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Features of Sperm Motility and Circadian Rhythm in Japanese Anchovy (Engraulis japonicus)

Dipak Pandey, Yong-Woon Ryu and Takahiro Matsubara*

Japanese anchovy (Engraulis japonicus) is a commercially and ecologically important fish that exhibits group synchronous and multiple spawning. However, the reproductive characteristics of the male in this species, especially sperm features and activation, are still largely unknown. In this study, we confirmed that features of the sperm and characteristics of the activations, regarding sperm motility and moving velocity. The average size of the sperm was 51 ± 1.3 μm in total length and possessed a normal structure with clockwise, anticlockwise, and linear motion. The initial motility at one minute after activation in seawater was 75 ± 12% during spawning time in this species (21:00– 22:00), and the initial moving velocity (196 ± 26 μm/sec) remained constant for fifteen minutes post activation. While, comparatively low motility (30 ± 10%) was found until 17:00, and the sperm was almost immotile in the morning (08:00–09:00). Swimming ability was also confirmed with sperm that swam for more than one hour in seawater without an exogenous energy supply derived from the ovary in females, suggesting the trigger for sperm activation in multiple spawning fish is possibly species dependent. This report is the first to demonstrate time specific activation, that is, circadian rhythm, in teleost males.