Drawing On Science
Prior to the European invasion of the east coast of North America, local Indians harvested clams and oysters without threatening the population of the species. The oyster was extremely abundant from Massachusetts to the Gulf of Mexico and locally along the coast of Maine and Canada to the Gulf of St. Lawrence. The animals were an important food source for the local inhabitants. Clams, especially the Hard Shell Clam, were also used to make belts of wampum for trading with other tribes. The arrival of the European, however, greatly affected the oyster population through their “modern” harvesting methods and by 1810 the oyster beds showed signs of decay.
From 1880 to 1920 oyster aquaculture was a thriving economic activity in Jamaica Bay and along the east coastal regions of North America. The local New York Oyster (also known as the Virginia or American Oyster), was known for its apparent anti-perishable abilities. Oysters are perhaps the most commercially valuable invertebrate and the local populations’ characteristics made New York the undisputed oyster capital of the United States. Harvested oysters would find their way to the plates of many restaurants throughout NYC and the US.
The ability to separate nutrients from silt allows oysters to survive in waters of high turbidity (a measurement of water transparency; high turbidity = less visibility) as occurs in many estuaries. It’s this highly efficient filtering ability of oysters that allows it to play an important role in removing not only suspended sediments from the water, but also the removal of various pollutants from water. Back in the early harvesting days of oysters this fact, the cleansing ability of oysters, was not considered as particularly important to the quality of the water in which the oysters thrived. Due to a number of factors including the growth of the city, the pollution that follows it and the untreated sewerage being dumped into the bay, the oyster population soon declined. The city’s growth also necessitated over-harvesting of the oysters to keep the restaurants supplied.
These facts, coupled with the belief that the oyster bounty in Jamaica Bay and other east coast areas was infinite, ultimately resulted in the decline of the oyster population. In 1890 oysters were linked to outbreaks of typhoid fever and gastro-intestinal distress. The polluted, local waters of Jamaica Bay were ultimately condemned in 1912 and oyster harvesting in the bay, for all intents and purposes, ceased.
Finally the hurricane of 1938 finished the job; the oyster population was completely decimated. Today the bay’s bottom is virtually devoid of oysters, although oyster larvae from outside waters may possibly be found in the bay.
There is at present a three-year study to determine if the Jamaica Bay environment is suitable enough to, once again, play host to the oyster. Professor Jeffrey Levinton, lead investigator and Marine Biology Professor at SUNY Stony Brook said, “Oysters are important components of marine ecosystems. They are natural filters of the water. If they were reintroduced to Jamaica Bay, they would form mounds and attract diverse plant and animal species. Our research will show us that oysters can survive and grow rapidly in Jamaica Bay. We hope it will also show us that if oysters were reintroduced to Jamaica Bay, they would be able to grow, reproduce, and establish sustainable oyster reefs.” In view of these possibilities the bay around Floyd Bennett Field and Hendrix Creek was seeded with adult oysters. It is hoped that these will serve as “growing chambers” for the larvae. (Hendrix Creek flows for just over a mile in Brooklyn through East New York. It passes under the Belt Parkway and between two dormant landfills before it empties into Jamaica Bay. Hendrix Creek was chosen because it’s protected from the stronger currents from the bay and it’s small enough for a relatively modest number of oysters and clams to have a measurable effect. The creek is directly fed by the 26th Ward waste water treatment plant. The experiment will also provide a good indication of how well the bivalves can filter water coming directly from the city’s own filtration system.)
To better understand how an oyster is able to filter the water let’s take a look at its external and internal anatomy. The anterior end of an oyster is pointed and is called the umbo, while the posterior end is rounded, and is called the bill (Diagram A). The left valve (shell) is the shorter and flatter of the two valves, which are wrinkled-like. An adult oyster may reach a length of three to four inches.
The adductor muscle holds the two valves together and its ability to keep the valves closed is obvious to anyone who ever tried to pry an oyster open (even that Hulk guy would have a problem). You can see the attachment points of this muscle on the inside of the valves (Diagram B). The thin layer of tissue that lines the inner part of each valve is called the mantle. This thin tissue contains glands that extract elements (if you find one with a pearl, let me know). The elements are then used to produce compounds that make up each valve. An oyster’s valve is made up of about 98 percent of calcium carbonate; the same material used to make chalk.
Oysters spawn during the early summer when the mantle cavity is loaded with eggs or sperm depending on the sex of the individual. The egg- or spermloaded fluid is emptied into the open water where fertilization takes place. It’s estimated that a single female may release up to 60,000,000 eggs at one time. Of course not all are fertilized and those that are may be devoured by predators or drawn out to the open ocean by currents; the eggs that survive hatch into tiny top-shaped, free-swimming larvae (trochophore larvae). In a day or two these larvae metamorphose into larger, clam-like larvae (veliger larvae). The veligers soon secrete materials necessary for the production of the valve. They then settle to the bottom or on some hard, rough surface and attach by their left valves forming clusters of oysters. A clam is adapted to a semi-sedentary life being able to move using its foot (If you’re thinking with toes, forget it. It’s not that kind of foot.) whereas an oyster is permanently attached to the bottom.
Oysters and clams are water-filtering animals, but an oyster is much more efficient in its filtering abilities cleaning as much as 50 gallons of water per day. The oyster feeds by filtering food particles from the surrounding water, but not through incurrent or excurrent siphons as in a clam. By opening and closing its valves controlled by the adductor muscle attached to each valve, water is pumped into the mantle cavity. The quantity of water pumped by a large, healthy oyster may approach four gallons per hour. Food, such as plankton and other particulate matter suspended in the water, is drawn into the oyster. Small, fringe-like whips called cilia located on the gills, trap food particulate which are held by mucus secretions. Food particles which are trapped by the gills are passed by the ciliary action to the mouth and then to the stomach, which is surrounded by a digestive gland. The food (partially digested) is passed into the intestine to complete digestion. Waste is excreted out through the anus into the mantle cavity and forced out into the surrounding water by the continuous pumping action of the oyster’s valves. Any material, such as silt, or large, non-nutritive particulate matter brought into the valve, but not passed into the mouth, is collected by mucus on the gills and then discharged into the surrounding water. I once watched an experiment where oysters were placed into an aquarium filled with turbid water. You could hardly see the oysters through the clouded water. It wasn’t long before the water started to clear up and in a few minutes it was crystal clear. Oysters are amazing filters, but there’s no way I would allow the slimy beast to just slide down my gullet. Munching on fried grasshoppers, however, is another story. Questions/comments? E-mail Steve: Drawingonscience@aol.com