Abstract
Water temperature is a key environmental factor shaping sex determination in many gonochoristic fishes, with direct consequences for population responses to climate warming. This study is the first examination of temperature effects on sex determination in an atherionid fish, a family belonging to a taxonomic group (Atheriniformes) with abundant examples of temperature-dependent sex determination (TSD). It integrates field observations with controlled rearing experiments to analyze the possibility of TSD in the bearded silverside Atherion elymus population of Tokyo Bay, near the northern limit of distribution of this species. Histological analysis of the gonads and daily age estimation using otolith increment analyses indicated that sex determination in wild fish was completed in both males and females at about 14 days after hatching and 10 mm standard length. Reconstruction of the thermal experience of individual fish during this period did not support the assumption of thermal effects on sex determination in wild individuals. Laboratory rearing experiments conducted with five families in environmentally-relevant temperatures also consistently failed to produce sex ratio biases. These results demonstrate that sex determination in the bearded silverside population of Tokyo Bay is largely insensitive to temperature and highlight the importance of integrative field- and experiment-based approaches for assessing temperature effects on sex determination under climate warming. Although the current findings negate the presence of TSD in the bearded silverside, examination of more southern populations is warranted to rule out the possibility of a latitudinal cline in thermal sensitiveness in this species.