A developmental reaction norm integrates three tightly linked factors of ontogeny, genotype, and environment to address the ability of an organism to deal with environmental change. This concept of organismic flexibility is termed plasticity, and is well characterized in coral reef systems. However, there has been little quantification of how phenotypic plasticity in scleractinian corals may modulate their response to ocean acidification. This thesis consists of two studies addressing the role of ontogeny, genotype, and environment as influences on phenotypic complexity in scleractinian corals that may affect their response to ocean acidification. In Chapter 2, to address ontogeny, I investigated the effects of elevated pCO2 on the movement and behavior of brooded Pocillopora damicornis larvae in Okinawa, Japan, in 2016. A change in behavior in this developmental stage may alter distribution and settlement patterns of adult colonies of P. damicornis. I found that brooded larvae freshly released from P. damicornis are able to regulate their vertical position in the seawater over at least 12 h, and that this response, likely driven by a combination of modified buoyancy and active swimming, is affected by high pCO2 . A change in vertical position of larvae due to elevated pCO2 has the potential to mediate pelagic larval duration (PLD) by determining their exposure to differing horizontal strata of water, thereby mediating the extent of larval connectivity among populations. In Chapter 3, to address genotype and environment, I first observed the effect of genotype-specific variation within adult colonies of P. damicornis in their growth response to elevated pCO2 in Moorea, French Polynesia, in 2016. In this preliminary experiment, I found differences among genotypes in mean growth rate that varied among trials conducted in different months, likely due to the environmental history of the corals. To quantify plasticity in two different environments, I conducted an experiment in 2017 that investigated how a plastic response in a coral to an environment change might modulate success in a fitness trait under elevated pCO2. I quantified plasticity using a suite of morphological traits in Pocillopora verrucosa at two different depths, and measured growth of plastic genotypes in high pCO2 . Results suggest that genotype-specific morphological plasticity does not influence success in growth in high pCO2 . Overall, the goal of this thesis was to better understand the scope of a coral’s ability to deal with environmental heterogeneity (e.g. increasing ocean acidity) based on the formation and flexibility of its phenotype. Results indicate that under projected ocean acidification conditions, the formation of a coral’s phenotype (e.g. larval behavior) will be affected by high pCO2 , but that a flexible phenotype in adult corals does not appear to modulate growth success in high pCO2.