Codium fragile
A siphonous green alga invasive in the Northern Atlantic.
Codium fragile, also called green sea fingers, dead man's fingers, felty fingers, forked felt-alga, stag seaweed, sponge seaweed, green sponge, green fleece, sea staghorn, and oyster thief, is a seaweed from the family Codiaceae. It originally comes from the Pacific Ocean near Japan and is now an invasive species along the coasts of the Northern Atlantic Ocean.
This dark green alga is siphonous and has two subspecies in Ireland and Great Britain that look alike. It grows long, erect, finger-like fronds that can reach over 40 cm, branching in a dichotomous pattern. The branch surface is made of tightly packed utricles—single-celled, club-shaped structures up to 1200 μm long. Their rounded tips give the frond a velvety feel. At low tide, the fronds hang down from rocks, which is why it’s called "dead man's fingers." The branches can be up to a centimeter wide and sometimes exceed 30 cm in length. It lives in the low intertidal zone and subtidal areas on high-energy beaches. There is no gametophyte stage; male and female gametes come from separate plants.
Subspecies can only be told apart under a microscope.
*Codium fragile* subsp. *atlanticum* has a short mucron (utricle tip) no longer than 20 μm. It was first recorded in Europe in southwest Ireland around 1808, likely arriving from the Pacific near Japan. It may have spread by rafting or floating. About 30 years later, it appeared in Scotland. Since 1840, it has spread the full length of Britain, including Shetland. Between 1949 and 1955, it moved 80 km between Berwick-upon-Tweed and St. Andrews, Fife. It is mostly found in northern Britain, but also in Norway, the Netherlands, France, Spain, and the Azores. This subspecies displaces the native *Codium tomentosum* and is used as food in the Far East.
*Codium fragile* subsp. *tomentosoides* has a very pointed mucron up to 68 μm long. It occurs along nearly the entire eastern U.S. coastline, from the Gulf of St. Lawrence in Canada to North Carolina. It is a fast-spreading invasive. Originating in the Pacific near Japan, it was introduced to New York from Europe in 1957. It was first recorded in the Gulf of Maine at Boothbay in 1964. It is now found around Ireland, the Scilly Isles, the Channel Islands, and the south and west coasts of England and Scotland. This subspecies dominates the subtidal zone, attaching to almost any hard surface.
Quick Facts
- Genus
- Codium
- Species
- fragile
Facts from the source article.
Lore & Background
This siphonous green alga is of two subspecies in Ireland and Great Britain. They are similar, both are dark green in color. It forms long erect finger-like fronds that grow to 40 cm or more long branching dichotomously. The cortex of the branches is formed by closely packed utricles, small cylindrical club-shaped structures formed from a single cell up to 1200 μm long. The rounded tips of these closely packed utricles give the frond a velvety texture. The fronds hang down from rocks during low tide, hence the nickname 'dead man's fingers'. The 'fingers' are branches up to a centimeter wide and sometimes over 30 centimeters long. Codium fragile occurs in the low intertidal zone, and subtidal on high-energy beaches. It has no gametophyte stage, and male and female gametes are both produced on separate plants. Subspecies of C. fragile can only be distinguished microscopically.
Reader's Guide
Codium fragile is notable primarily as an invasive species in the Northern Atlantic Ocean, originating from the Pacific near Japan. Its subspecies have distinct histories and impacts. Codium fragile subsp. atlanticum was first recorded in Europe in the southwest of Ireland around 1808, spreading to Scotland approximately 30 years later, and by 1840 had spread the entire length of Britain. Between 1949 and 1955 it spread 80 km between Berwick-upon-Tweed and St. Andrews, Fife. This subspecies displaces the native Codium tomentosum and is used as food in the Far East. Codium fragile subsp. tomentosoides is a rapidly spreading invasive along nearly the whole eastern US coastline from the Gulf of St. Lawrence to North Carolina. Introduced into New York from Europe in 1957, it was first recorded in the Gulf of Maine at Boothbay in 1964. It attaches to almost any hard surface in the subtidal zone, increasing maintenance labor for aquaculturists and reducing productivity. In established shellfish beds, it may attach to shellfish and float away, carrying the animals, inspiring the common name 'oyster thief'. Codium fragile subsp. scandinavicum was introduced from Asiatic coasts of the Pacific to Norway, and to Denmark in 1919. Codium fragile subsp. novae-zelandiae is found in low intertidal to subtidal zones around New Zealand and the Falkland Islands.
Did You Know?
- Codium fragile has no gametophyte stage; male and female gametes are produced on separate plants.
- Codium fragile subsp. tomentosoides was introduced into New York from Europe in 1957.
- The common name 'oyster thief' comes from the subspecies tomentosoides attaching to shellfish and floating away with them.
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