Oyster Thief

(Codium fragile)

What is it?

The “oyster thief” is a green alga that grows in subtidal, shallow waters along the coast.

Where did it Come From?

  • The oyster thief originated in the Pacific Ocean near Japan.
  • The first record in eastern North America was near New York in 1957.
  • The oyster thief was first seen in Canada in Mahone Bay, N.S., in 1989.
Oyster Thief
Oyster Thief

Where can it be Found?

World Wide

  • Europe
  • Both coasts of North America
  • Australia
  • New Zealand

Atlantic Canada

  • The Atlantic coast of Nova Scotia (south of Halifax)
  • Bay of Fundy (St. Mary’s Bay N.S.)
  • Southern Gulf of St. Lawrence (from Caribou N.S. to Lameque N.B., in P.E.I. and the Magdalen Islands)
  • St. Pierre & Miquelon

Why is it Called ‘Oyster Thief’?

Oyster Thief
Oyster Thief

The alga grows attached to the bottom of the ocean by a “holdfast” that sticks onto a solid surface and does not spread out into soft sediments like a real root. In places where the bottom is too soft for the alga to attach, it often attaches to shellfish such as oysters and mussels living on the bottom. The alga often becomes extremely buoyant due to production and trapping of gasses which may cause it to float away still attached to the shellfish or plant. This common observation is what led to the name “oyster thief”.

How/Where do They Live?

The adult oyster thief grows attached to the sea floor and looks like a green fuzzy branched “plant” reaching about 30-100 centimetres in height. This adult alga reproduces by production of spores and may also produce new plants when the “branches” break off or the whole plant is torn loose from the bottom (for example, by wave action) after which it may drift to a new area and reattach there.

Oyster thief washed up on a beach in Cape Cod, Massachusetts.
Oyster thief washed up on a beach
in Cape Cod, Massachusetts.

What are the Effects on Native Species and Habitats, or Human Uses of the Ocean?

Oyster thief has become a dominant member of the seaweed community in the low intertidal zone and subtidal waters off southwestern Nova Scotia. In this area, oyster thief can reach one metre in height and may replace entire kelp beds. This disrupts the normal cycle of kelp and sea urchins in that area. The loss of kelp beds affects the many native species that use kelp habitat for feeding, as nursery habitat, or as shelter from predators. Oyster thief does not appear to be used as food by many Canadian species and may affect the survival of species, such as sea urchins, that are dependent on kelp.

What can you do to Help?

Did you Know?

Oyster thief is known for floating away with not only oysters, but other species such as mussels and eelgrass that are sometimes used for attachment. Like kelp, eelgrass is a major habitat for many other species.

There are probably no effective ways to remove oyster thief that is already established in an area. Manual removal may result in breaking the alga into smaller pieces, each of which may begin a new plant. Preventing spread into new areas is the best thing you can do. Oyster thief may be picked up on boat hulls, motors, anchors or fishing gear in shallow waters. Ensure that equipment is clean when moving from one area to another and follow the general guidelines recommended by Fisheries and Oceans Canada on the weblink below.

For more information visit
www.qc.dfo-mpo.gc.ca/publications/envahissant-invasive/carnet_anglais.pdf [PDF 13.2 MB]