That sinking feeling: Suspended sediments can prevent the ascent of coral egg bundles

New research has demonstrated a previously unrecognised event that can markedly reduce the probability that coral gametes (egg-sperm bundels) reach the water surface and come in contact with each other.

Inshore reefs are regularly exposed to higher concentrations of suspended sediments during storms, runoff and dredging. While it has been recognised that suspended sediments can negatively impact fertilisation and later life cycle stages, the vulnerability of the egg-sperm bundles during their journey to the water’s surface has not been considered.

Lead author of the WAMSI research published in Scientific Reports Gerard Ricardo (AIMS/UWA) explained many corals reproduce through synchronised broadcast spawning of gametes and the buoyancy of egg-sperm bundles is critical to fertilisation at the ocean surface.

“We demonstrated that during their ascent to the surface, the egg-sperm bundles can run into suspended sediment grains that stick to their mucous coating, and that under certain water quality conditions the sediment grains are enough to cause a significant number of bundles to sink and never reach the water surface, or be slowed in their ascent” Gerard said.

A coral colony releases egg-sperm bundles into the water column. (G. Ricardo)

The detrimental effect this has on the reproduction of corals is made worse by the fact that the bundles carry both eggs and sperm and the bundles need to reach the water surface and break apart for fertilisation to occur. So if less bundles make it to the surface, the coral spawn slick will be less concentrated and therefore there is less chance two compatible gametes will bump into each other.

“Even if the egg-sperm bundles do make it to the surface, they may arrive too late and miss the party; as wind, waves and currents can quickly dilute and disperse a coral spawn slick,” Gerard explained.

Microscopy image of a coral egg-sperm bundle after failed ascent through elevated concentrations of suspended sediments, revealing considerable attachment of sediment grains (yellow) to the bundles (purple). (Gerard Ricardo)

The observations were captured using 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 metres deep, the model predicts that a coarse silt could reduce 10 per cent of egg-sperm encounters at suspended sediment concentrations of 35 milligrams per litre, and for a reef at 5 metres deep, it could reduce 10% of the encounters at 105 milligrams per litre.

“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 ability of a reef to maintain and recover its population,” Gerard said. “It adds to the list of stressors impacting early life history stages of corals and could prevent new corals from adding to the population on nearby reefs.”

It’s thought the mechanism and model used in this research could also be used to determine the effects of sediment grains on the reproductive success of other marine organisms that rely on positively buoyant eggs for fertilisation including some echinoderms, molluscs and fish.

Gerard Ricardo and researchers in the lab A microscopy image of an egg-sperm bundle showing eggs (orange) and sperm (white). (G. Ricardo)

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The WAMSI Dredging Science Node is made possible through $9.5 million invested by Woodside, Chevron and BHP as environmental offsets. A further $9.5 million has been co-invested by the WAMSI Joint Venture partners, adding significantly more value to this initial industry investment. The node is also supported through critical data provided by Chevron, Woodside and Rio Tinto Iron Ore.


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