Control of a teleost social signal. II. Anatomical and physiological specializations of chromatophores.

A prominent dark facial stripe, the 'eyebar', is an important component of the reproductive coloration and dominance displays of 'barred' territorial male Haplochromis burtoni (Teleostei; Cichlidae). 'Barless' territorial males are identical to barred, behaviorally and morphologically, except they completely lack an eyebar during agonistic encounters with conspecifics. Both anatomical and physiological differences characterize eyebar pigment cells of barred and barless males. Melanophores and iridophores, which form a single structural and functional unit in the eyebar, contain less pigment in barless males. Physiologically, eyebar melanophores are tonically expanded in barred males, contracted in barless males. This physiological difference is correlated with a difference in the alpha adrenoceptor-mediated aggregation response of the cells in vitro: eyebar melanophores of barless males are significantly more responsive to physiological concentrations of the sympathetic neurotransmitter norepinephrine than those of barred males, and this intermorph difference appears to be unique to the eyebar color pattern. Physiological and morphological characteristics of eyebar pigment cells are functionally related. When barless males first begin to use the eyebar as a social display, the physiological state of melanophores changes from aggregation to dispersion. In subsequent weeks, additional pigment develops, which enhances conspicuousness of the display.