Abstract
BACKGROUND: Aggregation affects the fertilization rate of species that utilize external fertilization. However, the process of aggregation has not been studied in detail, using either theoretical models or real world observations. We used the Ezo abalone Haliotis discus hannai as a model animal species to evaluate whether mucus trail following (MTF) facilitates aggregation by reducing the distance between members of opposite sexes. We also examined whether the fertilization rate increase with mucus-trail-following is an evolutionary driving force in mucus following behavior. METHODS: We used a y-maze to test whether H. discus hannai follows the mucus trails of other individuals. Distances between members of the opposite sex of MTF individuals were compared to non-MTF individuals using an individual-based model (IBM) consistent with the behavior of H. discus hannai. To examine whether MTF behavior evolved to reduce distances between members of the opposite sex, we constructed simple population genetic models of a diploid population with nonoverlapping, discrete generations. RESULTS: Haliotis discus hannai chose the y-maze arm with the mucus trail more frequently than the one without, regardless of the sex of the abalone that secreted the mucus or the reproductive season. In the IBM the distance between opposite sexes was significantly reduced by MTF behavior; however, the difference in distances between opposite sex compared to same sex individuals was only several centimeters. Simple population genetic models indicated that the aggregating effect of MTF between the opposite sex members could be an evolutionary driving force. CONCLUSIONS: These results suggest that observed MTF behavior might have evolved as a mechanism to increase the fertilization rates although other factors could also be involved.