For any animal, releasing sperm cells requires a bit of time and energy. Using up so much energy can keep them from the other required work of survival, including finding food and water or sleeping. But just what are the upper limits of gamete production? For a tiny fish called the medaka (Oryzias latipes), males have a mating capacity of 19 times per day. The findings are detailed in a study published January 8 in the journal Royal Society Open Science.
How do fish reproduce?
Most fish species mate using external fertilization. The female will deposit eggs outside of the body, usually near the bottom of a body of water or in a nest built out of rocks and other sediment. The male fish will then swim over the eggs and release sperm several times per day. This helps increase the chances that more eggs will be fertilized, leading to more offspring. Female fish may then collect the fertilized eggs and deposit them on plants.
Medaka–aka the Japanese rice fish–are only about 1.4 inches long and can be found in rice paddies, ponds, marshes, quiet streams, and tide pools in Japan. They are a popular aquarium fish and scientists have used them as a model to study the genetic basis of diseases. Understanding the limits of mating can help scientists gain insights into what it takes for a species to survive.
“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,” Yuki Kondo, a study co-author and an evolutionary ecologist at Osaka Metropolitan University in Japan, said in a statement.
Mating 19 times per day–in a lab
In this new study, Kondo and colleagues used a previously developed method to measure the sperm count of medaka. The day before the experiment, randomly selected males and females from different breeding tanks were put into separate glass tanks. The next day, a male and a female were put into a tank together and their behavior was observed until the end of mating or for about 20 minutes if mating did not occur. Then, the male was removed from the tank and placed in another tank with a new female. The team repeated this process until the male failed to mate with the three females in a row. Additionally, the eggs were gently removed from the females’ abdomens after they scooped them back up and brought them into Petri dishes to assess fertilization rates.
[ Related: Whale shark pre-mating ritual observed for the first time. ]
They found that male medaka can mate 19 times per day on average. In the first three mating sessions, the medaka released over 50 percent of their daily sperm output. The fertilization rate was nearly 100 percent in the early matings, but this significantly decreased after the 10th time. In some later cases, there was no confirmed fertilization.
Female medaka can produce eggs once per day. However, they release all of their eggs when mating and many of the eggs will go to waste when and if they mate with males who have already released most of all of their sperm.
According to the team, these results suggest that there are substantial reproductive costs for males and a potential for sexual conflict due limited sperm availability. However, it does not take into consideration the real world conditions faced by medakas in the wild.
“Our experiments were not intended to mimic natural conditions, but rather to identify the limits of males’ daily mating capacity and the potential reproductive rate of medaka by experimentally removing factors that limit their reproductive success, such as limited nutrition and mate availability,” the team wrote in the study.
