Abnormal skin pigmentation leading to full xanthochromism in a brown surgeonfish (Acanthurus nigrofuscus) from Okinawa Island, Japan

Xanthochromism, rare condition characterized by the dominance of yellow-orange pigment in animals, results from the disappearance of melanophores (black pigment cells) thus exposing xanthophores (yellow pigment cells) and revealing yellow coloration (Jawad and Ibrahim 2018).This abnormal coloration (as opposed to phenotypic color variation) has been documented in at least 68 fish species and has been suggested to reduce survival fitness in the wild and diminish commercial quality (Ban et al. 2023).Although its etiology is unresolved, it has been attributed to multiple factors, including genetic mutations, environmental conditions, nutrition, and diseases (Ban et al. 2023).However, most records are based on singular timepoint observations (e.g., fisheries specimens), which limits our understanding of the developmental mechanisms of this uncommon phenomenon.Here we report on an individual brown surgeonfish (Acanthurus nigrofuscus (Forsskl, 1775)) showing an abnormal skin pigmentation progressing to full xanthochromic coloration.Normal body coloration in A. nigrofuscus is melanistic, ranging from light brown to black, with black spots on the axil of the dorsal and anal fins, and occasional orange spots on the head (Fig. 1A).In Okinawa, A. nigrofuscus is one of the most abundant nominal herbivores and individuals cannot