A complex pigmentation disorder reveals energetic and ecological costs of coloration in a deep-sea sharpnose sevengill shark.

Pigmentation is a key functional trait influencing camouflage, predator-prey interactions and energetic efficiency in marine organisms, yet its physiological and ecological consequences remain poorly understood in deep-sea sharks. Here, we describe a deep-sea shark (Heptranchias perlo) exhibiting a mosaic pigmentation disorder characterized by the coexistence of hypermelanotic, hypopigmented and amelanotic regions, indicating disruption of normal melanophore distribution and regulation. Histological examination revealed no structural or inflammatory abnormalities, supporting a non-pathological origin of the pigmentation anomaly. In contrast, condition indices indicated pronounced energetic depletion, with reduced condition factor and hepatosomatic index, while lipid extraction and Fourier-transform infrared and ultraviolet-visible spectroscopy revealed substantial depletion and altered composition of hepatic lipid reserves consistent with chronic negative energy balance relative to phenotypically normal conspecifics. We propose that disruption of countershading in hexanchiform sharks may reduce camouflage efficiency and increase energetic costs, contributing to the observed physiological compromise in sharks. Despite being based on a single individual, this integrative analysis links pigmentation anomalies to functional and energetic consequences, and underscores the need to move beyond descriptive accounts toward mechanistic assessments of coloration in marine predators, particularly in deep-sea elasmobranchs that are inherently rarely encountered.