Abstract:The lateralization of animal behavior is widespread among vertebrates, which is particularly shown in the fish with ritualized fighting and display. The Black Paradise Fish (Macropodus spechti) is a species which shows agonistic and stereotyped displays among males. In this study, 37 male fish were kept in the individual glass tanks. All the fish were maintained isolated in their tanks which is divided by an opaque plate for 24 h before experimentation. Each fish was tested twice, once against a mirror and once against a real opponent, in a random order and with a gap of 30 minutes between each test. The mirror experiment was to replace the opaque plate with a single mirror so that the fish can see themselves in the mirror. When observations against the real opponents the opaque plate was replaced by glass plate so that the focal fish can see conspecifics (Fig. 1). Each experiment was lasted 20 minutes. The lateralization of eye-use (Fig. 2) and the number of fin display as well as, surface breaths were recorded during aggressive interactions. The data were not normally distributed and the nonparametric Wilcoxon matched-pairs signed test for data was used to compare responses to conspecifics and mirror images. It was found that the Black Paradise Fish had a great tendency of lateralization at 94.6 percent of individuals with a bias of displaying the left side and using the left eye when facing opponents. Meanwhile, there is an obvious tendency of left side in population (Fig. 3a), however, this tendency was reduced by mirror image (Fig. 4b) with no significant difference on the frequency of surface breathing (index for energy requirement and reaction intensity) (Fig. 4a). The results suggested that the laterality of display could be considered as a coordination in order to maintain the “head to tail posture” when facing opponents, while the change of this posture in the mirror image had a significant influence on the display behavior of the Black Paradise Fish (Fig. 5a), however, it did not show the intensity of air-breather difference. Therefore, the study indicated that the Black Paradise Fish lacked the self-recognition of the mirror image. Simultaneously, due to the potential effects of the existence of lateralization to our results, mirror image cannot obtain the consistent result, with conspecifics. This study could be a significant implication in guiding the use of mirror in future behavioral studies.