Ricardo, G. F., Jones, R. J., Clode, P. L., Humanes, A. & Negri, A. P. Suspended sediments limit coral sperm availability. Scientific Reports (2015).

I began my PhD in 2013 with my research project examining how dredge sediments might impact coral spawning and early developmental stages of corals. Generally, the research project was divided into three parts; how sediment impacts i) fertilisation, ii) larval development and iii) settlement. Coral spawning only occurs once or twice a year on a given reef, but represents an important opportunity for the population to replenish or recover from disturbance events like cyclones, and so management has usually taken a precautious approach by preventing turbidity-generating practises during these rare events.

Many different pathways in which sediment could impact coral fertilisation

The first question we wanted to help resolve was why previous research had sometimes found sediments were associated with low success rates of fertilisation, but other times not. The variation in fertilisation responses makes it difficult to determine what might be a safe (or risky) sediment concentration while the corals are spawning. We figured that, if we could understand how sediments actually led to a decrease in fertilisation, then we would have some insight into why there had been a range of responses, and perhaps even what sediments presented the greatest risk to coral fertilisation.

Coral eggs and embryos

So first we designed an experiment based on an ecotoxicology assay by (Marshall 2006) to help us determine whether sediment was impacting the sperm, or the eggs of the coral. We found that while the eggs were capable of fertilising in the presence of high sediment concentrations, far more sperm were needed to achieve an adequate level of fertilisation success. We then conducted sperm counts at the water surface (where the eggs float), which revealed a decrease in sperm numbers in presence of sediment. This indicated that the sediment was actually preventing the sperm from reaching the egg.

Video-microscopy still of coral sperm congregating around a sediment floc

The other thing we discovered when we first ran this experiment was that small flocs appeared on the bottom of the fertilisation containers just after we added the sperm. This gave us a pretty good hunch that the sediments might be sticking to the sperm and cause them to sink away from the eggs. We then used a range of microscopy techniques including scanning electron microscopy, video-microscopy and more recently fluorescence microscopy  to understand interactions between the sediment and the coral gametes. Through a process of confirmation (and elimination) we were able to confidently determine that sperm flocing and sinking was the dominant mechanism affecting coral fertilisation.

Our results have a few implications. Our results suggest that sediments may shrink the coral fertilisation window – which is a brief 1–2-hour period when sperm and eggs can fertilise before wind and waves spread them apart. The coral fertilisation window is nonlinear, meaning that while gametes remain in a saturated concentration, fertilisation remains high. However, when the gamete concentration passes under a certain threshold, fertilisation rates decrease sharply. Sediment can further decrease the sperm concentration. At high sperm concentrations this probably won’t be a problem, but if the sperm concentration is relatively low, the sediments are likely to have a strong impact on fertilisation rates.

Also, because the fertilisation response observed during sediment exposure is highly dependent on the sperm concentration, we urge that a more standardised approach to what sperm concentration is used in lab assays is needed. A lower sperm concentration will ultimately lead to a stronger response, whereas a higher concentration will buffer against the impact of the sediment. Preferably a single standard concentration or a range of concentrations should be used in absence of not knowing the in situ (in field) sperm concentrations.

We are finding some sediment types cause floccing more than others

We hope that this research will open up a few avenues for future research questions. It is very likely other marine organisms that broadcast spawn (some fish, echinoderms, molluscs) may also suffer from sediment-sperm floccing. One interesting finding was that carbonate sediment typical of most offshore reefs had no discernible impact on fertilisation rates and we are now investigating which types of sediments cause sperm to floc out of suspension, and therefore present the greatest risk to coral spawning events.

This work was funded by the Western Australian Marine Science Institution and partners.

References

Marshall DJ (2006) Reliably estimating the effect of toxicants on fertilization success in marine broadcast spawners. Mar Pollut Bull 52:734-738