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First published online November 1, 2006
Journal of Experimental Biology 209, 4574-4579 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02500
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Eggs regulate sperm flagellar motility initiation, chemotaxis and inhibition in the coral Acropora digitifera, A. gemmifera and A. tenuis

Masaya Morita1,2,*, Akira Nishikawa2, Ayako Nakajima3, Akira Iguchi4, Kazuhiko Sakai2, Akihiro Takemura2 and Makoto Okuno3

1 Department of Chemistry, Biology and Marine Sciences, University of the Ryukyus, 1 Senbaru, Nishihara, Okinawa 903-0213, Japan
2 Sesoko Station, Tropical Biosphere Research Center, University of the Ryukyus, 3422 Sesoko, Motobu, Okinawa 907-0227, Japan
3 Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, Meguro-ku, Tokyo 153-8902, Japan
4 Comparative Genomics Centre, Molecular Science Building, James Cook University, Townsville, Queensland 4811, Australia

* Author for correspondence (e-mail: h063107{at}sci.u-ryukyu.ac.jp)

Accepted 17 August 2006

Corals perform simultaneous mass spawning around the full moon. Most Acropora species release gamete bundles, which are a complex of eggs and sperm, into the seawater. Then, gamete bundles are separated into eggs and sperm. Eggs are fertilized when sperm and eggs come in contact with each other. However, it is still unclear how sperm meet the eggs of the same species in the presence of many eggs of different species and how eggs guard against the fertilization attempts by sperm of different species. In this study, we observed that A. digitifera, A. gemmifera and A. tenuis sperm showed motility initiation/attraction close to eggs. Sperm were completely immotile in seawater, but they began to swim in circular motion when they came in close proximity to eggs, and then approached the eggs in straightforward paths. Sperm flagellar motility was not activated by an egg from different species, suggesting that motility initiation by the egg is species specific. In addition, hybridization among these species did not occur under observed conditions. Furthermore, motility-activated sperm became quiescent when many sperm approached the eggs. This study is the first report to show that the egg secretes immobilization factor(s). Our results suggest that the flagellar motility regulation has evolved to avoid hybridization among different species during the mass spawning.

Key words: coral, Acropora, mass spawning, sperm, chemotaxis, flagellar motility






© The Company of Biologists Ltd 2006