Dead Man’s Fingers
by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station
I hadn’t planned to write about another type of invasive species, but lately on the north side of the island, beachgoers have run into huge piles of an alien plant that has established a foothold in our local waters, namely Codium fragile, otherwise known as “dead man’s fingers.” I got a call from my friend Edie Ray telling me that piles of stinking decaying algae are lapping along beaches from Washing Pond to Eel Point. I was able to see the thick C. fragile layers stretching in a 15-foot band bordering the shore north of Washing Pond early last week. Among the many common names for C. fragile such as “green fleece,” “green sea fingers,” “broccoli weed,” and “Sputnik weed,” the fisherman’s title of “oyster thief” is self evident when you see the piles of slipper shells (Crepidula spp.) attached to Codium washed up on the beach. Codium requires a rocky substrate, but a shell makes a perfect attachment point in a pinch. Inherently damaging to shellfish, Codium adheres to shells to serve as a home base. By wrapping its holdfast around shells, it impedes the locomotion and feeding of shellfish such as oysters, mussels, and scallops, and may dislodge them from the seafloor. As Codium gets a grip on a pile of slipper shells, before they know what’s up, they are up in the wrack line, slowly dying. When I bring groups of young kids down to the beach to see what lives there, they inevitably try to rescue as many of the slipper shells with the attached Codium that they can by throwing them back into the surf. I don’t tell them the natural balloon action and drag capacity of the Codium will bring those slippers shells right back to the same high and dry demise. The kids also love feeling the turgid green fingers of Codium, even if it is kind of “gross.”
Codium migrated to our waters attached to ship hulls, in ballast water, and through shellfish aquaculture from the Pacific waters around Japan via shipping ports in Europe and eventually to our shores in the 1950s. Codium is a genus of seaweed in the Phylum Chlorophyta and order Bryopsidales. There are about 50 species world-wide. The type that inhabits the water around Nantucket is most likely Codium fragile subspecies tomentosoides. According to the global invasive species database, C. fragile was first reported in the northwest Atlantic at Long Island Sound in 1957, presumably introduced by ships from Europe. It quickly spread up and down the coast and was seen first in Massachusetts in 1961. The species fouls shellfish beds and causes a myriad of impacts on shellfish communities, including clogging nets and dredges which greatly increases dredge time and equipment failure for commercial scallopers. This species also causes a nuisance to humans when it accumulates on beaches and rots producing a foul odor. C. fragile ssp. tomentosoides has been documented altering benthic (bottom) communities and habitats, causing serious environmental implications and is now listed as one of the most invasive species world wide. Unfortunately, there are not many options for fighting this invasion besides removal and composting. C. fragile quickly returns to an area swept clean of the pest because it can reproduce from fragments left behind. Currently scientists are looking at biological control measures like fungi and parasites. Although Codium is eaten in some Far East households and it also is used in some animal fodder, neither use is common enough to provide an economic incentive for harvesting it. Some scientists believe that Codium can harbor toxic metabolites and should not be used for animal or human consumption.
Codium fragile spp. Tomentosoides is a large branching green alga that can attain almost 1 m (3.0 feet) in length and weigh up to 3.5 kg (almost 8 pounds). The alga branches dichotomously (in two parts) and individual branches are three to ten mm in diameter. The plant is anchored to the substrate by a spongy basal holdfast (its “anchor”). This species exhibits various modes of reproduction, which is a common trait for many successful invaders. It can reproduce sexually, parthenogenetically (female based asexual reproduction), and vegetatively. Water currents can and will carry this species over long distances introducing it to new locations. Although Codium prefers warm water temperatures of approximately 24°C for optimal growth and reproductive success, it is tolerant of a variety of salinity and water temperature levels. It also thrives in sheltered habitats, such as harbors and marinas, which increases the possibility for human introduction on boat hulls. Codium can grow at greater depths than many other seaweeds and shading from phytoplankton blooms does not affect it as much as other plants. It can even grow under docks and piers.
Massachusetts eelgrass expert, Dr. Joe Costa, documented a huge Codium influx at Wareham, MA in 2005 in Buzzard’s Bay (http://www.buzzardsbay.org/codium-wareham.htm) and it is possible that the same thing is happening around Nantucket. Dr. Costa offers a very through update of the state of research into eelgrass habitat losses in the state and he explores the reasons behind these losses. As we know, “no eelgrass, no scallops” because bay scallops depend on eelgrass habitat during their juvenile stages. Area scientists have documented eelgrass destruction in Massachusetts embayments to be caused by coastal eutrophication via nitrogen loading from recent increases in development. When nitrogen makes its way into our harbor from septic systems, fertilizers, and cranberry bogs, phytoplankton blooms can occur shading out the bottom-dwelling attached eelgrass plants. Codium does not seem to be affected by reductions in water quality. Other threats to eelgrass include ice damage, storm damage, and isolated outbreaks of the killer wasting disease that almost completely wiped out eelgrass in the 1930s. If any of these causes contribute to an eelgrass bed’s decimation, the eelgrass will usually come back. But if it is caused by nitrogen loading, until that excess nutrient is no longer feeding voracious phytoplankton, the eelgrass has very little chance to recover.
In the mid 1990s, Charlie Costello with the Massachusetts Department of Environmental Protection started documenting eelgrass habitat using a combination of aerial surveys and field-based ground-truthing from boats. His research can be found in the updated 2007 Nantucket and Madaket Harbors Action Plan at http://www.nantucketharborplan.com. Complications can arise when using aerial surveys to identify eelgrass beds because codium patches can show up as eelgrass from the air. Additionally, if the surveys are done too early in the summer, the eelgrass may not have grown enough to be easily identified, and if the surveys are done later in the summer when the eelgrass meadows are lush and green, phytoplankton blooms in the water can diffract light and make it difficult to see the bottom. Ground-truthing from boats is the best method to verify the aerial surveys, but that takes time and is expensive. This summer at the field station, Sara Mack, a junior research intern, is doing detailed transects of eelgrass beds in the harbor and Town of Nantucket biologists and Maria Mitchell researchers are also documenting eelgrass in the harbor.
Because of its bush-like growth pattern, Codium is considered a "low-lying alga." Dense patches prevent larger species of fish and mammals from swimming freely through intertidal areas. The oyster thief is even more worrisome in oyster beds as Codium covers the oysters and smothers them, eventually destroying the beds. Codium has been documented overtaking kelp beds when either sea urchins or nutrient overload have reduced the kelp and allowed it to gain a foothold.
One of the things I find most fascinating about Codium is that each “finger” section is a huge single cell; no true cell walls exist, which allows it to have a turgid vacuole type interior that can bounce incoming light from chloroplast to chloroplast, using light more efficiently and increasing in size with little energy output. In biological terms, the entire thallus (plant body) of Codium fragile is composed of a single, multinucleate cell or "coenocyte," that has formed a tangle of branching filaments. The term "siphonous" describes this filamentous growth form. Now you can pass that SAT test! When Codium washes up on the beach, it loses its chlorophyll-based green coloring and turns a ghostly white as it bleaches in the sun.
Codium is a source of food for many invertebrate species, though not usually a primary food source. And it’s not all bad: in experiments done at the field station over the past few years, codium was pulled directly from the harbor and centrifuged to see what types of epiphytic (surface dwelling) creatures live on a thin layer of water that clings to each “finger.” Hundreds of different diatoms, zooplankton, larval species of shellfish and sponges live on the surface, which looks knobby or hairy under a microscope. The rugged exterior creates a high surface area for these phytoplankton and zooplankton to attach to and gather food. In another set of preliminary experiments, juvenile scallops were placed in tanks with eelgrass, codium, or both, to see if the juvenile scallops would use the codium plant body in the same way they use eelgrass (traveling upward to get higher in the water column and away from predators like crabs). In this experiment, the scallops only preference was for the sides of the tanks. Time to get some bigger tanks!
The jury is still out as to whether our eelgrass beds can hold their own and continue to provide a sustainable habitat for scallops. Hopefully new ideas will lead to successful eradication efforts. For more details, visit www.projectufo.ca/drupal/Dead_Mans_Fingers, which includes links for kids. Another informative website is http://www.columbia.edu/itc/cerc/danoff-burg/invasion_bio/inv_spp_summ/Codium_fragile.html.
