Interspecific Variation in Coral Settlement and Fertilization Success in Response to Hydrogen Peroxide Exposure.

Hydrogen peroxide (H2O2) is involved in the regulation of numerous reproductive and morphogenic processes across an array of taxa. Extracellular H2O2 can be widespread in oceanic waters, and elevated sea surface temperatures can cause increased levels of intracellular H2O2 within cnidarian tissue, but it remains unclear how this compound affects early life-history processes in corals, such as fertilization, metamorphosis, and settlement. To evaluate the effects of H2O2 on multiple stages of recruitment, experiments were conducted using Caribbean corals with various reproductive modes, including the brooders Porites astreoides and Favia fragum and the broadcast-spawning species Acropora palmata and Orbicella franksi. H2O2 accelerated settlement in all brooding species tested. Concentrations of 1000 µmol l-1 H2O2 caused close to 100% settlement in all larval age classes, regardless of exposure duration. As larvae aged, the required threshold of H2O2 capable of inducing settlement decreased. In contrast, H2O2 concentrations of 100 µmol l-1 or greater caused a significant reduction in metamorphosis and settlement in the larvae of spawners. Furthermore, fertilization of their gametes was inhibited in the presence of H2O2 concentrations as low as 100 µmol l-1. In Porites astreoides larvae, internal levels of H2O2 reached a maximal value of 75 µmol l-1 following 48 h of incubation at 31 °C. This concentration was found to significantly alter settlement rates in both brooding coral species and likely induced a cellular cascade in the settlement signaling pathway. The results of this study suggest that temperature stress influences H2O2 production, which in turn impacts coral settlement. While it is unlikely that the current levels of externally derived concentrations of oceanic H2O2 are affecting coral larvae, internal concentrations (produced under heat stress) have the capacity to impact recruitment under a changing climate.

Asymmetric conspecific sperm precedence in relation to spawning times in the Montastraea annularis species complex (Cnidaria: Scleractinia).

In broadcast spawners, prezygotic reproductive isolation depends on differences in the spatial and temporal patterns of gamete release and gametic incompatibility. Typically, gametic incompatibility is measured in no-choice crosses, but conspecific sperm precedence (CSP) can prevent hybridization in gametes that are compatible in the absence of sperm competition. Broadcast spawning corals in the Montastraea annularis species complex spawn annually on the same few evenings. Montastraea franksi spawns an average of 110 min before M. annularis, with a minimum gap of approximately 40 min. Gametes are compatible in no-choice heterospecific assays, but it is unknown whether eggs exhibit choice when in competition. Hybridization depends on either M. franksi eggs remaining unfertilized and in proximity to M. annularis when the latter species spawns or M. franksi sperm remaining in sufficient viable concentrations when M. annularis spawns. We found that the eggs of the early spawning M. franksi demonstrate strong CSP, whereas CSP appears to be lacking for M. annularis eggs. This study provides evidence of diverging gamete affinities between these recently separated species and suggests for the first time that selection may favour CSP in earlier spawning species when conspecific sperm is diluted and aged and is otherwise at a numeric and viability disadvantage with heterospecific sperm.