Medaka males can mate 27 times daily, but sperm performance drops fast

A new study reveals that male medaka fish, despite their ability to mate up to 27 times a day, experience a rapid decline in sperm performance following repeated mating sessions. Researchers from Osaka Metropolitan University found that while these small fish exhibit high levels of sexual activity during the breeding season, their fertility potential diminishes significantly after just a few mating encounters. The study, led by Specially Appointed Assistant Professor Yuki Kondo and Professor Satoshi Awata, examined the effects of repeated spawning on sperm quality in Oryzias latipes, commonly known as medaka. These fish, typically measuring between 2 and 3 centimeters in length, engage in frequent mating behaviors, especially in environments with a large number of females. Earlier research indicated that males could mate an average of 19 times per day, reaching up to 27 times under certain conditions. However, previous studies primarily focused on sperm quantity rather than quality. To investigate how repeated mating affects sperm function, the researchers conducted controlled experiments involving male medaka placed in tanks with either one or 15 females. After confirming successful mating, the females were returned to their stock tanks, while the males underwent sperm quality assessments. The findings, published in the Journal of Ethology, highlight a notable decrease in sperm velocity among males who engaged in multiple mating sessions. The data revealed that sperm from males who had mated repeatedly showed approximately a 20% reduction in swimming velocity within the first 30 seconds after activation, compared to sperm from males that had mated only once. This decline in velocity was not sustained beyond 40 seconds, suggesting that the impact of repeated mating on sperm performance might be temporary. Kondo emphasized the importance of considering both sperm quantity and quality in reproductive biology. “Previous studies have mainly focused on sperm quantity, but our findings show that it is also necessary to consider changes in sperm quality,” she stated. The research underscores the need for further investigation into how these factors collectively influence fertilization success. The study’s final dataset included sperm counts from 12 males in the single-mating group, averaging 2,538 sperm cells, and 10 males in the multiple-mating group, averaging 1,265 sperm cells. These figures indicate a substantial drop in sperm numbers following repeated mating, adding another layer of complexity to understanding reproductive efficiency in medaka. Awata highlighted the significance of these findings for future research. “In future studies, it will be necessary to determine the extent to which reductions in sperm count and decreases in swimming velocity each affect fertilization success. Another key challenge will be to determine the physiological mechanisms underlying the decline in sperm swimming velocity associated with repeated spawning,” he noted. The implications of this research extend beyond medaka fish, offering insights into broader reproductive strategies in animals. By examining the interplay between sperm quantity and quality, scientists can gain a more comprehensive understanding of how different species adapt to ensure genetic continuity. Future investigations may explore the specific biological processes responsible for the observed changes in sperm performance, potentially revealing new dimensions of reproductive physiology.