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Strategies for Sperm Chemotaxis in the Siphonophores and Ascidians: A Numerical Simulation Study

Makiko Ishikawa^1,*, Hidekazu Tsutsui^1,, Jacky Cosson², Yoshitaka Oka^1, and Masaaki Morisawa¹

¹ Misaki Marine Biological Station, Graduate School of Science, The University of Tokyo, Japan
² Observatoire Oceanologique de Villefrance-sur-Mer, CNRS, France

Author to whom correspondence should be addressed. Present address: Laboratory for Cell Function Dynamics, Brain Science Institute, RIKEN, Hirosawa 2-1, Saitama 351-0198, Japan. E-mail: tsutsui{at}brain.riken.go.jp

Chemotactic swimming behaviors of spermatozoa toward an egg have been reported in various species. The strategies underlying these behaviors, however, are poorly understood. We focused on two types of chemotaxis, one in the siphonophores and the second in the ascidians, and then proposed two models based on experimental data. Both models assumed that the radius of the path curvature of a swimming spermatozoon depends on [Ca²⁺]_i, the intracellular calcium concentration. The chemotaxis in the siphonophores could be simulated in a model that assumes that [Ca²⁺]_i depends on the local concentration of the attractant in the vicinity of the spermatozoon and that a substantial time period is required for the clearance of transient high [Ca²⁺]_i. In the case of ascidians, trajectories similar to those in experiments could be adequately simulated by a variant of this model that assumes that [Ca²⁺]_i depends on the time derivative of the attractant concentration. The properties of these strategies and future problems are discussed in relation to these models.

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