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One important source of phenotypic variation on which natural selection can act is developmental plasticity (the capacity of a single genotype to produce different environment-dependent forms). Therefore, studies of how the environment influences development can facilitate our understanding of how natural selection acts to yield phenotypic evolution. Using the Pacific barnacle (Balanus glandula Darwin), I explored how functionally independent appendages (the legs and unusually long penises of barnacles) respond to widespread spatial and temporal variation in water velocity and conspecific density. Through field surveys, reciprocal transplant experiments, and histological sectioning, I show that barnacle legs and penises appear remarkably well adapted to spatial and temporal variation in water velocity. Building on past work on leg form variation, I show that penises from exposed shores were shorter than, stouter than, and more than twice as massive for their length, as those from nearby protected bays (this effect holds true for artificially inflated penises as well). A transplant experiment confirmed that most of this variation in penis and leg form variation was due to developmental plasticity. Penises and legs of barnacles from an exposed shore also had thicker cuticle, and muscles with greater cross-sectional area (and shorter sarcomeres) compared to those from a protected shore. Form variation was consistent with numerous predictions from engineering theory suggesting that barnacles show dramatic, complex and likely adaptive variation in leg and penis form among sites that differ dramatically in water velocity. Additional experiments showed evidence for and against developmental limits to plasticity in barnacles. A transplant experiment identified an important (and asymmetrical) developmental limit to leg-length response time – likely mediated by food limitation – while a field survey showed that developmental coupling does not restrict adaptive plastic responses of legs and penises to multiple conflicting cues (conspecific density and water velocity). Finally, a two-year survey of natural populations revealed the first evidence that barnacles also change leg form seasonally. Together these results contribute valuable information on the mechanisms of phenotypic change. This research also sheds light on the circumstances that allow decoupling of developmental processes to produce novel combinations of characters on which natural selection can act. / Ecology