Introduction: The Impact of Ocean Acidification on Shellfish Calcification The health of the world's oceans is vital to the planet's overall well-being, as they play a crucial role in regulating climate, supporting biodiversity, and providing resources and livelihoods to millions of people worldwide. However, anthropogenic activities, particularly the burning of fossil fuels, have led to increased carbon dioxide (CO2) emissions, resulting in the phenomenon known as ocean acidification. This process involves the absorption of CO2 by seawater, leading to a decrease in pH levels and alterations in the carbonate chemistry of the ocean. Among the various marine organisms affected by ocean acidification, shell-forming organisms such as shellfish are particularly vulnerable. Shellfish, including mollusks like oysters, clams, and mussels, rely on calcium carbonate (CaCO3) to build and maintain their shells or exoskeletons. As the acidity of seawater increases, it becomes more challenging for these organisms to extract the carbonate ions necessary for shell formation, ultimately impacting their ability to calcify. This paper aims to explore the effects of ocean acidification on shellfish calcification and the potential implications for marine ecosystems and human societies. Effects of Ocean Acidification on Shellfish Calcification: Ocean acidification poses significant challenges to shellfish calcification through multiple pathways. Firstly, the decrease in carbonate ion concentration (CO32-) in seawater reduces the availability of building blocks for shell formation. Shellfish utilize carbonate ions along with calcium ions (Ca2+) to precipitate calcium carbonate in the form of aragonite or calcite, essential 2 components of their shells. With declining carbonate ion concentrations, shell-forming organisms face difficulties in maintaining and repairing their shells, leading to weakened shell structures and increased vulnerability to predation and environmental stressors. Secondly, ocean acidification can disrupt the physiological processes involved in shell formation. Studies have shown that elevated CO2 levels interfere with the metabolism of shellfish, affecting the activity of enzymes responsible for biomineralization and shell deposition. As a result, shellfish may exhibit reduced growth rates, thinner shells, and altered shell morphology under conditions of high CO2 and low pH. These physiological impairments not only compromise the integrity of shellfish populations but also have cascading effects on marine food webs and ecosystem dynamics. Furthermore, ocean acidification exacerbates other environmental stressors that impact shellfish calcification, such as ocean warming and hypoxia. Rising sea temperatures can enhance metabolic rates in shellfish, increasing their energy requirements for shell maintenance and repair. Combined with the effects of acidification, this can lead to a mismatch between energy allocation and shell growth, further compromising the resilience of shellfish populations. Additionally, hypoxic conditions, often associated with eutrophication and climate change, can impede shellfish growth and survival by reducing oxygen availability and exacerbating acidification through microbial respiration processes.