On the nervous and hormonal control of melanophores and their differential reactions in the catfish Ictalurus melas (Raffinesque)

Abstract

The melanophores in the catfish I. melas are arranged in 3 layers, one epidermal and two dermal. The former are the smallest, and also differ from those in the dermis in their shape and in their reaction-times in response to background change. Hypophysectomy impairs the ability of the animal to adapt to a black background but has apparently no effect on white adaptation. It severely affects the dispersion of pigment in the epidermal melanophores in comparison to the lower dermal ones. Equilibration on different greys and in darkness of hypophysectomised specimens indicates that the pituitary gland plays no role in the initial phases of dispersion or in adaptation to darkness. The removal of the pituitary also appears to accelerate the initial phases of black-to-white adaptation. The nerve fibres controlling the aggregation of pigment in I. melas follow in principle the same path as in Phoxinus. The equilibration to a white background following anterior spinal section requires 6-8 weeks. All melanophore layers of white-adapted chromatically spinal fish fully disperse their pigment in about 24 hr on transfer to a black background. On reversal the white-equilibrated condition is obtained in 7-10 days, epidermal melanophores being slowest to achieve full dispersion and being the first to aggregate it. Hypophysectomised spinal fish remain at an intermediate tint on all backgrounds and after enucleation. Observations suggest that the pituitary contributes to the dispersion of pigment, following transection of the anterior spinal cord. White background adaptation is impaired after interrupting the hypothalamo-hypophyseal tract. It is inferred that in I. melas, as in elasmobranchs and amphibians, the central control on the pars intermedia is of an inhibitory nature. Pinealectomy does not affect the ability of the animals to adapt to illuminated white or black backgrounds. The equilibrium Mi's of white/black adapted and of enucleated pinealectomised animals in darkness are strikingly similar to those of unoperated specimens. Noradrenaline and adrenaline cause pigment aggregation in isolated skin. Adrenergic blocking and anti-adrenaline substances cause a slight dispersion in dermal melanophores and moderate dispersion in epidermal melanophores. Acetylcholine has only a slight dispersion effect. The results of the present investigation can be explained on the assumption of one hormone and mononeuronic control of melanophores in Ictalurus. It is suggested that the epidermal melanophores differ from the dermals in having different threshold levels of response to MSH and to noradrenaline. Finally, it is concluded that melas in its chromatic physiology occupies a position between the teleost species in which the melanophores are predominantly neurally controlled (e.g. Phoxinus) and those in which their co-ordination is mainly hormonal (e.g. Anguilla).