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

## TL;DR

This study shows that rabbitfish larvae grow unevenly, with feeding and swimming structures developing early to help them survive after hatching.

## Contribution

The study identifies stage-specific allometric growth patterns in a herbivorous marine fish, differing from carnivorous species.

## Key findings

- Feeding and locomotion structures show strong positive allometry in early larval stages.
- Body proportions stabilize during the mid-larval phase before changing again for juvenile life.
- Growth patterns reflect an adaptation to the herbivorous lifestyle of Siganus fuscescens.

## Abstract

The early survival of marine fish larvae depends strongly on how quickly they acquire basic abilities such as vision, feeding, and swimming. In this study, we examined how different body parts develop during early growth in the mottled spinefoot rabbitfish (Siganus fuscescens), a coastal herbivorous fish important to both ecosystems and aquaculture. Our results show that rabbitfish larvae do not grow evenly across all body parts. Instead, structures involved in feeding and movement tend to develop earlier than overall body size, which likely helps larvae begin feeding successfully soon after hatching. As development proceeds, body proportions become more balanced, and later change again as fish prepare for juvenile life in shallow coastal habitats. This developmental pattern differs from that reported for several carnivorous marine fishes and reflects a growth strategy adapted to the rabbitfish’s herbivorous lifestyle. Understanding these growth characteristics provides useful biological insight for improving larval rearing practices and for better understanding how fish larvae adapt to their environments.

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.