Decoding reproductive strategies to guide restoration and conservation: the case study of Mediterranean red coral (Corallium rubrum)

Corallium rubrum is an octocoral endemic of the Mediterranean Sea, which suffers from population decline due to overfishing and warming-driven mass mortality events. For several years, scientists and managers have been interested in fostering the persistence of this resource, promoting various conservation and restoration actions. Understanding how the mating system influences the reproductive success of a species and, consequently, its persistence over time is crucial to planning efficient and successful restoration initiatives. In recent years, relatedness and kinship analysis suggested that breeding units in the red coral are highly restricted in space, even at a scale of half a meter. In this study, we aim to elucidate the processes underlying the genetic structure of C. rubrum, understanding the influence of fertilization distance and population density on reproductive success. By looking at a population in a semi-controlled environment (i.e., artificial caves) where the genetic print, sex, and morphology are known for all the individuals, we aimed to 1) estimate C. rubrum maximum fertilization distance, 2) evaluate the effect of sperm dilution on the fertilization of female colonies, and 3) verify if sperm limitation affects female larval production. After collecting all the larvae released during two breeding seasons (summer 2022 and 2023), we performed parentage analysis to assign each larva to its respective parents. Results show that polyandry is likely the norm in C. rubrum and that the species' potential fertilization distance can reach at least 13 meters. Notably, we detected a significant negative relationship between the number of larvae produced by females and the distance of the siring male, demonstrating that sperm dilution can play a crucial role in limiting fertilization success and that sperm limitation in sparse populations significantly impacts the coral fecundity. These findings have important implications for restoration, as they suggest that inter-colonial distance in transplantation projects has a significant role in ensuring the reproductive success of restored populations. Hence, we propose replicating the structural patterns characteristic of C. rubrum populations by restoring small, dense patches of reproductive colonies capable of sustaining breeding interactions and ensuring recruitment over generations.