Egg deposition substrate of Japanese medaka Oryzias latipes in a rice paddy.

Abstract

Although wild Japanese medaka Oryzias latipes is known to reproduce in rice paddies, reproductive ecology of medaka in rice paddies has not been studied. It is known that the female medaka deposits fertilized eggs on appropriate substrates, such as aquatic plants, after the spawning behavior by a pair of male and female. In the present study, we observed which plant species were used as egg deposition substrates in a rice paddy on a university campus, where medaka reproduce naturally. Eggs of medaka were observed on Chara braunii, Trapa jeholensis, and Spirogyra sp., but not on rice Oryza sativa or Monochoria vaginalis. The study revealed that medaka did not utilize O. sativa, but utilized mainly aquatic plants, so-called weeds, as egg deposition substrates in the rice paddy. Spraying herbicide may be not only directly toxic to medaka but also indirectly threatening their reproduction due to deprivation of egg deposition substrate.