The edible mussel is found attached by byssus threads to other mussels, to rocks and to piles, between tide lines and in shallow water. At Woods Hole, Mass., it is common and may be collected off docks and at Penzance Point and Pine Island. The sexes are separate but cannot be distinguished externally.

Spawning individuals have been obtained in the Woods Hole region from early June to mid-September (Bumpus, 1898; Field, 1922). Nelson (1928) noted that spawning of this species begins at temperatures of 10 to 12û C., which indicates the possibility of an earlier season than has been recorded. His findings are supported by the recent work of Sullivan (1948) at Prince Edward Island; Sullivan obtained larvae from late May to early August, when temperatures ranged from 10 to 24û C. Battle (1932) offers some evidence of a lunar periodicity for this species.

A. Care of Adults: The animals may be kept on a sea water table or in aquaria provided with running sea water. The sexes may be identified by examining the gonads, which completely fill the mantle folds of mature animals. The testes are orange-pink in color, the ovaries deep maroon; spent gonads are transparent.

B. Procuring Gametes: Field (1922) states that rough handling, such as shaking in a bucket or stirring rapidly in a dish of sea water, will induce spawning within an hour. This method has not proved successful in our experience. Gametes can be obtained, however, by removing the valves of an individual and placing the mantle folds in a small fingerbowl containing about 100 cc. of sea water. If the gonads are fully ripe, the gametes will ooze from them. The eggs should be strained through cheesecloth (previously washed and rinsed in sea water) to a bowl of fresh sea water. Berg and Kutsky (1951) report that they obtained Mytilus gametes by placing the animals in separate bowls of sea water and allowing natural spawning to occur. Their animals were stored for periods as long as one week at 4û C. before spawning occurred.

C. Preparation of Cultures: As soon as a microscopical examination of the eggs reveals germinal vesicle breakdown, they may be inseminated by the addition of four or five drops of active sperm suspension. Artificial insemination is not successful unless the eggs are readily released, the sperm very active, and germinal vesicle breakdown prompt. The eggs remain fertilizable for about three or four hours. Larvae may be cultured through metamorphosis; a method is described by Loosanoff (1954).

A. The Unfertilized Ovum: The egg is almost spherical, measuring 70 microns in diameter, and is enclosed by a vitelline membrane about one micron thick. It is pale brownish-pink in color, but when viewed in transmitted light, has a yellowish color. Although shed in the germinal vesicle stage, the ovum rapidly proceeds to metaphase of the first maturation division, becoming somewhat oval in shape meanwhile. It remains in this condition for three to four hours and then, unless fertilized, it dies (Field, 1922).

B. Fertilization and Cleavage: The head of the sperm enters, enlarges and moves toward the center of the ovum, which assumes a spherical form. Two polar bodies are given off in fairly rapid succession, the second directly below the first. Prominent polar lobes are formed prior to the first two cleavages. Cleavage is unequal and spiral, and gastrulation is by epiboly followed by invagination (Rattenbury and Berg, 1954). Figures of these early stages may be found in a paper by Field (1922).

C. Time Table of Development The following schedule of development at 20û C. was observed by Field (1922). The time is recorded from insemination.

Stage First polar body Second polar body Polar lobe formation, fusion of pronuclei First cleavage Second cleavage Free-swimming blastula Gastrulation, young trochophore Shell appears Well-formed veliger Metamorphosis

Time 18 to 20 minutes 30 minutes 50 minutes 70 minutes 80 minutes 4-1/2 to 5 hours 20 hours 43 hours 69 hours 1-1/2 weeks

D. Later Stages of Development and Metamorphosis: The young trochophore is a top-shaped organism with an extremely long apical flagellum and an equatorial circle of long cilia. About 40 hours after insemination, the shell gland appears as an invagination on the dorsal side of the larva; the apical flagellum is lost as the velar cilia develop (Rattenbury and Berg, 1954). The intestine is well developed, and the mouth, stomach and proctodeum are distinguishable. Within a few hours, the shell is formed, first as a single structure but soon becoming bivalved. It grows rapidly until it encloses the viscera. A conspicuous statocyst is present in the foot, which develops a "heel," enclosing the opening of the byssus gland. It also has a pair of eyespots. At metamorphosis, the larva, now asymmetrically shaped, settles to the bottom and becomes attached by byssus threads. The velum disintegrates, and the valves develop a blue rim as they increase in size. Stafford (1909) and Rattenbury and Berg (1954) present further details and diagrams of late larvae and metamorphosing forms.

SPECIAL COMMENTS:

Since this species is abundant at Woods Hole and the. female furnishes an adequate supply of ova, it should be more frequently utilized as embryological research material. The eggs resemble those of Cumingia and Ensis.

BATTLE, H. I., 1932. Rhythmic sexual maturity and spawning of certain bivalve molluscs. Contr. Canadian Biol. and Fish., 7: 255-276.

BERG, W. E., 1950. Lytic effects of sperm extracts on the eggs of Mytilus edulis. Biol. Bull., 98: 128-138

BERG, W. E., AND P. B. KUTSKY, 1951. Physiological studies of differentiation in Mytilus edulis. I. The oxygen uptake of isolated blastomeres and polar lobes. Biol. Bull., 101: 47-61.

BUMPUS, H. C., 1898. The breeding of animals at Woods Holl during the months of June, July and August. Science, 8: 850-858.

FIELD, I. A., 1922. Biology and economic value of the sea mussel Mytilus edulis. Bull. U. S. Burl Fish., 38: 127-259.

LOOSANOFF, V. L., 1954. New advances in the study of bivalve larvae. Amer. Sci., 42: 607624

MATTHEWS, A., 1913. Notes on the development of Mytilus edulis and Alcyonium digitatum in the Plymouth laboratory. J. Mar. Biol. Assoc., 9: 557-560.

MEVES, F., 1915. Über den Befruchtungsvorgang bei der Miesmuschel (Mytilus edulis L.). Arch. f. miter. Anat., 87: A, 2, pp. 47-62.

NELSON, T. C., 1928. On the distribution of critical temperatures for spawning and for ciliary activity in bivalve molluscs. Science, 67: 220-221.

RATTENBURY, J. C., AND W. E. BERG, 1954. Embryonic segregation during early development of Mytilus edulis. J. Morph., 95: 393-414.

STAFFORD, J., 1909. On the recognition of bivalve larvae in plankton collections. Contr. Canadian Biol., 1906-1910, pp. 221-242.

SULLIVAN, C. M., 1948. Bivalve larvae of Malpeque Bay, P. E. I. Bull. Fish. Res. Bd. Canada, no. 77, pp. 1-36.

WHITE, K. M., 1937. Mytilus. Liverpool Mar. Biol. Comm. Mem., vol. 31, pp. 1-117.