This text originally appeared in the Annual 2016 Australian Coral Reef Society Newsletter

From the paper: Ricardo, G. F., Negri, A. P., Jones, R. J. & Stocker, R. That sinking feeling: Suspended sediments can prevent the ascent of coral egg bundles. Scientific Reports 6, 21567 (2016). doi:10.1038/srep21567

Fig. 1. Predicted reduction in the number of coral egg-sperm bundles reaching the water surface due to sediment ballasting, and microscopy of the ballasted bundles after failed ascent. A) Percent reduction in bundle ascents. Green and blue contours denote the SS loads that reduce successful ascents by 10% (EC10) and 50% (EC50). B) EC10 values for the ascent failure (EC10, green and purple lines, referring to two different sediment grain radii) and encounter failure (EC10, red line). C) Optical microscopy image showing sediment grains attached to a Montipora digitata bundle. D) Coloured backscatter scanning electron microscopy micrographs of an Acropora nasuta bundle, showing sediment grains in yellow and biotic matter in purple. Scale bar = 200. 

Inshore reefs are regularly exposed to elevated concentrations of suspended sediments associated with natural resuspension events, runoff and dredging. While it has been recognised that suspended sediments can negatively impact coral recruitment, the vulnerability of critical reproductive stages prior to fertilisation have not been considered. In a recent paper, we demonstrate a previously unrecognized pre-fertilization mechanism that can markedly reduce the probability that coral gametes reach the water surface (where fertilisation takes place) in the presence of commonly encountered concentrations of suspended sediments.

Synchronised broadcast spawning of gametes is the dominant mode of reproduction in hard corals and the buoyancy of egg-sperm bundles is critical to maximise gamete encounters and fertilization at the ocean surface. We demonstrate that (i) during their ascent to the surface, the bundles can intercept suspended sediment grains that stick to their mucous coating, and (ii) under a broad range of realistic environmental conditions the ballasting effect of the sediment grains is sufficient to cause a sizeable fraction of bundles to sink and never reach the water surface. The detrimental impact of this loss of ascending bundles on egg-sperm encounters is magnified because the bundles carry both eggs and sperm, and the encounter rate is proportional to the product of their respective concentrations at the surface. Even for bundles that remain positively buoyant, reaching the surface is delayed, further reducing egg-sperm encounter probabilities, and hence fertilisation success. Observations of this mechanism are successfully captured by a mathematical model that predicts the reduction in ascent probability and egg-sperm encounters as a function of sediment load, depth and particle grain size. For reefs at 15 m deep, the model predicts that a coarse silt could reduce 10% of egg-sperm encounters at 35 mg L⁻¹, and for a reef at 5 m deep, reduce 10% of egg-sperm encounters at 106 mg L⁻¹.

This is the first study to examine the effects of environmental pressures on the success of coral gamete ascent, which can have important flow-on effects for the recruitment success of corals following disturbance. This new mechanism intensifies the cumulative risk posed by other sediment and climate stresses on early life history stages of corals and could contribute to recruitment failure on nearby reefs. Furthermore, the mechanism and model we propose provides a blueprint for related processes in other marine organisms, including some echinoderms, molluscs and fish that rely on positively buoyant eggs for fertilization and are thus vulnerable to sediment ballasting.