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Marine fish spermatozoa: racing ephemeral swimmers

J Cosson, CNRS, Villefranche sur Mer, France
A Groison, Biology Department, University of Bergen, Bergen, Norway
M Suquet, Ifremer, Argenton, France
C Fauvel, Ifremer, Palavas, France
C Dreanno, CNRS, Roscoff, France
R Billard, Laboratory of Ichthyology, National Museum Natural History, Paris, France

Correspondence: Jacky Cosson, Email: cosson{at}obs-vlfr.fr

Abstract

After a long period of spermatogenesis (weeks to months), marine fish spermatozoa are delivered at male spawning in sea water at the same time as ova. As the micropyle of the ova becomes quickly closed after release, these minute unicells, the spermatozoa, have to accomplish their task, that is reaching the micropyle within a very brief delay (several seconds to minutes), for delivery of the haploid male genetic information to the ova. For sustainment of such a goal, their highly performant equipment, the flagellum, must immediately fully activate as soon as it contacts the sea water and then propel the sperm cell at an unusually high initial velocity. The cost payed for such hyperactivity is a very rapid expense of intracellular ATP which cannot cope for the demand so that spermatozoa become rapidly exhausted because mitochondria cannot compensate this very fast flagellar energy consumption. Therefore, any spermatozoon ends up with two possibilities : either getting exhausted and immotile or taking a chance to reach the egg micropyle within its very short period of forward motility (ranging tens of seconds) before micropyle closure in relation to both contact of seawater and cortical reaction. The aim of the present review is to present step by step the successive events occurring to marine fish spermatozoa from activation until their full arrest of motility. The present knowledge about activation mechanism is summarized, as well as a description of the motility parameters characterizing the motility period.