Milt production in goldfish: regulation by multiple social stimuli.

Previous studies in goldfish (Carassius auratus) demonstrate that milt (sperm and seminal fluid) volume is increased both by a preovulatory steroidal pheromone, 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17,20 beta-P), that acts by increasing serum concentrations of gonadotropin II (GTH-II), and by a post-ovulatory prostaglandin (PG) pheromone, that acts by stimulating sexual interactions. Here, we show that male goldfish also increase milt volume when isolated for 24 h, or when placed with another male injected with human chorionic gonadotropin (hCG) for 12 h. In contrast to the milt increase induced by pheromonal 17,20 beta-P, the milt increase following isolation or exposure to hCG-injected males was not associated with increased serum GTH-II. Serum growth hormone also was unaffected, although serum testosterone increased in isolated males. The absence of GTH-II increase following isolation or exposure to hCG-injected males and the long latency time for these two types of responses, suggests these effects are mediated by a novel mechanism as yet undescribed. The present findings suggest that sperm competition has selected for complex mechanisms regulating milt production and fertility in goldfish: males regulate milt production not only in response to stimulatory pheromonal cues from ovulatory females, but also in response to unknown stimulatory and inhibitory cues from male competitors.