Hormonal regulation of colour change in eyes of a cryptic fish

Colour change of the skin in lower vertebrates such as fish has been a subject of great scientific and public interest. However, colour change also takes place in eyes of fish and while an increasing amount of data indicates its importance in behaviour, very little is known about its regulation. Here, we report that both eye and skin coloration change in response to white to black background adaptation in live sand goby Pomatoschistus minutes, a bentic marine fish. Through in vitro experiments, we show that noradrenaline and melanocyte concentrating hormone (MCH) treatments cause aggregation of pigment organelles in the eye chromatophores. Daylight had no aggregating effect. Combining forskolin to elevate intracellular cyclic adenosine monophosphate (cAMP) with MCH resulted in complete pigment dispersal and darkening of the eyes, whereas combining prolactin, adrenocorticotrophic hormone (ACTH) or melanocyte stimulating hormone (α-MSH) with MCH resulted in more yellow and red eyes. ACTH and MSH also induced dispersal in the melanophores, resulting in overall darker eyes. By comparing analysis of eyes, skin and peritoneum, we conclude that the regulation pattern is similar between these different tissues in this species which is relevant for the cryptic life strategy of this species. With the exception of ACTH which resulted in most prominent melanophore pigment dispersal in the eyes, all other treatments provided similar results between tissue types. To our knowledge, this is the first study that has directly analysed hormonal regulation of physiological colour change in eyes of fish.