Allometric Growth of Feeding and Locomotor Structures During Early Ontogeny of Rabbitfish (Siganus fuscescens)

Early survival of marine fish larvae depends on the timely development of feeding and swimming functions. This study examined ontogenetic changes in relative growth patterns of feeding- and locomotion-related body parts in the mottled spinefoot rabbitfish, S. fuscescens. Larvae and early juveniles were reared under controlled conditions, and morphometric measurements were analyzed using log-log segmented regression. Body length increased gradually during the early larval stage and accelerated after approximately 10 days post-hatching. Three developmental phases were identified, with breakpoints at approximately 5 mm, 7-9 mm, and 17-19 mm body length. In the early larval phase (NL < ~5 mm), eye diameter, upper jaw length, snout length, and caudal peduncle depth showed strong positive allometry, indicating rapid acquisition of feeding and swimming functions. This was followed by a mid-larval phase characterized by near-isometric growth and stabilized body proportions. During the late larval to early juvenile phase, body depth and caudal peduncle depth again exhibited positive allometry, reflecting reorganization toward juvenile morphology. These results reveal a stage-specific growth strategy in S. fuscescens and provide a morphological basis for improving larval rearing and feeding practices.