Unraveling The Secrets Of—Seaweed?

This sounds like the trailer for a bad horror movie: it’s green, slimy and smelly, and its decaying hulk sends beachgoers fleeing in disgust.  It’s codium, an invasive species of seaweed, and the threat it poses to the visitor economy is very real indeed.

            The problem is acute along the Nantucket Sound beaches of the Cape, and town officials have struggled with ways to dispose of the seemingly endless piles of codium that litter the beaches, making them unsuitable for swimmers or sunbathers.  That’s one reason that graduate student Chris McHan of Northeastern University decided to give the stuff a closer look.

            In a paper he co-authored with his advisor, Dr. Donald Cheney of Northeastern’s biology department, McHan notes that codium often reaches lengths of more than two feet, yet is actually composed of a single, multinucleated cell.  Known as “dead man’s fingers,” “green fleece,” or “oyster thief weed,” codium is thought to have originated from the waters of Southeast Asia, having been brought to North America on the hulls of cargo ships in the mid 1950s.  It spread by attaching to oyster seed, moving from its original foothold in Long Island Sound to the remainder of the Northeast U.S. coastline.

            From an ecological point of view, codium is a harmful invasive species because it crowds out indigenous plants along the shore of Nantucket Sound, McHan said.

            “It’s changing what’s there.  It is the dominant flora that’s in the area,” he said.  But it’s not its ecological effects that inspire people’s strong feelings about codium, he added.  It’s the foul stench and the sight of littered beaches.

            “I think people are pretty aware of the impacts of codium.  People hate the stuff,” McHan said.  With no practical, permissible way to remove, compost or burn the piles of seaweed, towns have resorted to burying it—or in the case of Red River Beach in Harwich, building dunes with it.  But there, and on other south side beaches, it’s a serious struggle for towns to deal with the weed.  Harwich officials estimated that as much as 1,000 tons of plant material can accumulate along one kilometer of beach each summer.

            What confounds scientists is that, by conventional wisdom, codium actually shouldn’t be thriving here.  That’s because codium is a macroalgae that uses a sticky foot, known as a “holdfast,” to cling to a surface.  Aside from a few rocks, there is little on the Cape’s south side beaches to serve as a foundation for codium.

            “It doesn’t have a root structure, like a tree might have, in order to grip loose particles,” McHan said.   Seagrass, the prevalent species in these waters before the arrival of codium, doesn’t require a hard surface substrate.

            With support from Thomas Leach and Heinz Proft of the Harwich Natural Resources Department, McHan surveyed a total of 827 codium plants from various sections of Red River Beach, and discovered the likely answer.  Ninety-eight percent of the plants were attached to the same kind of organism, a snail-like bottom dweller known as crepidula, or the slipper limpet.  Crepidula produces the common slipper shell often seen on south-side beaches.

            But McHan theorizes that crepidula provides more than a suitable foundation for codium.  The slipper limpet feeds by ingesting organic particles from the water column, and probably takes a certain amount of codium gametes, which germinate when they are passed through the limpets’ digestive system.  That means that the gametes are provided with plenty of fertilizer from the limpets’ waste, helping them grow in abundance. 

            It also appears that codium is reproducing most feverishly in the wintertime, when other species are more dormant.  By starting their lives when the competition for open space is low, young codium algae can get a good foothold, McHan theorizes. 

            Is it possible that any of this information might lead to a program to control codium on Cape beaches?

            “I hope so,” McHan said.  As an invasive species, codium should be controlled so that eelgrass and other indigenous species can thrive, he said.  That means either interrupting the codium or the slipper limpet, and the key to doing that might involve a third party: nutrients in the water.  Both crepidula and codium thrive in nutrient-rich water, and nitrogen from septic systems, lawn fertilizer and road runoff probably stimulates the growth of both species, McHan said.

            “If this [research] was going to be taken into an application sense, that’s the side we’d have to attack it from,” he said.

            While it may someday be possible to control codium, McHan said there won’t be a quick fix for the problem.

            “I don’t think you’ll ever get rid of it,” he said. 

Cape Cod Chronicle

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