Quantifying pigment cover to assess variation in animal colouration

The study of animal colouration addresses fundamental and applied aspects relevant to a wide range of fields, including behavioural ecology, environmental adaptation and visual ecology. Although a variety of methods are available to measure animal colours, only few focus on chromatophores (specialized cells containing pigments) and pigment migration. Here, we illustrate a freely available and user-friendly method to quantify pigment cover (PiC) with high precision and low effort using digital images, where the foreground (i.e. pigments in chromatophores) can be detected and separated from the background. Images of the brown shrimp, Crangon crangon, were used to compare PiC with the traditional Chromatophore Index (CI). Results indicate that PiC outcompetes CI for pigment detection and transparency measures in terms of speed, accuracy and precision. The proposed methodology provides researchers with a useful tool to answer essential physiological, behavioural and evolutionary questions on animal colouration in a wide range of species.

Melatonin does not affect the black pigment migration in the crab Neohelice granulata

N-acetyl-5-methoxytryptamine or melatonin is a multifunctional molecule. The main physiological function, at least in vertebrates, is to transduce to the animal the photoperiodic information and regulate rhythmic parameters. But studies have also observed the action of this molecule on pigment migration in ectothermic vertebrates. Thus the aim of this paper was to investigate in vivo and in vitro the influence of melatonin on the pigment migration in melanophores of the crab Neohelice granulate. Injections of melatonin (2 × 10−9 moles · crab−1) at 07:00 h or 19:00 h did not affect (p > 0.05) the circadian pigment migration of the melanophores in constant darkness. Additionally no significant pigment migration (p > 0.05) was verified in normal and eyestalkless crabs injected with melatonin (10−10–10−7 moles · crab−1) during the day or night. In the in vitro assay, the response of melanophores to the pigment-dispersing hormone in eyestalkless crabs injected with melatonin (2 × 10−9 moles · crab−1) 1 and 12 hours before the observations did not differ (p > 0.05) from the control group (injected with physiological solution). These results suggest that melatonin does not act as a signaling factor for pigment dispersion or aggregation in the melanophores of N. Granulate.