LIVING MATERIAL

The soft-shell clam is found between tide lines and in shallow water, on mud flats and under stones. It may be collected in many such areas around Woods Hole, Mass. (including Rand's Harbor), but is not abundant. The sexes are separate; however, they cannot be distinguished unless the gonads are cut open and the sexual products examined.

Spawning begins when the sea water temperature rises above 10 to 12û C. (Nelson, 1928), and, according to Just (1939), extends throughout the summer. Observations by Bumpus (1898), Kellogg (1899), and Mead (1901), however, limit it to May and June. gelding (1915) recorded spawning in June and July, but we have seen only spent females at the beginning of July. The discrepancy might be explained if there is a second peak of gamete production in late summer, as suggested by Battle (1932) and, more recently, by Sullivan (1948). The latter observed an increase of young larvae at the beginning of August in plankton taken at Prince Edward Island.

A. Care of Adults: The adults must be kept in clean aquaria and occasionally allowed to lie exposed. Foul water and continuous submergence are unfavorable (Kellogg, 1899).

B. Procuring Gametes: Mature animals of both sexes have a firm, hard foot and large creamy gonads, surrounding the digestive tract. It is probably best to use naturally-shed gametes obtained from isolated mature individuals. Eggs and sperm obtained by cutting open the gonads in sea water will, upon insemination, produce only about 10 to 20 per cent normal development (Mead, 1901); gelding (1915) also reported that artificial insemination is not very successful.

C. Preparation of Cultures: Inseminate a fingerbowl of eggs by adding two or three cc. of sperm suspension. Leave for about 15 minutes and then decant the supernatant fluid and replace with filtered sea water. In about 12 hours the upper layers of water, containing free-swimming larvae, should be poured off into a large battery jar. Fill the container with sea water filtered through coarse filter paper, and aerate. The sea water should be changed every second day and a small quantity of mixed microplankton should be added daily. Old shells will provide surfaces for attachment as the time for metamorphosis approaches. Prytherch (1937) and Loosanoff (1954) give details for mass culture of larvae.

NORMAL DEVELOPMENT

A. The Unfertilized Ovum: The egg is almost spherical; measurements of the diameter vary from 58 by 62 microns (Stafford, 1901) to 80 microns (Battle, 1932). Belding (1915) recorded 62.5 microns as the average diameter. The ovum is surrounded by a vitelline membrane enclosing a thin perivitelline space, and a jelly-case is often seen. A considerable amount of yolk is present, and the egg is grey. It is shed in the germinal vesicle stage.

B. Cleavage: The unequal, spiral cleavage resembles that of Pecten and Venus. A ciliated blastula is formed, and gastrulation is by invagination, probably preceded by epibolic movement.

C. Developmental Rate: The blastula develop in about 9 hours, and trochophores by 12 hours after insemination. The veliger larvae may be seen in about 36 hours. In Massachusetts waters, the larvae spend about two weeks at the swimming stage, before settling to the bottom as spat (Ayers, 1956).

D. Later Stages of Development and Metamorphosis: As the trochophore forms, the body becomes elongated and the cilia become confined to the anterior part of the body. A primitive mouth is present, and opposite it the shell gland develops. The 24-hour larva is a typical lamellibranch veliger with a transparent, bivalved, hinged shell, an apical flagellum, and a round, ciliated velum. In the older veliger, the velum degenerates and is replaced by the foot as an organ of locomotion. Mantle, gills heart and a pair of otocysts develop prior to metamorphosis. Figures of the older larvae and fixation stages may be found in papers by Kellogg (1899), Mead (1900) and Stafford (1909).

AYERS, J. C., 1956. Population dynamics of the marine clam, Mya arenaria. Limn. And Oceanog., 1: 26-34.

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

BELDING, D. L., 1915. A report upon the clam fishery. 50th Ann. Rep., Comm. Fish and Game, Massachusetts, pp. 93-234.

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

JUST, E. E., 1939. Basic Methods for Experiments on Eggs of Marine Animals. P. Blakiston's Son & Co., Inc., Philadelphia.

KELLOGG, J. L., 1899. Special report on the life-history of the common clam, Mya arenaria. 29th Ann. Rep., Comm. Inland Fish., Rhode Island, 1898, pp. 78-95.

LOOSANOFF, V. L. 1954. New advances in the study of bivalve larvae. Amer. Sci., 42: 607-624.

MEAD, A. D., 1900. Observations on the soft-shell clam. 30th Ann. Rep., Comm. Inland

Fish., Rhode Island 1899, pp. 20-42.

MEAD, A. D., 1901. Observations on the soft-shell clam. 31st Ann. Rep., Comm. Inland Fish., Rhode Island, 1900, pp. 21-44.

MEAD, A. D., 1902. Observations on the soft-shell clam. 32nd Ann. Rep., Comm. Inland Fish., Rhode Island, 1901, pp. 20-33.

MEAD, A. D., AND E. W. BARNES, 1903. Observations on the soft-shell clam (Mya arenaria). 33rd Ann. Rep., Comm. Inland Fish., Rhode Island, 1902, pp. 29-48.

MEAD, A. D., AND E. W. BARNES, 1904. Observations on the soft-shell clam. 34th Ann. Rep., Comm. Inland Fish., Rhode Island, 1903, pp. 29-68.

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

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., 1901. The clam fishery of Passamaquoddy Bay. Contr. Canadian Biol., 1901, pp. 19-40.

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.