Environmental control of morphological color changes in the Bigfin Reef Squid, Sepioteuthis lessoniana

Phenotypic plasticity enables organisms to adjust their traits in response to environmental conditions, yet studies of animal coloration--particularly in cephalopods--have largely focused on rapid neural control while overlooking slower, substrate-level changes. Here, we investigate whether morphological color change contributes to phenotypic plasticity in the bigfin reef squid (Sepioteuthis lessoniana). Hatchlings were reared in contrasting light (white) and dark (black) environments for 14 weeks, and chromatophore organization was quantified across developmental stages (4, 9, and 14 weeks) and body regions (dorsal and ventral) using high-resolution imaging and automated analysis. We find that environmental context and age interact to shape both the density and size of black, yellow, and red chromatophores, revealing a consistent trade-off between chromatophore number and expansion. Black-reared individuals maintain higher chromatophore densities, whereas white-reared individuals exhibit fewer but larger cells, particularly at later developmental stages. These effects are region-specific and align with the emergence of countershading, suggesting ecological tuning of pigmentation architecture. Our results demonstrate that the cellular substrate underlying color change is itself plastic and dynamically remodeled across development. We propose a hierarchical model in which slow morphological plasticity defines the functional limits of rapid neural color change. This work expands current frameworks of cephalopod coloration by integrating developmental, environmental, and cellular mechanisms, and highlights the importance of multi-timescale plasticity in adaptive visual signaling.