Cocconeis scutellum is a marine and brackish-water diatom in the family Cocconeidaceae, one of the most commonly encountered epiphytic diatoms in coastal ecosystems worldwide. The frustule is broadly oval and relatively large for the genus, with robust silica walls bearing coarse transapical striae that give the valve a distinctive, shield-like appearance—a morphology reflected in the species epithet 'scutellum,' Latin for small shield. This species is a strict epiphyte in marine environments, attaching to the surfaces of seagrasses, macroalgae such as Ulva and Zostera, coralline algae, and other benthic substrates in shallow intertidal and subtidal zones. Cocconeis scutellum has been documented from coastal habitats across the Atlantic, Pacific, and Indian Oceans, with records from temperate and tropical regions including South America, Europe, and the Indo-Pacific, reflecting its cosmopolitan marine distribution. The species is a basal component of coastal food webs: dense epiphytic biofilms including C. scutellum are grazed by amphipods, gastropods, sea urchins, and small fish, transferring photosynthetically fixed carbon up the food chain. In estuarine environments where freshwater and saltwater mix, C. scutellum may co-occur with freshwater and brackish Cocconeis species, and salinity tolerance experiments indicate the species survives across a broad range of salinities. Its silicon frustules contribute to coastal sediment biosilica content upon cell death. Conservation status is not formally evaluated.