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Island Science

Dead Man’s Fingers

from WikipediaI love fall when the kids are back in school and coming out to the Nantucket Field Station for nature walks. One of my favorite things to find along the beach and show them is Codium fragile, otherwise known as “dead man’s fingers.” Among the many common names for C. fragile such as “green fleece,” “green sea fingers,” “green sea sponge” 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 C. fragile washed up on the beach. We will learn below, there is an even more sinister story behind the nickname “oyster thief”. C. fragile requires a rocky substrate, but a shell makes a perfect attachment point in a pinch. Inherently damaging to shellfish, C. fragile 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 C. fragile 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 C. fragile that they can by throwing them back into the surf. I don’t tell them the natural balloon action and drag capacity of the C. fragile will bring those slippers shells right back to the same high and dry demise. The kids also love feeling the turgid green “dead man’s” fingers of C. fragile, even if it is kind of “gross.”

  1. fragile 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. C. fragile 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 C. fragile subspecies tomentosoides (used interchangeably here, C. fragile subsp. tomentosoides or C. fragile). 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 subsp. 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 worldwide. 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 C. fragile 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 C. fragile can harbor toxic metabolites and should not be used for animal or human consumption. Around Nantucket we don’t have much in the way of toxic elements in our harbors so I would say it is safe to use.
  2. fragile subsp. 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 C. fragile 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. C. fragile 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.

I just came back from a conference held on Hurricane Island on issues and research priorities for a group of Gulf of Maine field stations of which the Nantucket Field Station is the southernmost member. C. fragile first showed up in the Gulf of Maine in Booth Bay Harbor in 1964 due to transplantation of Long Island oysters. Research in the Gulf of Maine has found that it does best in warmer embayments although it has become dominant in subtidal areas near shore. It can quickly become a dominant species in their normally very diverse subtidal and intertidal communities such as tidal pools. When it moves into the neighborhood C. fragile subsp. tomentosoides may radically change subtidal community composition, structure, and function. The rapid growth of this species and its ability to regenerate from broken fragments assist it in outcompeting native eelgrass and kelp beds, the primary shelter for many finfish and invertebrates. Dense, low-lying C. fragile subsp. tomentosoides meadows on the seafloor make movement difficult for lobsters, fish, and other organisms. The proliferation of sea urchins and slowly increasing nutrient loads have reduced the kelp and allowed C. fragile to gain a foothold in some areas of Maine.

Massachusetts eelgrass expert, Dr. Joe Costa, documented a huge C. fragile influx at Wareham, MA in 2005 in Buzzard’s Bay ( fragile-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. C. fragile 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. He came back several times this summer to continue his research and he is collaborating closely with the town of Nantucket Shellfish Biologist Tara Riley. Complications can arise when using aerial surveys to identify eelgrass beds because C. fragile 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 or by snorkel surveys is the best method to verify the aerial surveys, but that takes time and is expensive. As I wrote about two years ago, we can use acoustic methods to map out vegetation on the sea floor and in some instance quickly tell between eelgrass and C. fragile .

Because of its bush-like growth pattern, C. fragile 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 C. fragile covers the oysters and smothers them, eventually destroying the beds! Indeed, the tendency of this species to overgrow and smother oyster beds is the reason fishermen call it “oyster thief”. This can be a real problem for oyster mariculturists and is no picnic for the poor smothered oysters either!

One of the things I find most fascinating about C. fragile 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 C. 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 C. fragile washes up on the beach, it loses its chlorophyll-based green coloring and turns a ghostly white as it bleaches in the sun.

  1. fragile 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, C. fragile 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 experiments created by my interns in 2006 and 2007, juvenile scallops were placed in tanks with eelgrass, C. fragile, or both, to see if the juvenile scallops would use the C. fragile 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 often preferred the sides of the tanks over either plant, great guys, real helpful. Fortunately several other New England shellfish researchers (John Carroll et al.) have verified that scallops can indeed survive just as well on C. fragile (

The jury is still out as to whether our eelgrass beds can hold their own and continue to provide a sustainable habitat for scallops. Some of the eelgrass loss last year was due to burial over the winter from the huge amounts of sand sloughed off of northerly facing banks in the harbor. We are looking forward to seeing eelgrass coverage on Charlie Costello’s new Nantucket maps. Dr. Peter Boyce with Maria Mitchell Association has also been documenting eelgrass coverage and the loss of eelgrass in the harbor. And maybe as Halloween approaches you’ll remember to look for and feel “Dead Man’s Fingers”.

Parts of this article were previously published at

Other references: “Expansion of the Asiatic Green Alga C. fragile subsp. tomentosoides in the Gulf of Maine” by A. C. Matheison et al., (2003)

“Codium fragile” by Original uploader was Flyingdream at en.wikipedia —