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For every animal, releasing sperm cells takes some time and energy. Expending so much energy can keep them from doing other necessary survival work, such as finding food and water or sleeping. But what exactly are the upper limits of gamete production? For a small fish called the Medaka (Oryzias latipes), males have a mating capacity of 19 times per day. The findings are detailed in a study published Jan. 8 in the journal Royal Society Open Science.
How do fish reproduce?
Most fish species mate with the help of external fertilization. The female will lay eggs outside the bodyusually at the bottom of a body of water or in a nest constructed from rocks and other sediment. The male fish then swims over the eggs and releases sperm several times a day. This helps increases the chance that more eggs will be fertilizedwhich leads to more offspring. Female fish can then collect the fertilized eggs and lay them on plants.
Medaka– also called the Japanese ricefish – are only about 1.5 inches long and are found in rice fields, ponds, swamps, quiet streams and tidal pools in Japan. They are one popular aquarium fish and scientists have used them as one model to study the genetic basis of diseases. Understanding the limits of mating can help scientists gain insight into what it takes for a species to survive.
“Medaka is one of the spawning fish, where fertilization occurs after the eggs and sperm are released into the water. Because these gametes are difficult to collect, the number of sperm released and the fertilization rate during successive matings have remained a mystery,” said Yuki Kondo, co-author of the study and an evolutionary ecologist at Osaka Metropolitan University in Japan. said in a statement.
Mating 19 times a day in a laboratory
In this new studyKondo and colleagues used a previously developed method to measure Medaka’s sperm count. The day before the experiment, randomly selected males and females from different breeding tanks were placed in separate glass tanks. The next day, a male and a female were placed together in an aquarium and their behavior was observed until the end of mating or for about 20 minutes if no mating occurred. The male was then 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. In addition, the eggs were carefully removed from the abdomen of the females after scooping them up again and placing them in petri dishes to assess fertilization rates.
[ Related: Whale shark pre-mating ritual observed for the first time. ]
They found that male Medaka can mate an average of 19 times a day. During the first three mating sessions, the Medaka released more than 50 percent of their daily sperm production. The fertilization rate was almost 100 percent in the early matings, but decreased significantly after the tenth time. In some later cases there was no confirmed fertilization.
Female medaka can produce eggs once a day. However, they release all their eggs during mating and many of the eggs will be lost when and if they mate with males who have already released most of all their sperm.
According to the team, these results suggest that there are significant reproductive costs for men and the potential for sexual conflict due to limited sperm availability. However, it is not taken into account real world conditions encountering 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 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.
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