Testis development in the Japanese eel is affected by photic signals through melatonin secretion

Abstract

Objective According to reported spawning characteristics of Japanese eel, Anguilla japonica, which exhibit spawning and migration patterns that are synchronized with lunar cycles and photoperiod, we hypothesized that a close association exists between specific photic signals (daylight, daylength, and moonlight) and endocrinological regulation. Given the photic control in melatonin secretion, this hypothesis was tested by investigating whether melatonin signals act as mediators relaying photic signals during testis development in the eel. Methods We examined changes in melatonin-secretion patterns using time-resolved fluorescence immunoassays in sexually immature and mature male Japanese eels under the condition of a new moon (NM) and a full moon (FM). Results The eye and plasma melatonin levels exhibited a nocturnal pattern under a 12-h light: dark cycle (12L12D) or under constant darkness (DD), but not with constant light (LL). Eye melatonin levels were similar under the 12L12D and short-day (9L15D) conditions. In the long-day condition (15L9D), secreted plasma melatonin levels were stable, whereas short-day melatonin secretion began when darkness commenced. Sexual maturation began at 8 weeks following intraperitoneal injection of human chorionic gonadotropin (hCG), and NM exposure led to significantly higher eye and plasma melatonin levels compared with those detected under FM exposure.

Effects of dopamine and melatonin treatment on the expression of the genes associated with artificially induced sexual maturation in Japanese eel, Anguilla japonica

“…In contrast, implanting melatonin over 5 months reduced mRNA levels of fshβ and lhβ in the pituitary of European eel, indicating a suppressive influence of melatonin on sexual maturation (Sébert et al, 2008a). Nevertheless, Hyeon et al (2021) reported that the spawning/spermiation behavior of Japanese eel during the new moon is related to melatonin secretion. An investigation of the effects of the new and full moons on immature and mature individuals revealed that the quantity of melatonin present in the eyes and plasma was higher during the new moon.…”