Scientific Excellence • Resource Protection & Conservation • Benefits for Canadians Underwater World 2 he soft-shell clam, Mya arenaria, are considerably lower due to decreases Thas been an important resource for in the size of commercial stocks result­ The Soft-shell a very long time. Early records and ing from over-exploitation and natural , archaeological evidence indicate that predation. Many previously open areas Clam Indian populations indigenous to the have been closed because of fecal Maritimes highly prized the soft-shell coliform contamination from human clam for food and jewelry. The pre­ or animal sources. Nevertheless with sence and extent of shell heaps, called increasing public interest, combined middens, some of which exceed two with better preserving and marketing feet in depth and over several hectares, methods, the demand for clams re­ testify to the early importance of soft­ mains strong. Now clams are sold for shell clams to these people. the fresh market and the canned meats Later records show that early settlers for the fried clam, chowder and soup also depended on the then abundant markets. clam stocks to subsidize their nutrition Although the term soft-shell clam is in periods of privation. Clams were, most commonly used for the species however. never a mainstay in their diet. Mya arenaria, thIS same species is also Since those very early times, soft-shell known as: clam, soft clam, squirt clams have been used as bait, fertilizer, clam, steamer and gaper (Fig. 1). The ornaments, dishes and in some instances soft-shell clam of the Maritimes a form of currency. One of the first belongs to that group in the animal commercial uses of clams was not for kingdom known as the phylum Mol­ food but for bait. In the mid-1800s, lusca. The Mollusca include all of the clams were harvested and preserved in bivalves (two shelled) as well as the brine to supply Grand Banks' fisher­ gastropods (snails) and, probably sur­ men with salt bait for the cod, mack­ prisingly to the non-biologist, the erel and halibut fisheries. Over the cephalopods (squids, cuttle-fish and years a more general and domestic octopii). usage and marketing of soft-shell clams took place. Landings increased and reached a peak of 10,525 metric Description Fig. 1 The soft·shell clam Mya arenaria. tons (t) in 1950. Now production levels In general, the shell of Mya has a chalky white appearance. This colora­ tion may appear somewhat gray or yellow in younger individuals due to an external covering called the perio­ stracum. This covering tends to erode as the clam ages, resulting in the general white appearance. In some areas Mya will appear dark - almost black in colour. This somewhat unusual appearance reflects the high organic content of the muds in which the clam is living. The outer surface of the shell is covered with somewhat eliptical markings, some more pronounced than others. These are the annual growth marks or rings which can be used to aid in aging clams. In general, the shell reaches a maximum size of about 10 em in length but some have been reported to exceed 15 cm. At the posterior end there are two siphons (Fig. 2). This region has great extendability and is sometimes referred to as the "neck" of the clam. At the opposite end to the Underwater World 3 cisco Bay, California, along with excurrent siphon shipments of seed oysters transplanted from the Atlantic coast. Since that time they have spread south to the Monterey \ region of California and northward to lncurrent siphon Alaskan waters. This clam is also found along the western Pacific coast of Asia from the Kamchatka Penin­ postenor adductor muscle pallial cavity sula, U.S.S.R., to the northern regions of the Japanese islands. They are eplbranchlal cavIty found in suitable substrates through­ kidney mass out the marine intertidal of the heart lower gIll leaves Maritimes. -----f,m~ brown or Keber's gland cut mantle edge digesllve gland Habitat crystalline style sheath Mya are found in bays and estuaries, stomach intertidally and subtidally, to depths of mouth ~::::1n~~~~r-;"fj~71- about 9 m. They require a salinity of palps, lower _ Intestine 5 parts per thousand to survive but gonad anterior adductor muscle do best at salinities in the range of 25 to -~8:~:a~( visceral outlme 35 parts per thousand and at tempera­ cut mantle edge tures from 6' to 14'C. Normal sea foot water has a salinity of 34 parts per foot opening thousand. Clams live buried in bottom sediments; often in fine sands mixed with clay or with black muds of high organic content. They als'o occur in Fig. 2 Internal anatomy of the soft· more gravelly soil and even in stony or shelf clam. siphons is the clam's foot. The foot is a rocky places, but in these latter areas muscular tongue-shaped organ capable they are generally rare and not in suffi­ of great expansion and contraction cient Quantities to be of economic which enables the clam to burrow value. Being buried to depths of up to quickly into the sediment. 10 em, the soft-shell clam must use its siphon to pump water from the sea bottom above it in order to respire and Distribution feed. The siphon may be up to three The earliest fossil records of Mya, times as long as the shell. the genus to which the soft-shell clam belongs, date back to the Miocene Period, approximately 25 million years Food and Feeding ago. Today, Mya arenaria is widely Soft-shell clams feed on microscopic distributed in both North American plant and animal matter which is and European coastal waters. On the suspended in the water column just east coast of North America the soft­ above the bottom. Through the beating shell clam ranges from the Labrador of small hairlike cilia, a current is region to Cape Hatteras. It is abundant created which draws water through the from Chesapeake Bay northward but incurrent siphon (Fig. 2). Up to becomes scarce south of North 54 L of water may be filtered per day Carolina, U.S.A. In European waters by each clam. The microscopic food, it has been recorded from Norway to such as .filamentous algae, diatoms, the Bay of Biscay, France. Surprising­ algal fragments and naked flagellates ly, throughout shellfish-loving Europe, floating in the water I are then trapped it is not a sought-after species. Mya within the body and passed to the arenaria was not a species of the mouth, again with the aid of cilia. Pacific coast of North America until, After digestion, fecal material is re­ around 1879, young Mya arenaria were leased at the anus and is flushed out of accidentally introduced into San Fran- the body through the exhalent siphon. Underwater World 4 Reproduction Enemies Soft-shell clams are dioecious, Soft-shell clams have many preda­ meaning that the sexes are separate. tors. Diving ducks, whistling swans, They mature at a shell length of about cormorants, gulls and even crows con­ 2.5 cm which may be reached at an age stitute some of the major bird preda­ of about two to three years. tors. Fish such as rays, flounders, cod As water temperature warms with and sculpins will eat whole clams or the onset of summer, the gonads begin sometimes just nip off their siphons. to grow rapidly and occupy much of Their invertebrate predators include the visceral cavity (the space enclosed starfish, clam drilling snails and green by the shell). The gonads of both sexes crabs. In the early 1950s the green crab are generally cream to yellow. By June was a major predator of soft-shell the maturation of sperm and eggs is clams in the Maritimes. Man is also a usually complete and spawning may severe predator both in direct harvest occur with the proper stimulus. and indirectly by killing many individ­ Fig. 3 The clam hack or hoe used by Generally, spawning peaks around uals as a result of the procedures used fishermen. mid-July, and, like many other inter­ in commercial harvesting. tidal invertebrates, is linked to the monthly tidal cycle and to water tem­ perature. Fertilization is external; egg Contamination • ... and sperm being released through the Over the years, as a result of coastal - excurrent siphon, into the water col­ development, a large number of .=-0 - umn where they unite. The resulting estuaries and intertidal flats have fertilized egg develops into a larvae become contaminated with fecal coli­ which remains in the plankton for a forms and other potentially harmful period of about two weeks. During this organisms. Because molluscan shellfish period the larvae feed on micro­ tend to concentrate these contami­ planktonic organisms in the water. At nants, many large and previously pro­ the end of this planktonic period, ductive clam beds have had to be closed larvae undergo a metamorphosis into to fishing. However, clams can rid juvenile clams and settle to the bottom themselves of the contaminants if they where they temporarily attach them­ are held in suitable conditions with selves to the sediment by byssal clean seawater for a brief period of threads. time; usually about 48 hours. This pro­ Soon they release their byssal attach­ cess is generally referred to as depura­ ment and begin to crawl about the tion. Depuration then, is the controlled Fig. 4 Fishermen using clam hack. bottom with the use of their highly purification of bacterially contami­ extensible foot. When animals reach nated shellfish. It has been widely the size of about 6 mm they establish a recognized as a means of increasing permanent burrow.