Most of what we call “seaweeds” are actually marine algae, not plants. Algae differ from marine vascular plants, such as eelgrass, in that they are more primitive and lack specialized structures such as seeds and flowers, although most do photosynthesize. Seaweeds are often categorized by color: red, brown or green. Codium fragile is a green marine algae that is easily identified by its thick dark green, rope-like, spongey branching form. These branches are soft with a velvety texture. Codium has a variety of common names including stag seaweed, sponge seaweed, and green fleece, but I’ve usually heard it referred to as dead man’s fingers. The latter derives from the swollen, bloated-looking finger-shaped branches that float in the water, or hang down seemingly lifeless from the sides of rocks or piers when the tide is out. 

My first memories of this seaweed are of using it as ammunition in seaweed fights with my brother at the beach in the summer, both of us ducking and squealing as wads of spongy green fingers hurtled our way. I assumed it had always been there. But that turns out not to be the case. Codium fragile is native to the Japanese region of the Pacific Ocean, but has become invasive throughout the north Atlantic. Dead man’s fingers were first seen in the United States in 1957 on Long Island, followed by Boothbay Harbor in Maine in the early 1960s. Since then, it has spread along the east coast, reaching as far north as the Gulf of Saint Lawrence and as far south as the Carolinas. It likely arrived as a hitchhiker, in ballast water, on fishing equipment or on transported shellfish. Now, according to the Massachusetts Office of Coastal Zone Management, Codium fragile is one of the most widespread non-native seaweed species in Massachusetts. 

Unlike true plants, seaweeds do not have roots. Instead, they have a structure called a holdfast that serves an anchoring function. Although dead man’s fingers’ holdfasts will readily attach to rocks when they are available, in the sandy marine environments surrounding Cape Cod, Codium must rely on other hard anchor points, such as empty shells or live shellfish. Where it occurs in dense colonies, it can have a choking effect on shellfish. It can grow over the filter-feeding siphons of clams, physically blocking them and it can weigh down scallops, inhibiting their movement and preventing them from feeding. Additionally, when it gets washed up on the beach in a storm, it often drags its mooring with it; in cases where it was anchored to a live shellfish, that organism is dislocated from its preferred habitat. 

There is seldom an easy answer to the control, management and eradication of invasive species, but the first step is to recognize that some species that we’ve assumed have always been there are actually recent interlopers and may have already caused significant changes to the native ecosystem.