Is intraspecific variation in diet and morphology related to environmental gradients?Exploring Liem’s paradox in a cichlid fish

Interspecific studies have demonstrated that trophic morphology and ecology are not always tightly matched:a phenomenon rarely reported at the intraspecific level.In the present study,we explored relationships among diet,morphology and the environment in a widespread cichlid fish,Astatoreochromis alluaudi(Pellegrin 1904),from 6 sites in southern Uganda to test for evidence of eco-morphological matching at the interdemic level.Previous studies of Astatoreochromis alluaudi have demonstrated developmental plasticity in trophic morphology in response to diet:a mollusk diet produces specimens with large pharyngeal jaws and muscles,whereas a soft-food diet produces smaller pharyngeal jaws and corresponding changes in musculature.Sites were chosen to maximize variability in environmental variables that might directly or indirectly affect trophic morphology.We found significant differences in pharyngeal jaw and muscle morphology among populations.Similarly,we found differences in diets among sites:mollusks were found in the stomachs of fish from only 2 populations sampled,despite the presence of mollusks in 5 of the 6 sites.Although trophic morphology did match the observed diet in 2 sites,diet did not correlate with either morphology or environmental variables across sites,nor were environmental variables correlated with morphological variation among sites.These results suggest that mismatch can occur among different populations of a single species for reasons such as seasonality in resources,developmental plasticity and/or complex indirect interactions.Intraspecific mechanisms should be further studied in order to better understand the complex relationships between morphological specialization and ecological generalization.

Phenotypic plasticity is maintained despite geographical isolation in an African cichlid fish,Pseudocrenilabrus multicolor

Gene flow among populations in different selective environments should favor the evolution of phenotypic plasticity over local adaptation.Plasticity in development is a common response to long-term hypoxia in some widespread African fishes,including Pseudocrenilabrus multicolor,a cichlid that exploits both normoxic(high oxygen)rivers/lakes and hypoxic(low oxygen)swamps.Previous studies have shown that fish from normoxic and hypoxic sites differ in many traits,including gill size,brain size and body shape,and that much of this vari-ation reflects developmental plasticity.However,these earlier studies focused on areas in Uganda where gene flow between swamp and river or lake populations is high.In this study we tested the hypothesis that P.multi-color from a relatively isolated lake population(Lake Saka,Uganda)exhibit low levels of plasticity in traits re-lated to oxygen uptake.Multiple broods of P.multicolor from Lake Saka were reared under low and high dis-solved oxygen,and traits related to gill size,brain mass and body shape were quantified.Surprisingly,both gill size and brain mass showed high levels of developmental plasticity.We suggest that high levels of plasticity,particularly in the gill size of P.multicolor,reflects low costs of maintaining the plastic response,even in rela-tively isolated populations.