Phenotypic plasticity is the capacity of an organism's phenotype to vary in different
environments. Although diet-induced phenotypic plasticity has been documented in New World
cichlids, it has been hypothesised that this type of plasticity would be limited in certain
Old World cichlids, because of the morphological constraints on the jaw imposed by
mouth-brooding. This hypothesis was experimentally tested by determining the effect of
different diets on the head and jaw morphology of split broods of several species of
haplochromine cichlids from Lake Malawi, Africa, and two substrate-spawning cichlids,
one from the Old World, Tilapia mariae Boulenger), and one from the New World,
Herichthys cyanoguttatum (Baird and Girard). Different feeding regimes resulted
in differences in head morphologies in both New and Old World cichlid species. Although
Old World mouth-brooding haplochromine cichlids exhibited phenotypic plasticity, the
magnitude of head-shape plasticity observed was greater in the New World substrate-spawning
cichlid, H. cyanoguttatum. The Old World tilapiine cichlid, T. mariae,
did not exhibit phenotypic plasticity of head morphology. Experiments with modified foods
demonstrated that the observed changes were unrelated to dietary nutrition, but were a
result of differing feeding modes. Phenotypic plasticity might have contributed to the
extensive trophic radiation and subsequent explosive speciation observed in Old World
haplochromine cichlids. The existence of phenotypic plasticity has implications for
morphology-based species descriptions as well.