Adaptive colouration of Mediterranean labrid fishes to the new habitat provided by the introduced tropical alga Caulerpa taxifolia

Since the introduction of the invasive tropical alga Caulerpa taxifolia in the Mediterranean Sea. a new, uniformly green habitat has been available to local fish assemblages. Underwater visual censuses of three species, Symphodus ocellatus, Symphodus roissali and Coris julis, revealed a higher proportion of green morphs in the C. taxifolia meadows compared to those inhabiting their usual indigenous habitats. In a fourth species, Symphodus rostratus, the distribution of the different colour morphs did not differ between the two habitats investigated. Experiments in aquaria proved that S. ocellatus and S. roissali were homochromic species able to adapt their colouration to the environment. In contrast, no colour change was observed in S. rostratus during the experiments.