Pigmentation anomalies in marine fishes: novel records from Spanish coastal waters and a global synthesis of wild flatfish ambicolouration.

This study documents unusual pigmentation patterns in marine fishes from Spanish coastal waters and provides a global synthesis of published records of ambicolouration in wild flatfishes. A mixed pigmentation anomaly was recorded in Diplodus sargus (Sparidae), severe hypopigmentation compatible with oculocutaneous albinism was documented in Halobatrachus didactylus (Batrachoididae), and near-complete ambicolouration was observed in Solea senegalensis (Soleidae). Three additional unusual pigmentation patterns involving Labrus bergylta (Labridae) and D. sargus were documented photographically during underwater observations and are regarded as putative pigmentation anomalies because natural colour variation and physiological colour change cannot be excluded. The specimens were obtained opportunistically through fishery bycatch, scientific surveys and underwater observations conducted in Galicia and Andalusia between 2025 and 2026. DNA barcoding confirmed the species identity of the three collected specimens. The discovery of an ambicoloured Solea senegalensis prompted a review of this condition in wild flatfishes worldwide. The literature review identified 100 studies reporting ambicolouration in 79 wild flatfish species from 10 of the 19 recognized families. Pleuronectidae and Paralichthyidae contained the largest numbers and proportions of species with published records, although these patterns may be influenced by uneven research and reporting effort. Pigmentation anomalies, although uncommon in nature, have been associated with genetic, developmental or environmental processes, but their environmental significance cannot be determined from opportunistic records alone. Standardized long-term monitoring accompanied by population and environmental data would be required to assess their potential value in ecosystem assessment.