Bivariate and multivariate analyses of allometric growth in larvae of farmed meagre (Argyrosomus regius): an integrated approach to define growth stages and ontogenetic transitions

Fish larval rearing represents a critical stage due to the extreme fragility of larvae, small size, and specific nutritional needs during rapid growth. Successful larviculture requires understanding ontogenetic changes to adapt rearing practices and optimize larval development. Thus, the objective of this study was to describe the ontogeny and allometric growth of hatchery-reared meagre ( Argyrosomus regius ) larvae from hatching to the juvenile stage by combining bivariate and multivariate morphometric analyses. Results showed that the early development of meagre was divided into three distinct growth stages marked by functional transitions. The first stage (≤3.21 mm standard length, SL) coincided with yolk absorption, onset of exogenous feeding, swim bladder inflation, and accelerated cranial development, particularly in the operculum and eye–operculum region, supporting respiration and feeding. The second stage (3.21–5.35 mm SL) was characterized by near-isometric cranial growth and positive allometry in trunk and caudal traits, reflecting gastrointestinal maturation and enhanced swimming capacity. The third stage (>5.35 mm SL) included substantial cranial and caudal differentiation, such as jaw ossification, fin formation, and sensory refinement, particularly visual enhancement, enabling efficient prey detection and capture. Principal component analysis identified two inflection points (3.21 and 5.35 mm SL), marking developmental transitions from cranial growth to locomotor enhancement and the onset of juvenile traits. These morpho-functional transitions highlight critical windows for aquaculture, such as ensuring optimal light conditions and swim bladder inflation during the first stage, providing highly digestible diets and adequate buoyancy during the second, and optimizing live prey presentation and water clarity during the third. This study showed that while traditional allometric approaches examine isolated characters relative to organism size, multivariate allometry offers comprehensive understanding of how multiple larval body regions scale together during size changes. In conclusion, this integrated approach highlights coordinated allometric transitions during meagre larval development, providing a framework to identify critical growth phases and refine rearing protocols for improved larval performance and survival.