Synthesizing larval competence dynamics and reef-scale retention reveals a high potential for self-recruitment in corals

Many organisms have a complex life-cycle in which dispersal occurs at the propagule stage. For marine environments, there is growing evidence that high levels of recruitment back to the natal population (self-recruitment) are common in many marine organisms. For fish, swimming behavior is frequently invoked as a key mechanism allowing high self-recruitment. For organisms with weak-swimming larvae, such as many marine invertebrates, the mechanisms behind self-recruitment are less clear. Here, we assessed whether the combination of passive retention of larvae due to re-circulation processes near reefs, and the dynamics of settlement competence, can produce the high levels of self-recruitment previously estimated by population genetic studies for reef-building corals. Additionally, we investigated whether time to motility, which is more readily measurable than competence parameters, can explain the between-species variation in self-recruitment. We measured the larval competence dynamics of broadcast-spawning and brooding corals and incorporated these in a model of larval retention around reefs to estimate the potential for self-recruitment and assess its variation among species and reefs. Our results suggest that the larvae of many corals, even those with an obligate planktonic phase, develop with sufficient rapidity to allow high levels of self-recruitment, particularly for reefs with long water retention times. Time to motility explained 77-86% of the between-species variation in potential self-recruitment in scenarios with a realistic range of retention times. Among broadcast spawners, time to motility was strongly and positively correlated with egg size, i.e., broadcast spawner species with small eggs developed more rapidly and exhibited greater potential for self-recruitment. These findings suggest that, along with water retention estimates, easy-to-measure species traits, such as egg size and time to motility, may be good predictors of potential self-recruitment, and therefore may be used to characterize the spectrum of self-recruitment in corals.