Codium

Codium is a genus of green algae in the class Ulvophyceae. It includes many species found in marine environments worldwide, from temperate to tropical seas. Members are characterized by siphonous, coenocytic thalli that are typically hollow and lack differentiated tissues such as true roots, stems, or leaves. The thallus is often soft and spongy, and may form upright finger-like fronds, flattened plates, or mats on substrates.

Species occur globally in coastal waters, from shallow intertidal zones to deeper subtidal habitats. They are commonly found on rocky substrates, coral reefs, seagrass beds, and man-made structures. Some forms are adapted to shaded or turbid environments; others require clearer water to thrive. The genus includes notable species such as Codium fragile, which has become invasive in parts of Europe and North America, where dense mats can displace native algae and alter habitat structure.

Introduction of Codium fragile and related forms is often linked to human activity, including ballast water discharge and ship fouling. Management focuses on prevention, monitoring, and manual removal, though recovery of native communities can be slow. Reproduction occurs chiefly by fragmentation, with detached pieces able to establish new individuals. In some species, sexual reproduction occurs via motile spores or other propagules, but life-cycle details vary across taxa. Ecologically, Codium contributes to primary production and habitat structure, supporting a range of marine invertebrates.