Early Ontogenetic Allometry and Shifting Modularity in Skull Development of the Red Sea Anemonefish, Amphiprion bicinctus.

Modularity is an inherent property of organismal design where organisms are subdivided into quasi-independent units. Studies have shown that patterns of modularity can dramatically shift during the ontogenies of direct developing organisms. However, it is unclear how modularity patterns shift in organisms that undergo metamorphosis which is a dynamic period of development where organisms undergo morphological changes in response to ecological or physiological cues. Here we examined the ontogenetic modularity of the skull in the Red Sea anemonefish, Amphiprion bicinctus, across four larval stages, preflexion, flexion, postflexion, and metamorphosis, using micro-CT scanning and 3D geometric morphometrics to assess ontogenetic allometry and modularity. We hypothesized that skull modularity reorganizes during developmental transitions, and that oral jaw elements exhibit strong allometric growth and integration linked to functional demands during larval stages. The mandible, premaxilla, and maxilla exhibited strong size-shape relationships while the lower pharyngeal jaw and parasphenoid were isometric. Morphological disparity peaked at preflexion, suggesting high developmental plasticity early in ontogeny. We tested modularity hypotheses based on developmental and functional interactions and found that the developmental hypotheses were favored across most stages. However, during flexion, a stage characterized by structural reorganization, support shifted to functional hypotheses and then reverted back to developmental hypothesis. This suggests that modular reorganization coincides with key functional transitions. Across all stages, the premaxilla and mandible remained highly integrated, underscoring the need for coordinated oral jaw development during early feeding. Our findings reveal how allometry, modularity, and integration interact during early skull development in a species undergoing rapid developmental and ecological transitions and highlight the functional importance of oral jaw coordination during early feeding.