A half-billion years after they appeared, mollusks – primitive animals that have achieved astonishing success through the simplest strategies – remain critical members of aquatic ecosystems the world over. Here on Martha’s Vineyard, shellfish have been a vital resource for humans since post-glacial sea-level rise made the Island an island, some five thousand years ago. It’s hard to think of a natural resource of more enduring importance to the Vineyard than shellfish, and that importance only grows as we learn more about the ecological roles these animals play. Shellfish – raising them, harvesting them, managing them, studying them, and eating them – inspire talent, effort, and passion here.
If you say the word “shellfish” to a Vineyarder, it’s bivalves that come to mind – mollusks that produce paired, tightly closing shells. In particular, it’s a half-dozen species of commercially important clams, oysters, mussels, and scallops. The shells, or valves, of these animals vary in shape and thickness depending on the species and enclose a soft body with a basic complement of organs: gills, gonads, a simple digestive tract and nervous system, and musculature to open and close the shell, circulate water through siphons, and in some cases control a primitive foot. The design is as simple as it is ancient, and since it appeared, continents have shambled around the globe, seas have filled and drained, ice ages have frozen and thawed, dinosaurs have come and gone. Across those eons, bivalves not very different from modern species enjoyed their quiet life in the sediment. The resilience of this basic body form marks the bivalve as one of evolution’s greatest innovations.
Estimates of the number of bivalve species extant today vary widely according to the taxonomic concepts they’re based on, but the tally is in the tens of thousands by any biologist’s count. Only a small fraction occurs here, but Island Life: A Catalog of the Biodiversity on and around Martha’s Vineyard (Allan R. Keith/Marine Biological Laboratories, 2008), compiled by Allan Keith and Stephen Spongberg, notes that the Island’s salt ponds and coastal waters hold more than sixty bivalve species. These range widely in size, habits, and abundance; the vast majority I’ve never seen and wouldn’t recognize. But this surprising variety indicates how prominently bivalves figure in local marine ecology.
An enduring harvest
Archeological excavations of local kitchen middens – debris piles left by the ancestors of today’s Wampanoags – have revealed shells in abundance, showing that clams and oysters have always been a major component of a Vineyarder’s diet. (And why not? Mollusks were abundant, easily harvested, nutritious, and delicious.) Clam shells were also laboriously shaped into wampum, colored beads that served as a revered trade product among Vineyard Wampanoags and related tribes. Even today, purple and ivory wampum is handcrafted by Island artisans to make jewelry. After they settled here in 1642, Europeans relied on shellfish as a source of sustenance accessible to all. Today, commercial and recreational harvests of bivalves produce millions of dollars of direct value for the Island economy, and millions more in indirect economic activity such as the sale of boats, fuel, and equipment.
But the role of shellfish on the Island goes beyond economics and touches on identity: Shellfishing is what we do, and teaching the next generation where and how to harvest registers among many Islanders as a central cultural value. “Many generations of Vineyarders have gone down and got their shellfish,” says Oak Bluffs Shellfish Constable Dave Grunden, noting with approval the undiminished vigor of this tradition. He issues about six hundred recreational licenses in a typical year. Dave recalls seeing four generations of a family shellfishing at once in Sengekontacket Pond, “with the great-grandmother of the family going into the water with her walker, digging quahaugs, and putting them in a basket she had attached to it.”
The Island supports robust commercial shellfisheries too, and digging clams or dredging for oysters or scallops offers an important economic alternative in the Island’s difficult seasonal economy. Isaiah Scheffer, a scalloper for all his adult life and now the shellfish constable for the town of Chilmark, explains that the work is hard and the level of commercial activity varies from year to year, depending on market prices and how plentiful the quarry is. “But the lifestyle is great when the fishing is good,” Isaiah says, adding that it’s a lifestyle well suited to a Vineyarder’s sense of self-sufficiency. “Being out on the water certainly has its romantic aspects. You’re one person working for yourself...and the harder you work, the more money you can make.”
Some Islanders, of course, don’t eat mollusks due to religious proscription, allergies, or squeamishness. But in general, shellfish – some imported but much of it local – figures prominently in Island dining and is valued for its simplicity, accessibility, and close link with the environment. “Everyone should eat [Vineyard] shellfish,” urges local-food proponent Jan Buhrman of Kitchen Porch catering in Chilmark. She ticks off reasons that include the support for the Island economy, the short supply routes, the assurance of knowing the origin and processing history of what you eat, and the health benefits of additive-free food. Clams or oysters breaded and deep-fried, steamed soft-shell clams, mussels simmered open in a wine or tomato sauce and served on pasta, minced cherrystones in chowder, bay scallops just hours from the water as ceviche or sautéed (very lightly, please) in butter, or best of all in the minds of many, quahaugs or oysters pried open and eaten unadorned off the half shell – these foods capture the very best of the ocean that surrounds us.
At risk in nature
Regrettably, all is not well with the Island’s iconic bivalves. Numbers of some species, like the notoriously finicky bay scallop, have always fluctuated unpredictably. But many of our shellfish populations clearly languish far below their historical levels. The pressures on bivalve populations, to the extent that we understand them, vary according to species and location. Exotic predators such as the European green crab, loss of eelgrass (which provides nursery habitat for some species), declining water quality, overharvesting, and destructive harvesting methods all are or have been part of the problem. But in general, the plight of our shellfish is symptomatic of a general decline in the health of our ponds and bays. As a result, the well-being of Island mollusks is steadily rising as an ecological priority.
One concern is health, both of humans and of mollusks. Bacteria in tainted water can concentrate in shellfish and cause human illness when the shellfish are consumed, and the Vineyard certainly has its share of bacteria sources, ranging from bird feces to road and farm runoff to leaking septic systems. Some Island shellfish habitat, such as Farm Pond in Oak Bluffs, is closed to harvest either permanently or intermittently due to the risk or reality of bacterial contamination. Meaningful statistics on the prevalence of illness caused by shellfish are hard to find, since most cases are minor and go unreported, but major outbreaks of illness linked to shellfish are exceedingly rare in the modern United States. Nevertheless, public health authorities uniformly describe raw shellfish as one of the riskiest things to eat, especially for people with underlying health problems such as an impaired immune system.
Shellfish aficionados note that the Vineyard still features ample areas of consistently high water quality, and that rigorous testing programs have made dubious specimens as rare as one could reasonably ask among legally harvested shellfish. The timid and those at special risk should consume their shellfish cooked; the rest of us, accepting that no portion of our food supply is absolutely safe, philosophically slurp our oysters off the half shell and find other things to worry about.
In addition to their ability to pass disease on to humans, shellfish suffer from ailments of their own, many of them harmless to people. Bay scallops and hard-shell clams aren’t particularly prone to disease, but soft-shell clams and especially oysters are susceptible, and shellfish pathogens can devastate affected populations. The worst culprit for Vineyard fisheries may be the protozoan Perkinsus marinus, cause of an oyster disease known as Dermo. Now established in both the Edgartown and Tisbury great ponds, Dermo emerged or at least was first noticed in the 1940s in oyster beds of the southeastern states. It causes progressive breakdown of the tissues of infected oysters, many of which eventually die from organ failure and then release more of the pathogen into the water as they decompose, passing on the infection. Oysters take several years to reach harvestable size, points out Rick Karney, a shellfish biologist and director of the Martha’s Vineyard Shellfish Group, and so a bad Dermo year depletes the harvest for multiple seasons.
Dermo is at its worst in warm water, and a couple of cooler summers in a row have helped rebuild oyster stocks in the south-shore ponds, Rick says. But the origins and ecology of many shellfish diseases remain troubling and somewhat mysterious. For example, Dermo, originally confined to warm waters, has spread into higher latitudes in recent decades. Whether this trend reflects a warming climate, evolution of a cold-tolerant strain of P. marinus, or some other factor is unknown. In any event, the ability of shellfish pathogens to spread and evolve, and the possibility of new pathogens emerging mean the threat of shellfish disease is unlikely to recede in the future. Encouragingly, shellfish aren’t taking this lying down. Oysters have shown some ability to develop resistance to disease, and this natural resilience, combined with human ingenuity, may be the salvation of our oyster fishery.
Farming efforts
Indeed, for decades now, artificial efforts have augmented populations of commercially important shellfish species on the Vineyard. While lightly fished areas are self-sustaining, according to Oak Bluffs’ Dave Grunden, the most accessible beds need periodic seeding with juvenile shellfish to compensate for heavy harvesting. And in recent years, the stocking of free-living mollusks around the Vineyard has been complemented by true shellfish aquaculture, “farming” oysters and mussels attached to artificial structures in the water. Edgartown’s Katama Bay is the current focus of this industry on the Vineyard, with more than a half-dozen growers there leasing space to raise oysters suspended from rafts, producing for both Island consumption and export.
Shellfish aquaculture is growing steadily in sophistication. Its stocks are selectively bred, just like the poultry and livestock of land-based farming, for traits such as disease resistance, rapid growth, flavor, and appearance. And growers, striving to provide the best conditions possible for shellfish growth, continue to tweak their culture methods. As aquaculturist Jack Blake of Edgartown explains, shellfish farming offers the same independence and water-based lifestyle as harvesting wild populations, only with greater control, predictability, and financial viability. The results are impressive: Katama Bay oysters I’ve eaten recently have been gorgeous mollusks, with rounder, thinner, paler shells than their wild cousins and plump meats nestled on deep half shells lined in pristine white. It is no wonder that cultured oysters often command twice the price of wild oysters at Island seafood stores.
The nexus of Island aquaculture has been the Martha’s Vineyard Shellfish Group (MVSG), a nonprofit corporation with roots going back more than thirty years. MVSG was “absolutely instrumental” to the establishment of shellfish aquaculture on the Vineyard, says Jack Blake, himself a pioneer in this business, citing grant funding and training programs MVSG has provided. Operating a cluttered, largely improvised, but incredibly productive hatchery atop a bluff on the Tisbury side of Lagoon Pond, MVSG produces literally tens of millions of immature shellfish annually to augment populations in Island ponds and bays. Crossing aquaculture stock with oysters that survived a Dermo outbreak in Edgartown Great Pond, MVSG has produced a strain that shows significant resistance to Dermo in laboratory tests. Seeded into the Edgartown and Tisbury great ponds, these shellfish haven’t yet been tested by a severe Dermo season, cautions Rick Karney. But a truly resistant strain could rescue the fisheries in those ponds.
MVSG has also supported Island fishermen in piloting an innovative deep-water mussel farm in Vineyard Sound, which has produced vigorous, healthy mussels, according to Rick. Expanding this approach to a full commercial scale faces daunting regulatory complications but could provide a major boost to the Island’s ocean-based economy. Tisbury’s Mark Lovewell, a Vineyard Gazette writer who has covered Island fisheries for many years, sees shellfish aquaculture as an activity that grows naturally from the central values of this place “where people are so highly conscious of their resources.” Buoyed by our affinity for everything aquatic, commercial farming of shellfish “looks like the future of the waterfront on Martha’s Vineyard,” says Mark.
A simple life
Artificial shellfish culture exploits the marvelously simple reproductive methods of bivalves. Each species puts its own stamp on the basic model, but in general, bivalves just dump their reproductive cells out into the water. (Bivalves are rather haphazard about sex, by human standards. Depending on the species, individuals may have a distinct sex, may have both male and female organs, or may even change from one sex to the other based on age or external conditions.) Typically, a regional cue such as water temperature prompts spawning, ensuring that most members of a species release their gametes around the same time, and hatcheries exploit this fact to persuade nursery stock to spawn under controlled conditions.
Fertilization is a random event, the happy collision of an egg and a sperm cell, and of the millions of eggs a wild bivalve might produce, most get eaten by something or simply drop unfertilized to the bottom. But fecundity trumps imprecision, and eggs that are successfully fertilized pass through one or more immature stages, depending on the species, while continuing to drift freely in the water, sometimes dispersing considerable distances from their points of origin. Upon reaching a certain developmental stage, immature shellfish sink to the bottom and, if they land on suitable substrate, settle in and grow to maturity. Many species of shellfish prefer something other than bare sand or mud to settle on. Oysters, for example, favor old shells to attach to, while bay scallops prefer to spend an immobile stage (temporary, in their case) attached to a sea-grass stem. Nursery-hatched juvenile shellfish can be distributed to bolster populations in shellfish beds; species that are immobile as adults can be transferred into boxes or cages and raised to marketable size off the bottom, amid steady water circulation and away from most predators.
Once in its adult form, a bivalve has little to do except feed, grow, hope it doesn’t fall prey to a starfish, gull, or clam-digger, and await sexual maturity so it can toss its own gametes into the lottery. Bay scallops, unusual among our bivalve species, live only a year or two and spawn just once. But at the other extreme, oysters and quahaugs, well protected in their shells and tapping an inexhaustible food supply, can reportedly live for decades if undisturbed, spawning annually.
If there is a weakness in the basic idea of a clam, it is immobility once the free-floating immature stages are past. Most mobile of our bivalves, scallops can jet across the bottom by snapping their shells shut (the robust muscle that powers these leaps furnishes the succulent white lump that is all that Americans typically eat of scallops). But scallop navigation is primitive, the motion imprecise and energy-intensive. Clams are equipped to burrow in the bottom sediment, thrusting a foot out, expanding it, and then hauling themselves after it. But their movements as adults are best measured in inches. And some bivalves, like mussels and oysters, spend their adult lives affixed to a single point (a habit that makes these species especially well suited for aquaculture). If conditions grow hostile, shellfish have no recourse but to clamp their valves shut and hope things improve before they suffocate.
A strained resource
In Vineyard salt ponds, a steadily growing risk to a bivalve’s environment is episodic reduction in the amount of oxygen dissolved in the water. Ordinarily, circulation of water and diffusion of gases through the pond ensure ample oxygen for the modest needs of a mollusk. But a process known as eutrophication can deplete oxygen levels, especially in water near the bottom of a pond, which is what matters to a clam. Paradoxically, eutrophication results from too much of a good thing. Excessive levels of nutrients, mainly nitrogen, trigger a bloom of algae, phytoplankton, and the tiny animals that feed on these simple plants.
Initially, this burgeoning of plant life can boost oxygen levels, since the process of photosynthesis gives off oxygen. But the hyper-abundant algae cloud the water, intercepting light needed by bottom-dwelling plants that furnish vital shellfish habitat. And when all that algae dies, drifts to the bottom, and begins to decay, the process of decomposition extracts oxygen from the water, threatening to suffocate animals that can’t move to areas with more oxygen. In extreme cases, all the bottom-dwelling organisms in the affected area die. Even if the effects of eutrophication fall short of fatal, bouts of low oxygen stress shellfish, reduce their growth rates, and make them vulnerable to disease.
The Vineyard’s salt ponds have always contained nutrients that are necessary to support aquatic life. But as human beings have proliferated on the Island so have septic systems, and effluent filtering into the ponds via ground water has steadily increased nitrogen levels in these vital bodies of water. Meanwhile, human proliferation has produced another source of nitrogen: Nitric acid, originating in the combustion of fossil fuels and arriving here in the form of “acid rain,” may be the largest single source of nitrogen now entering our ponds; other nitrogen sources include surface runoff containing lawn, garden, or crop fertilizer. Added together, these inputs have altered the nutrient balance in all of our ponds to varying degrees, increasing the frequency, extent, and severity of eutrophication.
The current condition of Vineyard ponds varies widely, says Bill Wilcox, the water resources planner for the Martha’s Vineyard Commission (MVC), with an evident relationship among water quality, the amount of development in a pond’s watershed, and how strongly ponds are connected to the open ocean. At one extreme, Menemsha and Cape Pogue ponds, with little surrounding development and powerful tidal flushing, are in good shape. At the other extreme, Lagoon Pond (with an intensively built watershed and a narrow opening relative to its water volume) and Tisbury and Edgartown great ponds (with only intermittent ability to flush nutrients out to sea) show impacts such as shrinking eelgrass beds and elevated levels of algae and phytoplankton. An ongoing monitoring program conducted by the MVC quantifies the murkiness and depleted oxygen levels in degraded ponds.
A linked future
Maintaining good water quality for shellfish will challenge Vineyarders in perpetuity. But it’s possible that the shellfish themselves could help. As filter feeders, shellfish literally do filter the water, inhaling through one tube, combing edible particles out as the water circulates across their gills, and pumping water, waste, and indigestible particles out another tube. In essence, digestible particles, largely phytoplankton, are then taken out of the aquatic system and converted into shellfish meat. One result is clearer water, greater light penetration, and healthier stands of bottom-rooted aquatic plants like eelgrass that are critical to inshore ecosystems. Another result is a leveling out of the “boom/bust” cycle of eutrophication. Abundant shellfish help limit algal blooms. And finally, the conversion of tiny plants into shellfish meat locks up some of the nitrogen in the system, improving water chemistry.
Bivalves, in other words, play an active role in maintaining the health of their own ecosystem. This ability to engineer their environment in their own favor may be one of the secrets behind the evolutionary success of this group of mollusks. The amount of water a single bivalve processes is modest, on the order of a gallon a day for an oyster, less for other species. But in a healthy aquatic system, bivalves can be incredibly numerous, and if you multiply that gallon a few million times, you are talking serious filtration. By one widely cited estimate, the (now much depleted) aboriginal oyster population in the Chesapeake Bay filtered a water volume equal to that of the entire bay in just three days!
This elegant bit of shellfish ecology raises an intriguing possibility: augmenting bivalve populations in our ponds and estuaries to improve the water quality of our coastal waters, thereby strengthening the entire aquatic ecosystem. Both free-living shellfish and cultured ones, suspended off the bottom, could do the job. Promoting shellfish, in other words, is an area in which economic development and ecological protection are not merely compatible, but downright complementary. If you have enough of them, says Dave Grunden, the Oak Bluffs constable, “shellfish are a great tool for maintaining clear water.” The MVC’s Bill Wilcox concurs. “The precise effect shellfish have on the clarity of the water column is still being thrashed out,” he says, but especially in the south-shore ponds, with their limited exchange of water with the ocean, shellfish could play a significant role in water-quality management (along with periodic pond openings and measures such as sewering or denitrifying septic systems to reduce the amount of nitrogen entering the ponds).
There are limitations to this scheme, and it would be foolish to think that simply planting ever more shellfish will allow Vineyarders to perpetually increase the amount of nitrogen we dump into our ponds. A pond only has room for so many shellfish, and since they are sedentary and bound to the bottom, wild shellfish have limited effect on clarity closer to the surface, where bright light is a major stimulant to algal growth. But it’s fair to say that water quality, shellfish health, and human ecological well-being are inextricably linked on Martha’s Vineyard; by improving one, we improve the other two.
Given their illustrious history, bivalves will likely outlast the human species, which is a recent upstart in evolutionary terms and shows a peculiar inclination toward self-destruction. But for as long as we have inhabited the Vineyard, human lives have been intertwined with shellfish, and the ecological challenges of the present and future will only intensify that interdependence. As the oyster goes, so goes the Island.
