The nomenclature Astronotus ocellatus includes a large number of varieties of oscar due to its differences in colouration and patterns over the body, but all of them have been reported to be under a single genus and species. There has been no report on the phenotypic variation of colour with culture periods and the exact taxonomic status of the reported varieties of oscar. In this context, an experiment of 90 days duration was conducted to assess the colour variation in different varieties of oscars (albino, common and red oscar), fed on non-carotenoid based feed and their subsequent molecular differentiation, The different dimensional characteristics of colour appearance (hue - H˚ab and chroma - C*ab) were calculated from the pectoral and caudal regions of each variety at 30 day intervals, wherein, no significant (p≤0.05) differences were recorded in the parameters throughout experimentation in albino oscar indicating stability of the yellow spectrum. However, common oscar recorded significant variation (p≤0.05) in the chroma and hue angle values between initial and subsequent period of experimentation. Red oscar recorded increasing trend in chroma and hue values, indicating decrease of red spectrum with time. Therefore, yellow spectrum in wild phenotypes may be concluded to be more stable than the red spectrum and such varieties having a larger proportion of the yellow spectrum (such as the albino) may be profitably cultured on non-carotenoid based diets. Stability of colour for longer periods without the supplementation of carotenoids is a subject for future research.
The molecular differentiation of different strains (albino, common and red oscars) using barcode and RAPD reveals genetic distance among the strains. The albino and common oscar show similar GC3 value (39.2) and red oscar shows variation from these values (38.6). The Nei's genetic distance clearly indicate that the red oscar is a different strain and genetically distant from both albino and common oscar (0.6161 and 0.5007 respectively). The present molecular study helps to resolve the ambiguity over the pure strains and hybrid varieties in this high value ornamental cichlid and will be helpful during selective breeding programs. The study shows that RAPD and barcoding can be used up to a considerable extend for identifying different strains. However, further researches using more specific markers will establish the exact taxonomic position of the fish.