Abstract
Environmental variation is a potent force affecting phenotypic expression. While freshwater fishes have provided a compelling example of the link between the environment and phenotypic diversity, few studies have been conducted with arid-zone fishes, particularly those that occur in geographically isolated regions where species typically inhabit intermittent and ephemeral creeks. We investigated morphological variation of a freshwater fish (the western rainbowfish, Melanotaenia australis) inhabiting creeks in the Pilbara region of northwest Australia to determine whether body shape variation correlated with local environmental characteristics, including water velocity, habitat complexity, predator presence, and food availability. We expected that the geographic isolation of creeks within this arid region would result in habitat-specific morphological specializations. We used landmark-based geometric morphometrics to quantify the level of morphological variability in fish captured from 14 locations within three distinct subcatchments of a major river system. Western rainbowfish exhibited a range of morphologies, with variation in body depth accounting for a significant proportion (>42%) of the total variance in shape. Sexual dimorphism was also apparent, with males displaying deeper bodies than females. While the measured local habitat characteristics explained little of the observed morphological variation, fish displayed significant morphological differentiation at the level of the subcatchment. Local adaptation may partly explain the geographic patterns of body shape variation, but fine-scale genetic studies are required to disentangle the effects of genetic differentiation from environmentally determined phenotypic plasticity in body shape. Developing a better understanding of environment–phenotype relationships in species from arid regions will provide important insights into ecological and evolutionary processes in these unique and understudied habitats.
Highlights
Species typically exhibit considerable phenotypic variation across their geographic range (Endler 1986; Wade and Kalisz 1990; Foster and Endler 1999), where much variation can be explained by climate, habitat type, and predation pressure (Losos et al 1998; Nagel and Schluter 1998; Schluter 2000; Langerhans and DeWitt 2004)
Ecology and Evolution published by John Wiley & Sons Ltd
We found that western rainbowfish exhibited considerable variation in body shape and marked sexual dimorphism, with males displaying deeper bodies and shorter caudal peduncles than females
Summary
Species typically exhibit considerable phenotypic variation across their geographic range (Endler 1986; Wade and Kalisz 1990; Foster and Endler 1999), where much variation can be explained by climate, habitat type, and predation pressure (Losos et al 1998; Nagel and Schluter 1998; Schluter 2000; Langerhans and DeWitt 2004). Phenotypic variation can be attributable to numerous factors, including (1) individual genotypes producing different phenotypes in response to changing environments (i.e., phenotypic plasticity), (2) populations exhibiting fixed differences in phenotypic traits in response to selection (adaptive differentiation), and (3) processes such as genetic drift and developmental constraints (Price et al 2003). Revealing the influence of environmental and ecological factors on phenotypic variation can provide valuable insights into evolutionary processes such as population differentiation and ecological speciation (Maynard Smith 1966; Schluter 1996; Rundle and Nosil 2005). Ecology and Evolution published by John Wiley & Sons Ltd. Ecology and Evolution published by John Wiley & Sons Ltd
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