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Environmental differences among populations are expected to lead to local adaptation, while spatial or temporal environmental variation within a population will favour evolution of phenotypic plasticity. As plasticity itself can be under selection, locally adapted populations can vary in levels of plasticity. Nine-spined stickleback ( Pungitius pungitius) originating from isolated ponds (low piscine predation risk, high competition) vs. lake and marine populations (high piscine predation risk, low competition) are known to be morphologically adapted to their respective environments. However, nothing is known about their ability to express phenotypic plasticity in morphology in response to perceived predation risk or food availability/competition. We studied predator-induced phenotypic plasticity in body shape and armour of marine and pond nine-spined stickleback in a factorial common garden experiment with two predator treatments (present vs. absent) and two feeding regimes (low vs. high). The predation treatment did not induce any morphological shifts in fish from either habitat or food regime. However, strong habitat-dependent differences between populations as well as strong sexual dimorphism in both body shape and armour were found. The lack of predator-induced plasticity in development of the defence traits (viz. body armour and body depth) suggests that morphological anti-predator traits in nine-spined stickleback are strictly constitutive, rather than inducible. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ••, ••-••. [ABSTRACT FROM AUTHOR]

1. Individual- and population-level variation in body size and growth often correlates with many fitness traits. Predation and food availability are expected to affect body size and growth as important agents of both natural selection and phenotypic plasticity. How differences in predation and food availability affect body size/growth during ontogeny in populations adapted to different predation and competition regimes is rarely studied. 2. Nine-spined stickleback ( Pungitius pungitius) populations originating from habitats with varying levels of predation and competition are known to be locally adapted to their respective habitats in terms of body size and growth. Here, we studied how different levels of perceived predation risk and competition during ontogeny affect the reaction norms of body size and growth in (i) marine and pond populations adapted to different levels of predation and competition and (ii) different sexes. We reared nine-spined stickleback in a factorial experiment under two levels of perceived predation risk (present/absent) and competition (high/low food supply). 3. We found divergence in the reaction norms at two levels: (i) predation-adapted marine stickleback had stronger reactions to predatory cues than intraspecific competition-adapted pond stickleback, the latter being more sensitive to available food than the marine fish and (ii) females reacting more strongly to the treatments than males. 4. The repeated, habitat-dependent nature of the differences suggests that natural selection is the agent behind the observed patterns. Our results suggest that genetic adaptation to certain environmental factors also involves an increase in the range of expressible phenotypic plasticity. We found support for this phenomenon at two levels: (i) across populations driven by habitat type and (ii) within populations driven by sex. [ABSTRACT FROM AUTHOR]