Cosmarium tumidum is a species of desmid, characterized by semicircular or crescent-shaped cells with a central constriction dividing each cell into two halves. These cells are typically 30 to 60 micrometers in length and have a rigid cellulose cell wall. Each cell contains disk-shaped or lobed chloroplasts responsible for photosynthesis, and a centrally located nucleus. Reproduction occurs through both asexual (binary fission) and sexual means. Cosmarium tumidum is found in freshwater habitats like ponds and lakes, preferring nutrient-rich environments where it can attach to submerged vegetation or sediment.