Thermal adaptation of energy allocation and reproduction during aestivation in northern and southern populations of western sand lance (Ammodytes japonicus).

Understanding how thermal environments affect energy allocation and reproduction is critical for predicting species responses to climate change. The western sand lance (Ammodytes japonicus) is a small cold-water fish distributed from the warm Seto Inland Sea (southern population) to the cooler Sendai Bay (northern population) in Japan. This species undergoes summer dormancy (aestivation) by burrowing into sandy substrates and ceasing feeding. Under three controlled summer temperature regimes (23 °C, ambient, and 28 °C), we examined lipid and protein consumption, and reproductive traits in males and females from both populations. Lipid content declined markedly under high temperatures in both populations. Even under identical summer temperature conditions, total energy reserves declined more in northern fish (56%) than in southern fish (46%). Protein content remained consistently higher in southern individuals throughout the experiment. During the spawning season, the southern population showed greater gamete production efficiency than their northern counterparts. Specifically, southern females produced approximately 2.2 times more oocytes per unit of energy, despite having slightly smaller yolk volume of ovulated eggs (84% of northern values). These findings indicate that the southern population exhibits a different pattern of reproductive energy allocation, producing more gametes per unit of consumed energy during aestivation. In contrast, although the northern population stored more lipids, it experienced greater energy loss and lower reproductive output per unit of consumed energy under elevated temperatures. These differences may explain the recent population collapse of the northern population and highlight potential vulnerability to ongoing ocean warming.