunds et al. 2024), and significant plasticity (Paz-García et al. 2015a, 2015b). The use of molecular detection tools (mtORF, (Johnston et al. 2017; Burgess et al. 2021, 2024; Harnay and Putnam 2023)) has highlighted the challenges of morphological identification due to the diverse morphology of Pocillopora spp. These corals are found at depths of up to 60 meters (Pérez-Rosales et al. 2021b), from forereef to fringing reefs (Burgess et al. 2024). Consequently, certain environments increase the likelihood of high plasticity (Todd 2008; Brambilla et al. 2021). Studies by (Chindapol et al. 2013) have shown the effects of currents on corals, while (Iwasaki et al. 2016) demonstrated the impact of marine environments on coral growth. These findings provide sufficient evidence to suggest that the habitats in which symbiotic colonies live are influenced by these factors.
During the growth process, elements such as the canals in the calcareous skeleton, volume ratio, and distribution (Li et al. 2021) are essential keys to understanding morphological growth. When comparing Pocillopora spp. with other species, it is observed that the genus Pocillopora possesses unique growth characteristics, such as a network of polyps supported by the coenosteum and interseptal spaces connecting all polyps within the same colony (Li et al. 2021). In P. verrucosa, the volume of the growth rings varies according to environmental factors (Li et al. 2021), which may also apply to other species within the Pocilloporidae family. The calices in P. verrucosa and P. meandrina are among the smallest, with diameters ranging from 0.5 to 1mm (Li et al. 2021).The distinct morphological features, such as an arborescent shape, cup-shaped calices separate from the coenosteum covered by verrucae, a system that elevates growth (building a new calice above the existing one, allowing the polyp to move into the newly available space), and a higher skeletal density distribution at the top (Li et al. 2021), are characteristics that differentiate this genus from others. However, these elements have been detected through computed tomography (CT), a technique applied to corals only in recent years (Beuck et al. 2007). They have not been applied to all Pocillopora spp, or following species identification with molecular tools.
In this context, increasing knowledge of skeletal adaptation and evolution in Pocillopora spp. is crucial for implementing conservation and restoration plans. Therefore, four species of Pocillopora spp. (n=5 P. meandrina, n=5 P. verrucosa, n=5 P. tuahiniensis, and n=5 P. grandis) were collected from two sites with different environmental conditions (high current vs. low current; (Edmunds et al. 2010)). This study aims to visualize the morphological differences at both the macro and microscopic scales in adult colonies by focusing on their calcium carbonate structures. To achieve this, we used a combination of synchrotron X-ray tomography, electron microscopy, skeletal morphometrics, and material structural analyses to assess skeletal properties and their variations according to environmental conditions.
