Matthias F. Wucherer; Nico K. Michiels

“ Most pigment-containing chromatophores (melanophores, erythrophores, xanthophores and cyanophores) are not only specialized in producing and storing, but also in translocating their numerous pigment particles [8] , [10] . They increase color visibility by radial particle dispersal or reduce it by concentric particle aggregation. In contrast to these pigment-based mechanisms, iridophores are chromatophores, which use refractive guanine platelets to create a silvery appearance or to generate structural colors (like most blue hues in fishes) [11] . ”

“ Rapid nervous control suggests that the fish might use this fluorescence for a purpose that requires rapid change in fluorescent color.
In addition, α–MSH and MCH, both hormones known for their melanophore-modulating properties, showed significant effects on the fluorescent cells. Fluorescent particles dispersed when stimulated with α–MSH and aggregated when stimulated with MCH. Again, this is similar to the response known from melanophores. In general, hormonal responses are much slower than those resulting from neuronal action. ”

“ If so, their fluorescence could be part of an adaptive body coloration system and may indicate the existence of communication on the long-wavelength side of the visible spectrum in marine fishes. We compare the fluorescent cells to other chromatophores on the basis of well-described characteristic features of melanophores [23] . The intrinsic neurotransmitter involved in this brain-controlled change of darkness of melanophores is noradrenaline [24] , [25] .
In addition to rapid neuronal changes, hormones can also influence body coloration. ”

“ UV-radiation might not be the only part of the light spectrum that is used more in visual communication than previously assumed. For instance, red displays cannot be generated by reflection in depths below 10 to 20 m, because water quickly attenuates the longer wavelengths (>600 nm) of the incoming sunlight [19] . As a consequence, pigments cannot reflect red and will appear grey or black in most of the euphotic zone of many marine environments. ”

“ These cells are on average 85 μm in diameter (±16.8 μm SD, n = 12) and their intracellular fluorosomes measure 1.3 μm (±0.2 μm SD, n = 10) . Spectrometric measurements show that the red coloration produced by these cells is pure fluorescence with no reflectance. Optimal excitation occurs at around 500 nm, whereas most red light is emitted at 595 nm (Figure 3) . Figure 4 shows red erythrophores and melanophores in bright field microscopy. The fluorescent cells are almost invisible under these conditions. ”

“ Note that erythrophores, melanophores and fluorescent chromatophores are spatially distributed and can be distinguished. Scale bar = 400 μm. In all three types of manipulation, the fluorescent cells have translocated their fluorosomes either towards the nucleus or away from it (Figures 5 and 6) . There was a significant reduction of fluorescent area after (neuronal) K+ stimulation (paired t-test, t = 10.5, df = 5, p<0.005) . ”

“ The Lidocain-treatment effectively inhibited this effect (paired t-test, t = 1.86, df = 4, p>0.13) . Neurotransmitter-induced aggregation of fluorosomes (NA) was highly significant (paired t-test, t = 4.67, df = 9, p<0.001) . Aggregation induced by MCH was significant (paired t-test, t = 5.19, df = 3, p<0.013) as well as α–MSH significantly induced dispersal in pre-aggregated cells (paired Wilcoxon, Z = 10.5, df = 5, p = 0.03) . Bars include standard errors. In the hormonal treatments the fluorescent chromatophores also act analogous to melanophores. ”

“ Then the Solution was Replaced According to the Three Tested Hypotheses Saline with elevated K+ was given to stimulate action potentials in neurons (in mM: NaCl 78, KCl 50, CaCl2 1.8, MgCl2 1.8, D (+) -Glucose 5.6, Tris-HCl 5.0, pH 7.2; the total ion concentration was kept constant relative to the physiological solution) . ”