Coral embryos make next-gen mucous cocoons

By Gerard Ricardo, UWA

The phenomenon we call ‘coral spawning’ actually involves five primary early life-history stages, from the release of the egg-sperm bundle, fertilisation, embryogenesis, larval development until finally settlement – each with their own challenges to the impacts of sediment.

The development of the coral embryo occurs on the water’s surface and lasts about 36 hours from the point of fertilisation until the larvae become free-swimming. During this time, the embryos are part of a coral spawn slick, a buoyant slurry of sperm, fertilised and decomposing unfertilised eggs…it gives a smell that takes a while to get used to.

When we exposed the embryos to suspended sediment, we noticed an interesting response that took us a bit by surprise. The embryos cocooned themselves in a mucous sac which sunk to the bottom of the chamber. Within the confines of safety, the embryo maintained development until ciliation (fine-like hairs used for swimming). When the larvae were moved to clean sediment-free water, the larvae could be seen rotating within the cocoon and eventually would rupture and emerge from it – hardly a beautiful butterfly but amazing nonetheless.

Scanning electron micrographs of the mucous cocoon. Coloured backscatter image of an embryo with part of the mucous cocoon removed (left). Orange = sediment, purple = embryo and mucus. Inset A) Mucous web observed under secondary electron mode B) Sediment grains observed under backscatter electron mode.

Once ciliated, the larvae seem to pretty capable of deflecting sediment grains with their new energy-efficient brooms, but while they are embryos, mucous cocooning maybe the only mechanism they have to protect against abrasive and sticky sediment grains. Overall, early-life stages of corals can be very sensitive to sediment, but for these two development stages, mucous cocooning and cilia beating bring a welcome reprieve.

The progression of the coral embryos through the cocoon formation stage



Ricardo GF, Jones RJ, Clode PL, Negri AP (2016) Mucous Secretion and Cilia Beating Defend Developing Coral Larvae from Suspended Sediments. PLoS ONE 11(9): e0162743. doi:10.1371/journal.pone.0162743


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