Abstract
The success of invasive species is frequently attributed to phenotypic plasticity, which facilitates persistence in novel environments. Here we report on experimental tests to determine whether the intensity of cryptic coloration patterns in a global invader (brown trout, Salmo trutta) was primarily the result of plasticity or heritable variation. Juvenile F1 offspring were created through experimental crosses of wild-caught parents and reared for 30 days in the laboratory in a split-brood design on either light or dark-colored gravel substrate. Skin and fin coloration quantified with digital photography and image analysis indicated strong plastic effects in response to substrate color; individuals reared on dark substrate had both darker melanin-based skin color and carotenoid-based fin colors than other members of their population reared on light substrate. Slopes of skin and fin color reaction norms were parallel between environments, which is not consistent with heritable population-level plasticity to substrate color. Similarly, we observed weak differences in population-level color within an environment, again suggesting little genetic control on the intensity of skin and fin colors. Taken as whole, our results are consistent with the hypothesis that phenotypic plasticity may have facilitated the success of brown trout invasions and suggests that plasticity is the most likely explanation for the variation in color intensity observed among these populations in nature.
Highlights
In the broadest sense, phenotypic plasticity is the ability of an individual to respond to an environmental stimulus with a change in behavioral state, morphological form, or physiological functioning [1]
Consistent with the observation that morphological color change allows individuals to match their surroundings, empirical studies have revealed reduced predation on individuals that were acclimated to substrate colors similar to conditions they would later experience in the wild [13,25]
Counter to predictions based on habitat similarity and potential gene flow, the Middle Rocky and Parkers Pond Brook did not exhibit more similar color intensity or plastic responses than the other populations, which in turn were predicted to be more similar to each other
Summary
Phenotypic plasticity is the ability of an individual to respond to an environmental stimulus with a change in behavioral state, morphological form, or physiological functioning [1]. The intensity of fish coloration is often assumed to be largely the result of phenotypic plasticity This likely stems, at least in part, from the observation that carotenoid pigments responsible for yellow and red colors are primarily dependent on uptake from the environment [15], though some species of fish (e.g., guppies, Poecilia reticulata) can supplement carotenoid colors through selfsynthesis [16,17]. The melanin-based colors are thought to be involved primarily in cryptic camouflage [23,24], though they may have a role in spawning displays and apparently can influence reproductive success [21]. Plasticity in melanin-based cryptic coloration in freshwater fishes is under both neural and hormonal regulation [22]. Consistent with the observation that morphological color change allows individuals to match their surroundings, empirical studies have revealed reduced predation on individuals that were acclimated to substrate colors similar to conditions they would later experience in the wild [13,25]
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