II THE MUSSEL

— A Bivalve for All Seasons

by JOHN

B. PEARCE

Of all the marine animals that the beach walker might observe, the mussel and its colleague in the intertidal envi­ronment, the barnacle, are by far the most common in those coastal habitats that have pilings, rock outcroppings, and other hard surfaces where these animals can attach. The common mus­sel, often referred to as the blue mussel, is a medium size bivalve which can grow to three or four inches long. It has an elongated foot which can be extended from between the two shells and can actually be used to allow the animal to move about. This foot has a special modification called the byssus, made up of several threads which are pro­duced by a gland in the foot. These threads are used to attach the mussel to hard surfaces. The mussel is a highly social animal and generally occurs in aggregations of hundreds or thousands where extensive rocky coastlines occur, as in Maine or California. They can occasionally be found in beds that cover hundreds of acres.

The mussel is extremely adaptable, found throughout the intertidal and sublittoral zones. In areas which receive heavy wave spray, the mussel will occur high in the intertidal environment. Moreover, because of its adaptability, the blue mussel is circumboreal in its distribution, i.e., it is found in temperate waters throughout the northern hemi­sphere. The blue mussel occurs on the west and east coasts of the United States, throughout the waters of Great Britain, Europe, and Scandinavia, and even in certain polar waters. In addition to being found throughout the entire intertidal range, from the splash tone to waters over a hundred feet in depth, the mussel is also adapted to living in varying degrees of salinity, from estuaries with salinities of 15 parts per thousand to the open sea.

Two other mussels are common to the east coast. The horse mussel is a much larger species, most often found in deeper waters and to the north of Cape Cod. The horse mussel, like the blue mussel, is circumboreal in its distribution, but because it usually occurs in deeper waters, it is generally not available to man.

The other common mussel on the East coast is the ribbed mussel, a com­mon denizen of marshes. It can be observed nested between stalks of marsh grass in most of the common east coast wetlands. Neither the horse mussel nor the ribbed mussel is regarded as suitable for human consumption. They often have a bitter taste, and the ribbed mussel frequently lives in environments which receive wastes or natural organic debris and is therefore not considered fit to eat.

The blue mussel is, however, an important food organism. It also func­tions within an important ecological niche in terms of other marine animals. As previously mentioned, the blue mus­sel will often form dense beds on rock surfaces or pilings, creating a habitat for many smaller species of marine life. Polychaete worms, amphipods or small shrimplike animals, snails, hydroids, small brittle-stars, and other forms requiring protection from predation will often live within and beneath the dense mussel beds. These small animals can move about under the layer of mussels and thus receive considerable protection from predators and from strong wave action and currents which might sweep them from their preferred habitat. Be­cause of the strong byssal attachments, the mussels themselves are difficult to remove and are able to withstand all but the strongest waves that pound exposed shorelines.

The mussel is an important prey for many species of marine fish and inver­tebrates. Mouths of the temperate water cunner and tautug are specially equip­ped to tear smaller mussels from their substrate. Other fish, such as dogfish and rays, also actively prey on mussels. In turn, as the mussels are gradually removed through grazing, other prey species are exposed and different forms of fish and invertebrates can then feed on them. In addition to fish, sea-stars, lobsters, and crabs, other invertebrates may use mussels for food. Thus, this important marine bivalve, which itself obtains food by filtering water for tre­mendous quantities of particulate foods, is able to grow very rapidly and, as a population, serves as an important element of intertidal and sublittoral food chains.

       In many instances, mussels in some parts of the world have been avoided during the summer months because of the possible toxic seafood poisoning which can develop. Traditionally, it has been held that mussels, along with other filter-feeding shellfish, should be col­lected or eaten only during a month with an “r” in its name. Marine scientists now understand that seafood poisoning develops when some species of single-celled phytoplankton are present in the water. Toxic forms of these phytoplank­ters produce a poisonous substance which is distributed throughout the tissues of bivalves when ingested. Sci­entists now are able to detect the pre­sence of such phytoplankton species and in many parts of the world waters are routinely analyzed to determine if they are free of toxic substances. This allows the mussels to be collected and prepared for consumption without undo concern for mussel poisoning.

The ecological requirements for mus­sel recruitment and growth, as previously noted. are extremely broad. For this reason many cultures throughout the world have for more than a century had commercial operations in which floats, ropes, and other devices are used to collect the early larval stages of the bloc mussel in shallow coastal waters and embayments. By properly placing such collecting devices, the mussel “farmer” is able to gather crops of mussels numbered in the millions. Because the mussel tends to reproduce most of the year, there is often a steady supply of larvae, and it is possible to have a more or less continuous crop of mussels developing within a given area all year long. Through the management of such extensive natural systems it becomes possible to provide food for large segments of the world’s popula­tions. As previously noted, mussels are extremely nutritious and even one or two individual animals eaten several times a week would provide the minimal protein allocations necessary for normal body maintenance and growth. Because the mussel can be grown in extensive numbers in compact areas, it becomes possible to provide food to extensive populations near mussel growing areas, cutting transportation and processing costs. It also ensures that the nutrients from runoff from the land are picked up as they pass through estuaries and before they are carried into the open oceans where they are effectively lost from a food web which culminates in man.

   The mussel has not only provided food for other forms of marine life, but has also been a delicacy in the diet of many human cultures. Romans regarded mussels as highly appetizing. Today, people throughout much of Europe also savor the mussel. The French have developed a wide range of recipes which the mussel is the principal ingre­dient; the thick French seafood soup, bouillabaisse, is an example. The man in the street in Italy or Portugal will often go to a seafood “bar” and have chilled fresh mussels as an appetizer or as a treat during the day. As with many of the more supposedly mundane foods, the numbers of ways to prepare mussels is virtually unlimited. In a recent book, “The Mussel Cookbook” by Sara Hurlburt (Harvard University Press), scores of recipes are provided on how to prepare mussels, in addition, the au­thor provides an extremely interesting life history on the blue mussel. She writes that about 3.5 ounces of common blue mussel contains 95 calories as opposed to 395 calories for the same amount of steak. The amount of protein in the common blue mussel is about the same as steak, and there is a much greater amount of energy-providing carbohy­drate. Trace minerals essential to human growth and development are far greater in a similar amount of T-bone. Perhaps most important, mussels are available throughout the year in most temperate coastal regions.

   To ensure that mussels are suitable for use, however, it is essential that their environment be free of sewage and in­dustrial wastes. As is true for many bivalves, mussels can take up sewage wastes, including pathogenic micro­organisms, and transfer them to the ultimate consumer, man. Mussels are also capable of biomagnification of industrial wastes. For instance, toxic levels of heavy metals and organic substances such as DDT and PCB's can accumulate in bivalves.

     Given the foregoing, and the benefit of seafood easily grown in coastal wa­ters, it is obvious that we must preserve water quality in estuaries not yet pollut­ed and, further, we must upgrade the water quality of major urban estuaries that are already burdened with patho­genic microorganisms and industrial wastes. This can and should be done. It will require recognition that coastal waters and estuaries are suitable for purposes other than waste disposal and industrialization. It will take a concerted effort by scientific and federal and state agencies to identify problems, the sources of contaminants, and solutions. It will require massive amounts of money to upgrade the sewage treatment and industrial waste treatment systems with­in such areas. The alternative, however, is to allow continued degradation of coastal waters and to allow areas that are already heavily polluted to remain relatively unproductive. Since estuarine and coastal water areas are limited, it is obvious that mankind must make the best use possible of habitats that are available to it for the production of seafoods. The mussel is one of the more productive organisms available, a rela­tively inexpensive seafood which has not only luxury connotations, but can also he a staple in the diet of mankind.

 

2 The Mussel                                   

1. What is a byssus

2. Where, geographically (range) are mussels found?

3. What are the three types of mussels listed....What use is each for food for man?

4. How do mussels benefit other organisms in a mussel bed?

5. What organisms feed on mussels?

6. What can cause poisoning in mussels?

7. What is the nutrition value of mussels?

8. What is the main drawback of raising mussels in coastal waters?

9. What is the main idea of the last paragraph?