Metabolic Rates of Japanese Anchovy (Engraulis japonicus) during Early Development Using a Novel Modified Respirometry Method

SIMPLE SUMMARY: Ontogenetic phase shift (i.e., a sudden change) in metabolism has been observed in several fish species during their early development, and understanding such metabolic shifts helps to understand their development trajectory. However, it is still unclear whether this sudden change in metabolism occurs typically in various fish species since larval-stage fish are vulnerable to physical stimuli due to their small size. Here, I developed a novel modified respirometry method by combining a peralstatic pump and the traditional semi-closed measurement method to measure metabolic rate changes in a pelagic fish species, the Japanese anchovy (Engraulis japonica). As a result, sudden changes in metabolism occurred in this species at a body weight of approximately 0.001 and 0.01 g. Furthermore, these changes coincided with their morphological and behavioral changes in this species. These findings confirm that a pelagic species undergoes similar metabolic changes during early development as the more widely studied non-pelagic species and will be useful in future metabolic studies. ABSTRACT: The allometric relationship between metabolic rate ([Formula: see text]) and body mass ([Formula: see text]) has been a subject of fascination and controversy for decades. Nevertheless, little is known about intraspecific size-scaling metabolism in marine animals such as teleost fish. The Japanese anchovy Engraulis japonicus is a planktotrophic pelagic fish with a rapid growth and metabolic rate. However, metabolic rate measurements are difficult in this species due to their extremely small body size after hatching. Herein, the metabolic rate of this species during its early developmental stage was measured for 47 individuals weighing 0.00009–0.09 g (from just after hatching to 43 days old) using the micro-semi-closed method, a newly modified method for monitoring metabolism developed specifically for this study. As a result, three distinct allometric phases were identified. During these phases, two stepwise increases in scaling constants occurred at around 0.001 and 0.01 g, although the scaling exponent constant remained unchanged in each phase ([Formula: see text] = 0.683). Behavioral and morphological changes accompanied the stepwise increases in scaling constants. Although this novel modified respirometry method requires further validation, it is expected that this study will be useful for future metabolic ecology research in fish to determine metabolism and survival strategy.