Working out the kinks of mating in the animal kingdom helps to gain insights into the survival of species. Among animals that have multiple partners who deposit eggs outside their body, such as most fish, the males release sperm several times a day, but producing these gametes requires energy and time.

Osaka Metropolitan University experts on fish behavior have recently uncovered a daily mating capacity for medaka. In findings published in Royal Society Open Science, Graduate School of Science Specially Appointed Dr. Yuki Kondo, Specially Appointed Professor Masanori Kohda, and Professor Satoshi Awata detailed the effects of continuous mating by medaka on the number of sperm released, fertilization rates, and the behavior of both males and females.

“Medaka are among the fish that spawn, where fertilization occurs after the eggs and sperm are released in water. As these gametes are difficult to collect, the number of sperm released and the fertilization rate during successive matings had remained a mystery,” Dr. Kondo explained. “Our research group previously developed an accurate method for measuring the sperm count of medaka, which is why we were able to successfully conduct this experiment.”

The group’s latest experiment showed that male medaka on average can mate 19 times a day. During the first three mating sessions, the medaka released more than 50% of their daily sperm output. While the fertilization rate was nearly 100% in the early matings, this decreased significantly after the 10th time, with some later cases where there was no fertilization confirmed.

Female medaka can produce eggs once per day, but they release all their eggs when mating. This means that many eggs go to waste when females mate with males who have already released most or all their sperm.

“This is the first study to quantitatively show the clear daily mating capacity of male medaka, as well as the volume of sperm released during each mating, fertilization rate, and the behavior of males and females during this process,” Professor Awata proclaimed. “Our research provides important insights into the relationship between the cost of gamete production and sexual selection.”

Funding
KAKENHI [grant numbers 22K20666 to Y.K., 20K20630 to M.K., and 23H03868 to S.A.]
The 2021 Osaka City University Strategic Research Promotion Project [grant number OCU-SRG2021_BR10 to S.A.]

Competing interests
The authors declare that they have no competing interests.

###

About OMU
Established in Osaka as one of the largest public universities in Japan, Osaka Metropolitan University is committed to shaping the future of society through “Convergence of Knowledge” and the promotion of world-class research. For more research news, visit https://www.omu.ac.jp/en/ and follow us on social media: X, Facebook, Instagram, LinkedIn.