| Summary: | The integument of the minnow phoxinus phoxinus (L.) was studied using both light and electron microscopy and found to be similar in general structure to that of other teleosts. The various types of chromatophores and their location in the integument are described. In general, melanophores have well organized microtubule systems and their possible roles in the mechanism of pigment granule movement are discussed. Fine structural and histochemical studies of melanophores suggest that their innervation is single and is probably adrenergic. A continuous observation apparatus was employed to study in the living fish the responses to black and white background reversals of melanophores with intact innervation and of similar cells disconnected from the central nervous system by spinal nerve section. Results indicate that both pigment aggregation and pigment dispersion are active processes. Electrical stimulation of the spinal cord, which has been shown to result in pigment aggregation, evoked pigment dispersion in paled chromatically spinal fish pretreated with bretylium, an adrenergic neuron blocking agent. The effects of alpha- and beta-adrenoceptor agonists and antagonists on chromatically normal and chromatically spinal fish were studied. Noradrenaline, adrenaline (alpha agonists) and, in relatively higher concentrations, isoproterenol (a beta agonist) were found to be potent pigment-aggregating agents on melanophores of chromatically normal and chromatically spinal black-adapted fish. Neither isopro-terenol nor the more specific beta agonists fenoterol and isoxsuprine, injected in various concentrations were able to evoke pigment dispersion in chromatically normal white-adapted fish. However, they resulted in marked pigment dispersion (isoproterenol in relatively lower concentrations) in melanophores of chromatically spinal, prolonged white-adapted fish. It is generally concluded that the melanophores of the minnows have only an adrenergic innervation and that the mechanisms of pigment aggregation and pigment dispersion are mediated by alpha-and beta-adrenoceptors respectively. The effects of the incident light intensity on the rate and magnitude of the fish's response to black and white background reversals were also studied. The response of the fish was found to be constant over a wide range of incident light intensities. In the complete absence of light, or under very dim overhead illumination, melanophores in the lateral stripe showed some primary response. The fine structure of the minnow retina was studied. The various types of photoreceptor and their responses in different conditions of illumination are described.
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