( PELECYPODA )
Venus (Now Mercenaria) mercenaria
Venus mercenaria, the hard-shell clam, is also known as the "little-neck" or quahog. It burrows in mud or sand flats near the lower part of the intertidal zone. The quahog is common at Woods Hole, Mass., and can be dug from the mud off Devil's Foot Island. Another clam, Callocardia, obtained in the same areas, is similar in appearance, but has a smooth margin to its shell; it is considerably smaller than the mature Venus.
Venus is "partially hermaphroditic," but is never self-fertile. It is usually protandrous until the second year, when the animal either remains male or reverses its sex (see the paper by Loosanoff, 1937b, for details).
Mid-June to mid-August, with a peak period extending from late June through early July. Individual batches of animals will shed for about 20 days only (Belding, 1911). The temperature of the sea water must be above the critical level of 23 to 25ûC. (Loosanoff, 1937b).
A. Care of Adults: These animals will remain in good condition indefinitely in the laboratory, if they are kept in large aquaria provided with flowing sea water.
B. Procuring Gametes: Since all attempts to prepare cultures by cutting up the gonads have proved unsuccessful, it is necessary to use naturally-shed gametes. Animals kept in the laboratory have been known to spawn three different times during a single season, extruding the gametes through the excurrent siphon in a fine stream. gelding (1911) stated that mature quahogs spawn at night when the water temperature reaches 24.5û C., and do best when they are not covered with sand. To induce shedding, place a beaker containing an adult into a waterbath, raising the temperature rapidly to 32-33ûC. Spawning usually occurs when the animal is cooled slowly. If the normal sea water temperature is above 20û C., the procedure is not effective until the animal has been pre-treated by chilling in an ice box for 24 to 36 hours (Zinn, personal communication).
C. Preparation of Cultures: The ova should be inseminated immediately. Add four or five cc. of sperm suspension to a fingerbowl of eggs and mix well. In a few minutes pour the contents through a #80 stainless steel sieve with 177-micron openings, into a fingerbowl of sea water. This will remove large debris. Repeat, using a finer sieve, which will retain the eggs and allow excess sperm and other small particles to be eliminated. Place the eggs in large battery jars of sea water and aerate continuously. Leave them undisturbed until the early veliger stage, then change the sea water every second day. Add a small quantity of mixed microplankton to the cultures daily; shells should be added to older cultures, to provide surfaces for attachment at metamorphosis. Using this method, Loosanoff and Davis (1950) reared cultures through metamorphosis without difficulty.
D. Extension of the Loosanoff and Davis (1950) have conditioned quahogs so that they spawn in winter. They are taken from their natural beds at a temperature of about 0û C., and placed in trays of running sea water having a temperature of approximately 5 to 7û C. At intervals of three to five days, the temperature is increased by several degrees, up to 20 to 22ûC. This conditioning period takes about three weeks. Actual spawning may now be induced by the method outlined above. Not all egg batches will have the same vitality, but with experience, poor batches can be recognized and eliminated.
A. The Unfertilized Ovum: Due to pressure within the ovary, the newly-shed egg is often irregular in shape, and one axis may be longer than the other. When spherical, the egg measures about 70 to 73 microns in diameter (Loosanoff and Davis, 1950). It is surrounded by a gelatinous membrane which swells markedly on contact with sea water; the diameter across this membrane measures 167 to 170 microns. The ovum is very yolky and appears white and opaque. It is shed in metaphase of the first maturation division. The germinal vesicle apparently will not break down in sea water (Loosanoff and Davis, 1950).
B. Fertilization and Cleavage: A fertilization membrane is formed, cleavage is spiral and unequal. Poorly defined polar lobes are developed. Gastrulation is by epiboly, followed by invagination.
C. Time Table of Development The following schedule of development at about 22û C. is presented by Loosanoff and Davis (1950). Time is calculated from insemination.
|
Stage First cleavage Second cleavage Ciliated blastula Early gastrula Trochophore Straight-hinge veliger Fully-formed veliger Mature larva, ready to set |
Time 45 minutes 90 minutes 6 hours 9 hours 12 hours 24-36 hours 32-44 hours 12 days |
D. Later Stages of Development and Metamorphosis: The gastrula elongates to form a top-shaped trochophore, which has no apical flagellum, but which is propelled through the water in a spiral direction by a crown of cilia at its anterior end. A mouth and shell gland are formed. The shell of the early veliger is bivalved and has a straight hinge line. Two adductor muscles are present. The mouth, stomach and small intestine are clearly visible, but the mantle is inconspicuous. The velum is elliptical and well ciliated. In older veligers, the velum degenerates and is replaced by a prominent ciliated foot, with its associated byssus gland. The digestive tract becomes increasingly complex, developing a liver, palps, and a long, coiled intestine. A well-formed mantle is present. At metamorphosis, the larva settles and becomes temporarily attached by the byssus.
Diagrams and further details of the larval stages can be found in papers by Stafford (1909), gelding (1911), Loosanoff and Davis (1950) and Loosanoff, Miller and Smith (1951).
BELDING, D. L., 1911. The life history and growth of the quahaug (Venus mercenaria). Ann. Rep., Comm. Fish and Game, Massachusetts, 1910, pp. 18-128.
LOOSANOFF, V. L., 1937a. Spawning of Venus mercenaria (L.). Ecology, 18: 506-515.
LOOSANOFF, V. L., 1937b. Development of the primary gonad and sexual phases in Venus mercenaria Linnaeus. Biol. Bull., 72: 389-405.
LOOSANOFF, V. L., 1937c. Seasonal gonadal changes of adult clams, Venus mercenaria (L.). Biol. Bull., 72: 406-416
LOOSANOFF, V. L., 1949. Method for supplying a laboratory with warm sea water in winter. Science, 110: 192-193.
LOOSANOFF, V. L., AND H. C. DAVIS, 1950. Conditioning V. mercenaria for spawning in winter and breeding its larvae in the laboratory. Biol. Bull., 98: 60-65.
LOOSANOFF, V. L., W. S. MILLER AND P. B. SMITH, 1951. Growth and setting of larvae of Venus mercenaria in relation to temperature. J. Mar. Res., 10: 59-81.
STAFFORD, J., 1909. On the recognition of bivalve larvae in plankton collections. Contr. Canadian Biol., 1906-1910, pp. 221-242.