Japanese scientists have demonstrated that male medaka fish can mate an average of 19 times a day, releasing more than half of their sperm production in the first three copulations.
Among animals that have multiple partners and deposit eggs outside their bodies, like most fish, males release sperm several times a day, but producing these gametes requires energy and time.
Study Details and Results
Fish behavior experts from the Osaka City University discovered the daily mating capacity of medaka fish.
In findings published in Royal Society Open Science, Dr. Yuki Kondo, appointed by the Graduate School of Sciences, Professor Masanori Kohda, and Professor Satoshi Awata detail the effects of continuous mating on medaka in terms of sperm released, fertilization rates, and the behavior of both males and females.
The Japanese rice fish or medaka is a freshwater fish from the Adrianichthyidae family, distributed in rivers and lakes in the river basins of Japan, Korea, China, and Vietnam, where its population does not seem to be threatened.
Importance of Gametes in Medaka Reproduction
“Medaka are among the fish that spawn, where fertilization occurs after eggs and sperm are released into the water. Since these gametes are difficult to collect, the amount of sperm released and the fertilization rate during successive matings remained a mystery,” explained Dr. Kondo in a statement.
“Our research group previously developed a precise method to measure the sperm count of medaka, so we were able to successfully carry out this experiment.”
Reproductive Strategies and Medaka Behavior
The group’s latest experiment showed that male medaka fish can mate an average of 19 times a day. During the first three mating sessions, medaka fish released over 50% of their daily sperm production.
While the fertilization rate was nearly 100% in the initial matings, it significantly decreased after the tenth time, and there were some subsequent cases where fertilization was not confirmed.
Female medaka can produce eggs once a day but release all their eggs during mating. This means that many eggs are wasted when females mate with males who have already released most or all of their sperm.
Study Implications for Conservation
“This is the first study to quantitatively demonstrate the clear daily mating capacity of male medaka fish, as well as the volume of sperm released during each mating, fertilization rate, and the behavior of males and females during this process,” said Professor Awata.
The study highlights the importance of understanding reproductive strategies and energy allocation in animals, which may have implications for conservation and breeding programs.
By examining the mating behaviors and capabilities of medaka fish, researchers can better understand how these fish and similar species manage their reproductive efforts in natural environments.
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