Apparent semi‐lunar spawning rhythmicity in a brackish cardinalfish

<jats:p>The purpose of the present study was to examine the lunar‐related spawning rhythmicity of the amboina cardinalfish <jats:italic>Apogon amboinensis,</jats:italic> which is a euryhaline species inhabiting mangrove creeks in the Indo‐Pacific area. During the reproductive season, the gonado‐somatic index increased towards and peaked within a week of the first and last quarter moon. Histological observations of the ovaries also showed development of yolk‐laden oocytes towards the first and last quarter moon; postovulatory follicles, an indicator of spawning, appeared around these lunar phases. Fish with immature oocytes in the ovary were also collected in the same lunar phases, suggesting that at least two fish populations with different oocyte development stages exist at any given time. The levels of two steroid hormones secreted from the ovarian pieces into a culture medium with human chorionic gonadotropin were measured using enzyme‐linked immunosorbent assay (ELISA). The level of oestradiol‐17β increased around the new and full moon periods. On the other hand, 17α,20β‐dihydroxy‐4‐pregnen‐3‐one levels increased around the first and last quarter moon, suggesting that the fish are actively undergoing vitellogenesis and final oocyte maturation around the respective lunar phase. These results suggest that with the 1 month ovarian development process, <jats:italic>A</jats:italic>. <jats:italic>amboinensis</jats:italic> exhibits a semi‐lunar spawning pattern with peaks around the first and last quarter moon. When <jats:italic>A</jats:italic>. <jats:italic>amboinensis</jats:italic> were reared under laboratory conditions without the tidal stimuli, mouth‐brooding was initiated in 13 of 20 males within a week of the first and last quarter moon. The possibility of a lunar‐related endogenous rhythm in <jats:italic>A</jats:italic>. <jats:italic>amboinensis</jats:italic> is discussed.</jats:p>