On a recent field trip with my students to DowsesBeach in Osterville, I noticed a large population of seaweed washed up on the beach. However, instead of finding a wide diversity of green, brown, and red algae, we observed the dominance of a single species of green algae, namely Codium fragile, subspecies tomentosoides, also called dead manís fingers or green fleece.
Codium is an introduced and highly invasive alga. It originated from Asia, most likely Japan. It might have arrived on these shores by attaching to ships, or possibly oyster shells from Europe, where it was also introduced. Once here however, it began to spread. The young plants can attach to practically any hard surface. Thatís part of the problem. It can also be very damaging to shellfish beds. Codium can grow over the filter-feeding siphons of clams and block them. It can also weigh down the motile scallops which also prevents them from feeding. I lifted several large clumps of Codium and could easily see that a tiny scallop would be unable to move if attached to this pest. In fact, Codium can remove shellfish from their natural habitats by floating away with the clam or snail still attached. Large quantities of slipper snails attached to Codium were also seen at Dowses.
Codium can also harm shellfish indirectly, particularly in valuable eel grass beds offshore. Codium gets so thick that it blocks sunlight to the eel grass, thus reducing their ability to photosynthesize and provide food to the numerous animals that live there. If Codium gets established in aquaculture beds, it can sharply reduce their productivity.
Codium is easy to identify. Branches are bright green, and spongy in their texture. A thick, flat root-like structure called a holdfast at its base attaches to different objects such as clams, snails, and rocks. The alga can reproduce quickly and abundantly through a vegetative process called fragmentation. Small pieces can brake off the adult plant and grow into new ones. Itís this ability to fragment that makes it so difficult to control. It can also reproduce through a process called parthenogenesis. In this species, swimming female cells called gametes are released into the water, attach to a hard surface, and grow into a new plant without fertilization from a male cell.
Codium has adapted to the waters of Cape Cod, particularly the warmer water on the southern side of the Cape. It has fouled many beaches, and appears to be increasing in number. So what can be done to control it? Unfortunately, very little has been successful. Unlike the more notorious invasive plant, Phragmites, Codium has not received the same amount of attention or research. The manual removal of the massive amounts along the beaches has been the most commonly used method. Controlling nitrogen loading into bays and estuaries will also help. Some parts of the world harvest Codium as a source of fertilizer, even food. Iíve used it in my garden as an inexpensive source of compost material. At any rate, this invasive alga deserves our attention for the simple reason that it is already having a negative impact on the financially important shellfish industry, as well as the visual enjoyment of many of the Capeís beaches.
First Published in
Cape Cod Times
the Cape Cod Times, April 27, 2009
Copyright Gil Newton 2009 Thanks to Chris Dumas for logo image.