The integumentary chromatophone patterns in the plaice Pleuronectes platessa L., with special reference to their responses towards different variegated backgrounds

Abstract

The integumentary pattern of the plaice Pleuronectes platessa is divisible into the dark patches, the dark zones, the pale zone, the pale flecks, the white spots, orange and black spots and the black dots. These colour pattern---units differ in size and density and in the arrangement of their chromatophores. The ultrastructure of the chromatophores has been observed.

The complex phenomenon of "pattern-matching" in the plaice is largely attributable to the differential responses of the upper dermal melan-ophores in the different pattern-units. The plaice responds to very small changes in the visual composition of its background: ; reflectivity; type of pattern; size of the figures comprising the pattern; proportion of black and white in the under-lying substratum; distance of the components of the pattern from both eyes. An analysis of these differential responses has been proposed.

"Pattern-matching" in the plaice appears to be predominantly neurally initiated and is later assisted by its hormonal system. The chromatic responses of long white- and long black-adapted fishes are strikingly different from each other and from those of the fishes adapted to a middle tint. An account is given of the neurally controlled responses of the long white- and black-adapted fishes towards patterned backgrounds after their background reversal and their subsequent return to the original backgrounds. The effect of length of background adaptation on the neurally controlled responses of the plaice has been described. It has been suggested that in relation with its nervous activity, the plaice adapts to a new background in three phases: Phase I, II and III.

The chromatic activity of intact and unilaterally enucleated fishes suggests the existence of a central integrating mechanism for most of the retinal information. Some retinal stimuli are capable of bringing about melanosomal movements'contralaterally.'

Melanization of the unpigmented 'ventral' side of the plaice can be caused under experimental conditions. The pattern thus produced does not correspond with that of the 'dorsal' side. The neurally controlled responses of the 'ventral' melanophores suggest a mode of innervation somewhat different from that of the 'dorsal' melanophores.

The differential responses of the melanophores in different pattern-units can be observed not only by a differential retinal stimulation, but also by the administration of some adrenergic drugs. An adrenergic innervation of the melanophores and a differential distribution of adrenoceptor-populations have been suggested to account for the differential responsiveness of these cells.

Integumentary transformations in the plaice during background pattern adaptation may be brought about by the selective stimulation of melanophores in various pattern-units through nerve-fibres originating from a central integrating mechanism.