Ecological and physiological role of the estuary in sexual maturation and spawning in the amphidromous goby Rhinogobius similis

Estuaries are important habitats not only for estuarine-resident species, but also for temporary inhabitants, such as diadromous species that migrate between marine and freshwater environments. However, the role of estuaries in the reproductive events of diadromous fish remains largely unknown. To address this gap, this study focused on an amphidromous (a diadromy subtype) goby species, Rhinogobius similis, which frequently spawns in estuaries in Japan, although its primary adult habitat appears to be in freshwater. This study investigated the degree of dependence on estuaries as spawning grounds and the effects of salinity on reproductive physiology in this species through field surveys and laboratory experiments. Otolith microchemical analysis combined with the dynamic time warping method revealed that otoliths of 29 out of 40 reproductive adults collected from an estuary (northern Kyushu, Japan) exhibited consistently low strontium/calcium (Sr/Ca) ratios throughout their juvenile and adult stages, suggesting that these individuals spent most of their time in freshwater prior to collection. A laboratory rearing experiment showed that testis development was greater in brackish water condition (salinity 10) than in freshwater, with no significant effect on ovary development. Consistently, testis development of the reproductive adults from the river estuary was marginally positively correlated with otolith Sr/Ca ratios (i.e., the degree of estuarine dependence). These results suggest that salinity partly influences gonadal development in R. similis. Overall, the series of surveys and experiments conducted in this study highlight the importance of estuaries as habitats for sexual maturation and spawning in this amphidromous goby species.