Coral eggs signal to sperm

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The eggs of mass spawning corals avoid hybridisation by releasing chemical cues to attract sperm of the same species and repel the sperm of others.

According to the results of a new study by Japanese scientists, Acropora sperm are attracted to eggs of their own species but not by those of their relatives.

The study, which was undertaken by a team of scientists from the Department of Chemistry, Biology and Marine Sciences at the University of the Ryukyus in Japan, is due to be published in the Journal of Experimental Biology.

The scientists observed three species of hard coral, Acropora digitifera, A. gemmifera and A. tenuis, and found that their sperm were normally completely immotile in seawater.

However, when they came in close proximity to the eggs of their own species, the sperm suddenly began to swim in a circular motion, before eventually swimming straight at the egg to fertilise it.

This "flagellar motility" wasn't seen when they approached the eggs of other species, providing evidence to suggest that the eggs produce a cue to initiate motility and guide nearby sperm in the plankton towards them.

Acropora, like most hard corals, spawn in lunar synchrony and release gamete bundles at full moon.

The gamete bundles contain complexes of eggs and sperm which separate and float around in the plankton.

The eggs become fertilised when they meet a sperm of the same species, but it was not previously known how eggs guarded themselves against fertilisation against other coral species.

For more details see the paper: Morita M, Nishikawa A, Nakajima A, Iguchi A, Sakai K, Takemura A, Okuno M (2006) - Eggs regulate sperm flagellar motility initiation, chemotaxis and inhibition in the coral Acropora digitifera, A. gemmifera and A. tenuis. J Exp Biol. 2006 Nov 15;209(Pt 22):4574-9.