The variable colour patterns of Tilapia zillii (Cichlidae): integrating ethology, chromatophere regulation and the physiology of stress

Tilapia zillii, a substrate breeding cichlid fish, practising bi-parental care, displaysvariable colour patterns during social behaviour. This study aims at gaining moreinsight in the causation of social behaviour through ethological analysis of these dis-plays. The colour patterns are composed of a restricted number of, independentlymodifiable, melanistic marking elements (MEs) in the skin of body and fins, which cancontrast or blend with the greyish-silver ground colour, that in its turn is variable.Histological survey shows that melanophores and light-reflecting chromatophores areunequally distributed in the skin. Differential regulation of the chromatophores canexplain the variability of the colour patterns. The MEs develop during ontogeny in thesemi-transparent larvae. Initially the melanophores, being continuously in the dispersedstate, effectuate crypsis. After development of the ground colour, later in ontogeny,variability of the MEs becomes apparent. Next to crypsis, other aspects of the colourpatterns in relation with the contextual situation then start playing a role in socialbehaviour.The variable black markings do not relate directly with attack or escape tendencies. Aparticular ME becomes visible when an inclination to maintain a specific spatialposition is activated and, concomitantly, the fish is endangered (falls into a 'perturbedstate'). The greater the unreliability of current events, the darker the MEs become. Thus'perturbed state' harbours both cognitive characteristics and 'conflict': a discrepancybetween the existing situation and a reliable one. Since the same physiological factorsunderlying the regulation of melanophores are involved in the regulation of stress, theresults open prospects towards ethological implications of stress.KEYWORDS:Tilapia zillii, social behaviour, colour patterns, chromatophores, physio-logy of stres