The Variable Colour Patterns of Tilapia Zillii (Cichlidae): Integrating Ethology, Chromatophore Regulation and the Physiology of Stress

Abstract

Tilapia zillii, a substrate breeding cichlid fish, practising bi-parental care, displays variable colour patterns during social behaviour. This study aims at gaining more insight in the causation of social behaviour through ethological analysis of these displays. The colour patterns are composed of a restricted number of, independently modifiable, melanistic marking elements (MEs) in the skin of body and fins, which can contrast or blend with the greyish-silver ground colour, that in its turn is variable. Histological survey shows that melanophores and light-reflecting chromatophores are unequally distributed in the skin. Differential regulation of the chromatophores can explain the variability of the colour patterns. The MEs develop during ontogeny in the semi-transparent larvae. Initially the melanophores, being continuously in the dispersed state, effectuate crypsis. After development of the ground colour, later in ontogeny, variability of the MEs becomes apparent. Next to crypsis, other aspects of the colour patterns in relation with the contextual situation then start playing a role in social behaviour. The variable black markings do not relate directly with attack or escape tendencies. A particular ME becomes visible when an inclination to maintain a specific spatial position is activated and, concomitantly, the fish is endangered (falls into a 'perturbed state'). The greater the unreliability of current events, the darker the MEs become. Thus 'perturbed state' harbours both cognitive characteristics and 'conflict': a discrepancy between the existing situation and a reliable one. Since the same physiological factors underlying the regulation of melanophores are involved in the regulation of stress, the results open prospects towards ethological implications of stress.