Shades of Plasticity: Tank Color Modulates Somatolactin Cells and Pigmentary Phenotypes in Tropical Gar (Atractosteus tropicus) Larvae

During early development, environmental conditions may influence fish morphology and physiology. Among these, tank color represents a critical yet underestimated variable in research and aquaculture. This study evaluates the effect of tank color on skin pigmentation and pituitary hormones linked to stress—adrenocorticotropic hormone (Acth)—and background adaptation—melanocyte-stimulating hormone (α-Msh) and somatolactin (Sl)—in Atractosteus tropicus (Lepisosteidae, Holostei) larvae. Larvae were reared in triplicate tanks per color (white, light blue, and grey) until 15 days post-hatching. Body pigmentation was assessed using digital image processing and head melanophore counting (n = 15 per treatment; 5 per replicate). Immunohistochemistry (n = 2 replicates per treatment) was performed to quantify Acth-immunoreactive (Acth-ir), α-Msh-immunoreactive (α-Msh-ir), and Sl-immunoreactive (Sl-ir) cells. Regarding pigmentation, total relative pigmentation was significantly higher in fish reared in light-blue and grey tanks compared to white tanks (p < 0.01), with no differences between the light-blue and grey tanks (p > 0.05). Conversely, mean grey intensity was highest in fish reared in white tanks (p < 0.01). Head melanophore counts and dark pigmentation were highest in grey tanks, intermediate in light-blue tanks, and lowest in white tanks (all pairwise p < 0.05), whereas intermediate-intensity pigmentation was highest in light-blue tanks (p < 0.05). Concerning the pituitary hormone response, Sl-ir cell abundance was significantly higher in fish reared in light-blue and grey tanks compared to white tanks (p < 0.001), with no differences between light-blue and grey tanks (p > 0.05). No significant differences were observed in Acth-ir or α-Msh-ir cell abundance (p > 0.05). In conclusion, tank color modulates Sl-producing cells and induces a distinct pigmentary phenotype for each background condition in A. tropicus larvae. These findings provide the first evidence for the involvement of somatolactin in physiological background adaptation in holostean fishes and highlight tank color as a key environmental variable in aquaculture and research.