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

Oryzias latipes, commonly known as medaka, is a small fish measuring about 2–3 cm (0.8–1.2 inches) in length that exhibits highly active spawning behavior during the breeding season.

Previous research by an Osaka Metropolitan University research team showed that males can spawn an average of 19 times per day and up to 27 times in environments with many females.

It also found that after around 10 consecutive mating events, sperm count per spawn decreases and is associated with a sharp decline in fertilization rates.

However, changes in sperm velocity with repeated spawning have remained unclear. ...