Oyster Thief

Oyster Thief, or codium fragile, is a large, green, spongy macroalga (seaweed) with thick, irregularly cylindrical branches which are usually 3 to 10 mm in diameter and 15 to 20 cm long, but can be up to 90 cm in length.

This alga grows on any hard surface including rocks, boulders, cobbles, wharves, boat hulls and shellfish in both intertidal and subtidal zones. It is tolerant of a wide range of conditions and can grow in the brackish or full-strength seawater. In general the plants die back during the winter and begin to grow again from the base in the spring.

It reproduces by sexual means in the spring and summer by releasing free-swimming gametes into the water column, and by asexual means (fragmentation) during the colder months.

(Photo credit: D. Mouland, DFA)

Oyster Thief is known to attach and grow on live scallops, clams, oysters and mussels in cultures and in wild populations and can impair these species growth. It is referred to as the “oyster thief” because when growing on cultured oysters they can be buoyant enough to move the shellfish from their beds.

It has been found to outgrow and exclude native kelps from base substrates and foul fishing nets. Large amounts of this seaweed have been known to wash up and rot on beaches making them unusable for recreation.

Other common names for this species of alga are Green Fleece or Dead Man’s Fingers.

Means of introduction

Oyster Thief can be spread though natural means, such as whole plants floating on ocean currents and by other vectors. Shipping can contribute to the spread through the spawning or fragmentation of mature alga attached to ship hulls. The transport of algae, fish and shellfish can also contribute to the spread since this alga can regrow from small parts of the organism which may remain attached to shellfish, fishing gear or algae.

Management of the species

As with most invasive species, once Oyster Thief has become established in an area eradication is not practical. Physical removal from the environment may reduce numbers but populations soon recover as a result of regrowth from remaining tissue or fragments and by the release of gametes from detached mature algae.

Since some of it’s life stages are hard to see or even microscopic, fish and shellfish harvesters should avoid transferring harvested shellfish and fishing gear to another area. If shellfish or gear must be moved it should be dipped into a brine solution prior to transport.

Boat hulls should be inspected and if necessary, thoroughly cleaned before moving to other areas. Any organisms removed from gear or boat hulls should be disposed of on land. Report sightings by using the contact information on the main AIS page.

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