Exploring the mechanisms and impacts of melatonin on fish colouration

The pineal hormone melatonin is a multi-functional molecule with a recognized role in pigment aggregation in chromatophores, mediating its actions through binding to subtypes of its specific receptors. Since its discovery, melatonin has been known to be responsible for pigment aggregation towards the cell centre in fishes, including their embryos, as an adaptation to reduced light and thus results in pale body colouration. Diversity exists in the sensitivity of melanophores towards melatonin at interspecies, intraspecific levels, seasons, and amongst chromatophores at different regions of the animal body. In most of the fishes, melatonin leads to their skin paling at night. It is indicated that the melatonin receptors have characteristically maintained to show the same aggregating effects in fishes and other vertebrates in the evolutionary hierarchy. However, besides this aggregatory effect, melatonin is also responsible for pigment dispersion in certain fishes. Here is the demand in our review to explore further the nature of the dispersive behaviour of melatonin through the so-called β-melatonin receptors. It is clear that the pigment translocations in lower vertebrates under the effect of melatonin are mediated through the melatonin receptors coupled with other hormonal receptors as well. Therefore, being richly supplied with a variety of receptors, chromatophores and melanocytes can be used as in vitro test models for pharmacological applications of known and novel drugs. In this review, we present diverse effects of melatonin on chromatophores of fishes in particular with appropriate implications on most of the recent findings.

Intertidal gobies acclimate rate of luminance change for background matching with shifts in seasonal temperature

Rate of colour change and background matching capacity are important functional traits for avoiding predation and hiding from prey. Acute changes in environmental temperature are known to impact the rate at which animals change colour, and therefore may affect their survival. Many ectotherms have the ability to acclimate performance traits such as locomotion, metabolic rate and growth rate with changes in seasonal temperature. However, it remains unclear how other functional traits that are directly linked to behaviour and survival respond to long-term changes in temperature (within an individual's lifetime). We assessed whether the rate of colour change is altered by long-term changes in temperature (seasonal variation) and if rate of colour change can acclimate to seasonal thermal conditions. We used an intertidal rock-pool goby Bathygobius cocosensis, to test this and exposed individuals to representative seasonal mean temperatures (16 or 31°C, herein referred to cold- and warm-exposed fish respectively) for 9 weeks and then tested their rate of luminance change when placed on white and black backgrounds at acute test temperatures 16 and 31°C. We modelled rate of luminance change using the visual sensitives of a coral trout Plectropmus leopardus to determine how well gobies matched their backgrounds in terms of luminance contrast to a potential predator. After exposure to long-term seasonal conditions, the warm-exposed fish had faster rates of luminance change and matched their background more closely when tested at 31 than at 16°C. Similarly, the cold-exposed fish had faster rates of luminance change and matched their backgrounds more closely at 16°C than at 31°C. This demonstrates that rate of luminance change can be adjusted to compensate for long-term changes in seasonal temperature. This is the first study to show that animals can acclimate rate of colour change for background matching to seasonal thermal conditions. We also show that rapid changes in acute temperature reduce background matching capabilities. Stochastic changes in climate are likely to affect the frequency of predator-prey interactions which may have substantial knock-on effects throughout ecosystems.

Regulation of eye and jaw colouration in three-spined stickleback Gasterosteus aculeatus

Fish can change their skin and eye colour for background matching and signalling. Males of Gasterosteus aculeatus develop ornamental blue eyes and a red jaw during the reproductive season, colours that are further enhanced during courtship. Here, the effects of different hormones on physiological colour changes in the eyes and jaws of male and female G. aculeatus were investigated in vitro. In an in vivo experiment, G. aculeatus were injected with a receptor blocker of a pivotal hormone (noradrenaline) that controls colour change. In males, noradrenaline had aggregating effects on melanophore and erythrophore pigments resulting in blue eyes and a pale jaw, whereas melanocyte-concentrating hormone (MCH) and melatonin resulted in a pale jaw only. When noradrenalin was combined with melanocyte stimulating hormone (MSH) or prolactin, the jaw became red, while the eyes remained blue. In vivo injection of yohimbine, an alpha-2 adrenoreceptor blocker, resulted in dispersion of melanophore pigment in the eyes and inhibited the blue colouration. Altogether, the data suggest that noradrenalin has a pivotal role in the short-term enhancement of the ornamental colouration of male G. aculeatus, potentially together with MSH or prolactin. This study also found a sex difference in the response to MCH, prolactin and melatonin, which may result from different appearance strategies in males, versus the more cryptic females.