Courtship and spawning behaviour of medaka in a semi-outdoor environment initiating at midnight

Abstract

Reproductive timing is a critical ecological trait that directly influences fitness. Medaka (Oryzias latipes), a small freshwater fish, is widely used as a model organism in various scientific fields. However, ecological studies conducted under (semi-) natural conditions remain limited. Although, spawning has been reported to occur within 1 h before and after sunrise, direct observations remain scarce. We investigated the timing of spawning initiation and associated courtship in medaka through 24-h observations using infrared cameras under semi-natural conditions. During the experiments, sunrise occurred at approximately 4:45. Observations of the 31 pairs revealed that spawning occurred between 1:05-9:48, with a peak at 2:00-4:00. Unlike previous reports but like the most recent fieldwork, only 26% of the total spawning events (8/31) were observed within 1 h before and after sunrise. Male courtship behaviours, including following females and quick circle displays, increased from midnight, peaking between 2:00-5:00. This study provides new insights into the natural reproductive timing of medaka, suggesting a possible adaptation to nocturnal spawning, likely as a strategy to reduce predation on both parents and eggs. It also underscores the importance of investigating the ecology of model organisms under (semi-)natural conditions to gain a more comprehensive understanding of biological phenomena observed in laboratory settings.

Keywords: Oryzias latipes; Courtship; Medaka; Model organism; Spawning; Video observation.

Fig. 1

(a) Medaka spawning at night, captured using an infrared camera. The female was on the right, and the male was on the left. The male wrapped the female with his fins during egg and sperm release. See Supplementary Movie S1 for the video. (b) Following at night, captured by an infrared camera. The female was in the front right, and the male was behind her. The male was following the female. See Supplementary Movie S2 for the video. (c) Quick circling at night, captured by an infrared camera. The female was in the lower left, and the male was in front of her. The male was circling the female. See Supplementary Movie S2 for the video.

Fig. 2

(a) Frequency distribution of spawning events observed each hour over 24 h in medaka, Oryzias latipes (n = 31). The peak of spawning events was determined using gamma distribution fitting. (b) and (c) Changes in courtship behaviours from 18:00 to 17:00 over 24 h. (b) The total following duration (sec / 10 min) and (c) the number of quick-circle behaviours (number/10 min). Each plot (green circle: pre-mating male; red triangle: post-mating male) signifies the observed values from the analysed videos. The regression curves were based on the generalised additive mixed models (GAMMs) using all data (n = 744 observations from 31 males), and the shading indicates the 95% confidence intervals.