Colour affects shoal choice in rainbowfish

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Leeds researchers have shown that environmentally-induced changes in body colour affect shoal choice in the Westen rainbowfish, Melanotaenia australis.

The experiments, carried out by Gwendolen Rodgers, Jennifer Kelley and Lesley Morrell explored whether fish that had undergone an environmentally-induced colour change would prefer to shoal with fish of the same colour, or if would they be “colour blind” to their shoal mates.

Rainbow fish live in small shoals with colour varying between populations. One of the main reasons for shoaling is that it offers safety in numbers: it is harder for a predator to pick a single individual out of a shoal than if a fish was swimming alone.

The exception to this is the “oddity effect”  where animals that have a different appearance from the rest of their group are more readily preyed upon. It would be expected that similar looking fish would shoal together, but would this hold true after environmental colour change, rather than genetic differences in colour?

By keeping fish in either dark or pale coloured aquaria, the researchers were able to alter the colour of the fish – black pigments increased in those in the dark tanks (above top) and decreased in those in the pale tanks (above bottom). When given the choice between two separate shoals in a sub-divided tank, overwhelmingly, fish preferred to shoal with those of a similar colour, even if they had been the opposite colour to start with.  

The authors are particularly pleased with the results as they give further evidence of the complex behaviours fish can exhibit: "These combined processes of morphological and behavioural background matching amount to a sophisticated suite of colour-mediated anti-predator defences", commented Rodgers.

For more details, see Colour change and assortment in the western rainbowfish, Gwendolen M. Rodgers, Jennifer L. Kelley, Lesley J. Morrell. Animal Behaviour 79 (5): 1025-30.