Spectral data reveal unexpected cryptic colour polymorphism in female sailfin silverside fish from ancient Lake Matano

Persistent colour polymorphisms can result from natural and/or sexual selection, and may occur in males, females, or both sexes. Contrary to conspicuous patterns frequently observed in courtship colouration, differences in cryptic colouration are not always perceived by the human sensory system. In sexually dimorphic sailfin silversides fishes, males show conspicuous colour polymorphisms whereas females appear monomorphic and cryptic. We measured the spectral composition of body, fin and peduncle colouration in male and female Telmatherina antoniae ‘small’, a sailfin silverside species endemic to ancient Lake Matano, and found evidence for a colour polymorphism in both sexes. The three colour morphs distinguished by spectral data correspond to those commonly reported for males, and are also present in the visually (to a human eye) cryptic females. Females show hue value patterns similar to those present in males, but differ from males substantially in chroma and brightness. This is, to the best of our knowledge, the first example of a cryptic colour polymorphism in fishes; however, its significance for the mating system remains unknown. The present finding highlights the need for incorporating female spectral data into analyses of colour patterns, and suggests that colour analyses should include cryptic sexes.