* We are indebted to Dr. V. L. Loosanoff for some of the information reported here.

These small white bivalves are found with Venus mercenaria, burrowing on muddy or sandy flats near the lower part of the intertidal zones, and also in deeper waters. Although somewhat similar to Venus in appearance, Callocardia is much smaller and, unlike Venus, does not have a crenulated shell-margin. The species is moderately plentiful at Woods Hole, Mass., and may be collected by digging or dredging. The sexes are separate and similar in appearance.

Although the limits of the breeding season have not been ascertained, it is known that mature animals are available during the early part of July at Woods Hole. Loosanoff (personal communication) states that animals brought from their natural beds in Long Island waters during the month of January were apparently ripe.

A. Care of Adults: Adults may be kept on a water table or in large fingerbowls supplied with a continuous stream of sea water.

B. Procuring Gametes: Open the valves and excise the visceral mass, discarding the gills and mantle. After rinsing in running sea water, place the visceral mass in a slender dish filled with about 25 cc. of sea water. If mature, the gametes will ooze from the gonads which are located dorsal to the foot, on either side of the liver. The testis is light yellow in color; the ovary is creamy-white (Loosanoff, personal communication). The eggs appear as tiny flecks, the sperm as a milky suspension. Using a pipette, transfer the eggs to a large fingerbowl of sea water. They are very fragile and can stand neither rapid changes of temperature nor high temperatures.

Loosanoff (personal communication) states that he and his associates have been unsuccessful in all attempts to induce spawning in these animals, nor has he observed spontaneous spawning of Callocardia in the laboratory.

C. Preparation of Cultures: When a microscopical examination reveals that the eggs have become spherical, inseminate them by adding two or three drops of sperm suspension. Place the culture on a water table, and leave for about 15 to 17 hours. At the end of this time, use a mouth-pipette to remove the free-swimming larvae to a fresh dish of sea water; this procedure should be repeated daily

This culture method has yielded veligers which lived for a week or more. It should be noted, however, that only a very small percentage of the inseminated eggs developed.

Loosanoff ( 1954) has described another method for culturing older larvae, by rearing the embryos in large (five-gallon) earthenware jars; the cultures are left undisturbed until swimming larvae are present. Then the water is changed every day or two, by passing it through fine sieves which hold back the larvae. Older larvae should be fed; it is convenient to use mixed plankton for this purpose.

A. The Unfertilized Ovum: The egg is small, measuring from 49 to 60 microns m diameter, and is surrounded by a thick jelly-hull. It contains prominent yolk granules and a large germinal vesicle, which breaks down after entrance of the sperm. When first shed, the ovum often has a protrusion at one side (undoubtedly the remnant of the ovarian attachment), but after standing in sea water, it soon rounds up.

B. Cleavage: Cleavage is unequal and spiral, and gastrulation is probably by invagination. The early development of Callocardia is almost identical to that of other pelecypods (Loosanoff, personal communication).

C. Time Table of Development: The developmental rate of naturally-shed eggs has not been determined. Those eggs which have been artificially procured from the gonads seem to be slow in maturation and rather irregular in cleavage. The speed of development is apparently dependent to a large extent on the temperature (Loosanoff, personal communication). Loosanoff observed the following schedule of events at room temperatures; times are recorded from insemination.

Stage Polar bodies Ciliated swimming larvae Late trochophores Straight-hinge larvae Setting

Time 30-45 minutes 6-8 hours 18 hours 24 hours About 14 days

D. Later Stages of Development The larvae are quite small; Loosanoff reports (personal communication) that normal straight-hinge larvae measure approximately 78 x 64 microns. The trochophores are top-shaped, and have a long apical flagellum. Typical bivalve veligers are formed, similar in structure to those of Mactra and Pecten, but smaller and more opaque. After setting, the young clams grow quite well, according to Loosanoff.

REFERENCES

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

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