The pollution of the marine environment with microplastics is pervasive. However, microplastic concentrations in the seawater are lower than the number of particles entering the oceans, suggesting that plastic particles accumulate in environmental sinks. Yet, the exact long‐term sinks related to the “missing plastic” phenomenon are barely explored. Sediments in nearshore biogenic habitats are known to trap large amounts of microplastics, but also the three‐dimensional structures of coral reefs might serve as unique, living long‐term sinks. The main framework builders, reef‐building corals, have been shown to ingest and overgrow microplastics, potentially leading to a deposition of particles in reef structures. However, little is known about the number of deposited particles and the underlying processes determining the permanent deposition in the coral skeletons. To test whether corals may act as living long‐term sink for microplastic, we exposed four reef‐building coral species to polyethylene microplastics (200 particles L⁻¹) in an 18‐month laboratory experiment. We found microplastics in all treatment specimens, with low numbers of particles trapped in the coral tissue (up to 2 particles per cm²) and much higher numbers in the skeleton (up to 84 particles per cm³). The numbers of particles accumulated in the coral skeletons were mainly related to coral growth (i.e., skeletal growth in volume), suggesting that deposition is a regularly occurring stochastic process. We estimate that reef‐building corals may remove 0.09%–2.82% of the bioavailable microplastics from tropical shallow‐reef waters per year. Our study shows for the first time that microplastic particles accumulate permanently in a biological sink, helping to explain the “missing plastic” phenomenon. This highlights the importance of coral reefs for the ecological balance of the oceans and reinforces the need to protect them, not only to mitigate the effects of climate change but also to preserve their ecosystem services as long‐term sink for microplastic.

Traditional surface area and volume quantification techniques for scleractinian corals are often destructive or inaccurate. Therefore, non‐destructive 3D scanning methods have been applied as minimally invasive alternative. However, it remains largely unknown how the reproducibility of the measurement is affected by the complexity of the coral colonies. It is also unclear how the scanning procedure (handling, exposure to air, and light of the scanner) impacts the corals’ health. In this study, we used a high‐end handheld 3D scanner, which combines a structured‐light with an image‐based approach, to investigate the reproducibility of surface area and volume measurements as well as handling effects. Corals with different shape complexity, covering branching and massive species, were used as model organisms. The variance of repetitive scans ranged from 0.13% to 1.31% for volume and from 0.09% to 0.58% for surface area calculations. Linear regression models indicated that reproducibility decreases with increasing complexity of the coral. Excessive scanning caused an increase or decrease of growth rates, depending on the studied species. However, it did not impair coral health. We conclude that 3D scanning is a highly precise, reproducible, and minimally invasive method for coral surface area and volume measurements, which allows for quick processing of large datasets. Detailed technical recommendations for the application of 3D scanning in coral research are provided in the manuscript.