top of page
Codium (Codium fragile ssp. tomentosoides)


(Codium fragile ssp. tomentosoides)

Report this Species!

If you believe you have found this species anywhere in Pennsylvania, please report your findings to state and federal authorities.

Species at a Glance

Codium, also referred to as the “oyster thief”, has been introduced around the world through shellfish aquaculture, recreational boating, and ballast water transport. It will often make its home on the shells of oysters, scallops, and clams, smothering them and causing damage to aquaculture industries. Codium is also a nuisance when it accumulates and rots on beaches, producing a foul odor.


A large, dark green, branching alga that can reach up to 1 m (3 ft) in length and weigh up to 4 kg (8 lbs). In wave exposed areas, it tends to be shorter as it undergoes more frequent fragmentation. Codium is composed of cylindrical, dichotomous, spongy branches that are 3-10 mm in diameter. A spongy basal holdfast anchors the plant to hard objects. The cortex of the alga is formed of club-shaped, bladderlike swellings called utricles that are tightly compressed together. The alga is upright when small, but droops as it gets larger. Juvenile stages appear as fuzzy, moss-like mats. Codium bleaches white when washed onto shore.

Similar Species

While Codium is often mistaken for a sponge, there are no native sponges in the northwest Atlantic Ocean with a similar appearance to this alga. While it’s difficult to distinguish the different subspecies of Codium, it can be done by looking at variations in the size and shape of the utricles.


Able to colonize a wide range of environments, Codium tolerates large variations in salinity and temperature. It thrives in sheltered habitats such as harbors, estuaries, and bays and generally prefers to attach itself to hard surfaces such as rocks, ropes, jetties, wharf pilings, and shellfish.


Codium was most likely introduced through ballast water, hull fouling, aquaculture, and through its use as a packing material for shipping live marine organisms. Once introduced, it has multiple methods of reproduction that can help it establish and spread, including sexual reproduction, parthenogenesis, and fragmentation. Water currents can also carry this species over long distances, introducing it to new locations.


Native to coastal Japan, Codium has established itself worldwide. On the East Coast of North America, it is found from the Gulf of Saint Lawrence in Canada, down to South Carolina and in all Mid-Atlantic states except for Pennsylvania.


Note: Distribution data for this species may have changed since the publication of the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016), the source of information for this description.

Environmental Impacts

Considered a fouling organism, Codium causes problems when it attaches itself to shellfish beds, fishing nets, and other man-made structures. This alga can smother the shellfish it rests upon, and can float away with oysters attached to it. It can also impact the benthic environment by making it difficult for invertebrates and fish to move among it and forage on the ocean floor. It has been known to displace native kelp forests and can therefore disrupt the food web in these habitats.


Information for this species profile comes from the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016).

bottom of page