Fifty trillion microplastic particles are floating in the sea. Although microplastics alter the development of a range of marine invertebrates, the consequences of these developmental effects are likely underestimated, and the effects for ecosystems are unknown. No systematic studies have characterized the range of developmental defects elicited by microplastic contamination on marine invertebrates. Using the sea urchin Paracentrotus lividus as a model, this project aimed at: (1) explore the morphological and genetic consequences of microplastic exposure on development; (2) identify which characteristics of the plastic particles cause the effects; (3) assess if the effects are specific, altering developmental pathways, or a consequence of general toxicity. This multidisciplinary, novel work will transform our understanding of the effects of microplastic contamination on development, including the underlying genetic mechanisms, and their implications for other animals exposed to MP contamination. Plastic waste is a topic of major societal interest, and media coverage around plastics and marine pollution suggests that this project is of great interest to the general public, and is meant to help stakeholders (fishermen/farmers and third sector (NGOs) along with plastic manufacturers) and regulatory bodies avoid damage to stocks and livelihoods.