( PELECYPODA )
Crassostrea virginica
(Formerly Ostrea virginica)
(Syn. O. virginiana Gould)
LIVI NG MATERIAL:
The American oyster is found in shallow and brackish water and may be collected at West Falmouth, Rand's Harbor, and Hadley Harbor, Mass. The animals are raised commercially at Cotuit, Mass. The species is protandrous in these northern waters, but in one-year-old animals the sexes are essentially separate, although externally similar in appearance.
Mid-June to mid-August in the North Atlantic states (Galtsoff, 1937), provided that the temperature is 20 to 21û C. or above. Loosanoff and Davis (1952) state that in Long Island Sound, spawning begins at the end of June or early in July. They also demonstrated that animals could be induced to spawn at temperatures as low as 15û C.
A. Care of Adults: These animals will live indefinitely in aquaria provided with running sea water. If they are to be kept for any period of time, the sexes should be segregated, since the presence of members of the opposite sex acts as a stimulus to spawning. To ascertain the sex of an adult, one can drill a small hole in the anterior portion of one of the shell valves, and pinch off a small piece of the gonad for microscopic examination. This procedure does not seem to disturb the animal (Galtsoff, 1937), and the eggs or sperm are easily recognized in such a preparation.
B. Procuring Gemetes: Galtsoff (1937) gives several methods for procuring gametes and fertilizing the eggs of the oyster. The following account is abstracted from his article.
Remove one of the shells and locate the gonads which, in a ripe animal, appear as a creamy-white layer surrounding the visceral mass. Cut out a portion of an ovary and shake it gently into a fingerbowl of filtered sea water. As soon as the requisite number of eggs are released, the gonad should be removed. Sperm can be obtained by cutting up 0.5 gram of testes into 50 cc. of sea water.
C. Preparation of Cultures: One or two cc. of sperm suspension is sufficient to inseminate a fingerbowl of eggs. According to Brooks (1880), the sperm retain their full vitality for several hours; the eggs, however, should be fertilized immediately after they are released. Mix the eggs and sperm, allow the eggs to settle and then decant the top layers, containing superfluous sperm. Fill the fingerbowl with fresh, filtered sea water. In about four to six hours the larvae will rise to the surface. They should be transferred by means of a wide-mouthed pipette to a large battery jar of fresh sea water, and left undisturbed on a water table until the following day. Sea water filtered through coarse filter paper will contain enough microplankton to feed the larvae, but will be free of predators and competing species. The sea water should be changed every day. The larvae may be concentrated by the use of filter paper or bolting silk. For details of culture methods for older larvae consult the papers of Prytherch (1937) and Loosanoff (1954) .
D. Extension of the After the natural breeding season, oysters pass into an "indifferent" stage. This is followed by sex differentiation and then by a winter inactive stage which normally persists until April or May. Precocious gonad development may be induced in mid-winter during the inactive stage. The following conditioning method has been developed by Loosanoff (1945, 1954): The animals are brought in from water in which the surface layer is frozen or nearly frozen, and placed in trays of sea water maintained at 7-8û C.; the temperature is gradually increased by several degrees per day, until it reaches 20-22û C. (after two to three weeks). Then the animals are removed from the conditioning trays and placed in small glass dishes (containing sea water) which, in turn, are set in a large tray into which warm water is poured. This procedure rapidly increases the temperature of the sea water in the small dishes, and the animals are induced to spawn. The fertilized eggs are freed of debris by passing them through a series of fine screens; they are then transferred to large (five gallon) earthenware jars containing sea water, and left undisturbed until they reach the swimming larva stage (Loosanoff, 1954).
A. The Unfertilized Ovum: When first shed, the egg is irregular, hexagonal or pear-shaped, but on exposure to sea water it quickly becomes spherical, measuring 45 to 54 microns in diameter. It is somewhat opaque. The ovum is shed in the germinal vesicle stage.
B. Fertilization and Cleavage: The germinal vesicle breaks down shortly after insemination and two polar bodies are quickly formed by two maturation divisions. A thin fertilization membrane is present. Cleavage is unequal and spiral, with large polar lobes forming during the first two divisions. Gastrulation is accomplished chiefly by invagination, preceded by epibolic movements (Brooks, 1880).
C. Time Table of Development: The following schedule of development at 18 to 21û C. is given by Amemiya (1926). The time is calculated from insemination.
|
Stage First polar body Second polar body First cleavage Second cleavage Rotating blastula Free-swimming gastrula; Trochophore Shell well developed Feeding begins |
Time 40 to 50 minutes 45 to 60 minutes 75 to 90 minutes 80 to 120 minutes 6-1/2 hours 8 hours 24 hours 52 hours 60 hours |
At 20û C. the free-swimming stage lasts about 17 days; according to Galtsoff (1937), the free-swimming stage is reached in 4-1/2 to 5 hours at 23û-25û C.
D. Later Stages of Development and Metamorphosis A rounded trochophore is formed, with an aboral circle of cilia. It rapidly develops into a veliger by the expansion of the prototroch into a round, flat velum and by formation of a shell, bivalved from the time of its origin. The straight-hinge larva is a delicate pink in color. This color darkens to reddish-brown in the older veliger, which has a prominent foot, digestive tract and gills before metamorphosis. Diagrams and further details of the younger forms are given by Brooks (1880) and of later developmental stages by Stafford (1909).
AMEMIYA, I., 1926. Notes on experiments on the early developmental stages of the Portuguese, American and English native oysters, with special reference to the effect of varying salinity. J. Mar. Biol. Assoc., 14: 161-175.
BROOKS, W. K., 1880. Development of the American oyster (Ostrea virginiana List.). Stud. Biol. Lab., Johns Hopkins Univ., l: no. 4, 1-81
CLELAND, K. W., 1950. Respiration and cell division in developing oyster eggs. Proc. Linn. Soc., N. 5. Wales, 75: 282-295.
COE, W. R., 1936. Environment and sex in the oviparous oyster Ostrea virginica. Biol. Bull., 71: 353-359.
DAVIS, H. C. 1953. On food and feeding of larvae of the American oyster, C. virginica. Biol. Bull., 104: 334 350.
GALTSOFF, P. S., 1937. Spawning and fertilization of the oyster, Ostrea virginica. In: Culture Methods for Invertebrate Animals, edit. by Galtsoff et al., Comstock, Ithaca, pp. 537-539.
LOOSANOFF, V. L., 1945. Precocious gonad development in oysters induced in midwinter by high temperature. Science, 102: 124-125.
LOOSANOFF, V. L., 1954. New advances in the study of bivalve larvae. Amer. Sci., 42: 607-624.
LOOSANOFF, V. L., AND H. C. DAVIS, 1952. Temperature requirements for maturation of gonads of northern oysters. Biol. Bull., 103: 80-96.
PRYTHERCH, H. F., 1937. The cultivation of lamellibranch larvae. In: Culture Methods for Invertebrate Animals, edit. by Galtsoff et al., Comstock, Ithaca, pp. 539-543.
STAFFORD, J., 1909. On the recognition of bivalve larvae in plankton collections. Contr. Canadian Biol., 1906-1910, pp. 221-242.