Harry C. Davis

Rearing of Bivalve Mollusks

Publisher Summary This chapter focuses on the rearing of bivalve mollusks. The rearing of larval and juvenile bivalves requires an adequate supply of sea water of proper salinity and free of substances that may interfere with their normal development. To condition mollusks for out-of-season spawning, it is necessary to keep them in running sea water at temperatures of 18°C to 20°C or sometimes higher. Warm sea water is also needed for rearing larvae and juveniles during the cold season. Conditioning of bivalves to develop mature gonads during the cold part of the year is relatively simple. It consists of placing mollusks, brought from their natural environment where water temperature may be near freezing, into somewhat warmer water, and then gradually increasing the temperature several degrees each day until the desired level is reached. Development of the egg of a bivalve is also discussed and the rearing of different species like Crassostrea virginica , Modiolus demissus , and Crassostrea gigas is reviewed.

THE pH TOLERANCE OF EMBRYOS AND LARVAE OF MERCENARIA MERCENARIA AND CRASSOSTREA VIRGINICA

1. The pH range for normal embryonic development of oysters was 6.75 to 8.75, and for clams, 7.00 to 8.75.2. More than 68% of the larvae of both clams and oysters survived at pH 6.25 to 8.75. The lower pH limit for survival of oyster larvae was 6.00 and for clam larvae, 6.25.3. The pH range for normal growth was 6.75 to 8.50 for clam larvae and 6.75 to 8.75 for oyster larvae. The rate of growth of both species dropped rapidly at pH levels below 6.75.4. The optimum pH for growth was 7.50 to 8.00 for clam larvae and 8.25 to 8.50 for oyster larvae.5. At pH 9.00 to 9.50 the percentage of eggs that developed normally, the percentage of larvae that survived, and the percentage increase in mean length of both species decreased rapidly.

CONDITIONING V. MERCENARIA FOR SPAWNING IN WINTER AND BREEDING ITS LARVAE IN THE LABORATORY

Rapid progress in studies of the morphology, physiology and ecological requirements of lamellibranch larvae, as well as other forms, can be achieved only if these organisms are easily available for study. This is seldom so in the case of lamellibranchs because the majority have a breeding period of comparatively brief duration lasting only a few months (Lebour, 1938; Thorson, 1946). During this period the larvae can be collected in plankton tows but, unfortunately, the early stages of many lamellibranchs are so alike in size and appearance that it is often impossible to distinguish with any degree of certainty the larvae of different species, or even genera. Thus, unless these larvae can be grown to metamorphosis their specific identity may remain in doubt. It is preferable, therefore, to work with larvae raised in the laboratory from fertilized eggs because in that case their identity cannot be questioned. In obtaining material for morphological and physiological studies of larvae the advantages of the direct method, i.e., the method of raising larvae in the laboratory, over the indirect method of collecting them in plankton and often only guessing their identity, are clear. However, because lamellibranch larvae are small, and because their free-swimming period is relatively long, they are difficult to culture. As a result, with the exception of a few successful attempts, mostly confined to commercially important species, such as oysters and mussels, few lamellibranchs have been grown in laboratories past the early veliger stage (Thorson, 1946). Therefore, the need for a simple but reliable method for culturing them is, of course, obvious. It is hoped that the simple but efficient method described in this article for conditioning the hard shell clam, V. mercenaria, to spawn out of season and for culturing its larvae will be applicable to many other species of lamellibranchs permitting their successful cultivation in the laboratory, where their morphological features and various aspects of behavior can be studied under controlled conditions. Realizing the importance of having a good supply of larvae for studies of the life history of Venus mercenaria, Belding (1912) tried to raise them in the laboratory. He was not successful because most of the larvae in his cultures died either before they reached the straight hinge veliger stage or soon afterwards. Belding concluded that there was no practical method for raising hard shell clams to the setting stage because of the small size and delicate nature of the eggs. Several years later, nevertheless, Wells (1927) succeeded in growing clam larvae to the setting stage. Wells, however, was mostly interested in oysters and did not continue the clam work.

EFFECTS OF TURBIDITY-PRODUCING MATERIALS IN SEA WATER ON EGGS AND LARVAE OF THE CLAM (VENUS (MERCENARIA) MERCENARIA)

1. Some clam eggs developed normally in concentrations of 4.0 g./l. of clay, precipitated chalk or finely ground Fullers earth, although the percentage developing normally decreased as the concentration of these suspended materials increased.2. In silt concentrations below 0.75 g./l. the percentage of clam eggs developing normally was not significantly different from that in control cultures but decreased progressively in successively higher concentrations.3. None of the clam eggs developed normally in silt concentrations of 3.0 or 4.0 g./l.4. Larvae resulting from clam eggs developing in high concentrations of each of the suspended materials were reared to metamorphosis after being returned to normal sea water at 48 hours.5. Clam larvae were unable to grow in concentrations of clay, chalk or Fullers earth as high as those at which some eggs developed.6. The highest concentration of chalk was 0.25 g./l. and 0.5 g./l. was the highest concentration of clay and Fullers earth at which clam larvae showed an...

ON FOOD AND FEEDING OF LARVAE OF THE AMERICAN OYSTER, C. VIRGINICA

1. None of the 13 species of marine bacteria tested to date was utilized as food by oyster larvae.2. Five species of flagellates, Dicrateria inornata, Chromulina pleiades, Isochrysis galbana, Hemiselmis rufescens and Pyramimonas grossii, were utilized as food by oyster larvae, while another, an unclassified chrysomonad, in addition to the three flagellates previously reported, was not.3. Chlorella sp. was not utilized as food by young oyster larvae but was utilized during later larval stages.4. None of the combinations of foods tried gave any evidence of providing a more balanced diet or more rapid larval growth than could be obtained by feeding equivalent quantities of a single food. The effects of all foods tested, including Chlorella, are additive.5. When equal numbers of cells are fed, different species of flagellates induce different rates of growth of oyster larvae.6. Species of flagellates also differ in the number of cells needed to induce the maximum rate of growth of oyster larvae.7. The maximum...

Survival and growth of larvae of the European oyster (Ostrea edulis L.) at different temperatures.

1. The temperature range for satisfactory growth of O. edulis larvae (70% or more of optimum) was from 17.5° to 30° C.2. The temperature range for satisfactory survival (70% or more of optimum) was from 12.5° to 27.5° C. Even at 10° and 30° C survival was poor, perhaps, because the unfavorable temperatures weakened the larvae, making them more susceptible to bacterial toxins and diseases.3. In these experiments approximate setting times were as follows: 17.5° C—26 days, 20° C—14 days, and at 25°, 27.5°, and 30° C beginning of setting varied from the 8th to the 12th days.4. More spat were obtained at 20° to 22.5° C than at higher temperatures.5. It is suggested that larvae be reared to setting size at temperatures from 25° to 27.5° C, then kept at 20° to 22.5° C during setting to obtain fastest growth of larvae and highest percentage setting.6. Spat kept at 10° C showed virtually no growth; at temperatures from 12.5° to 27.5° C growth of spat increased with each increase in temperature.

Mass Culture of Phytoplankton as Foods for Metazoans

An apparatus for mass culture of photosynthetic microorganisms has been developed to grow algae for use as foods for larval and juvenile mollusks in studies of their physiological requirements. The apparatus consists of a series of 5-gal growth chambers, and the system can be enlarged to yield any desired volume of algae by replication of basic units. Approximately 50 lit. of algal suspension, averaging about 0.5 ml of packed wet cells per liter, are produced daily.

SURVIVAL AND GROWTH OF LARVAE OF THE EUROPEAN OYSTER, O. EDULIS, AT LOWERED SALINITIES

1. At salinities of 25 and 22.5 ppt growth of larvae of O. edulis and intensity of setting was not significantly different from that in control cultures at our normal salinity of 26-27 ppt.2. At 20 ppt growth of larvae was appreciably slower than at higher salinities and the intensity of setting was reduced.3. At 17.5 and 15 ppt larvae lived for some time and showed appreciable growth, but they all died prior to metamorphosis.4. At 12.5 ppt larvae showed no growth and by ten days after swarming they had suffered 90% or higher mortality.5. At 10 ppt all larvae died in less than four days.6. Some larvae that had been reared to setting size at a salinity of 26-27 ppt were capable of setting in salinities as low as 15 ppt.7. No normal larvae were obtained from adult oysters kept at salinities of 20 or 17.5 ppt.

MORTALITY OF OLYMPIA OYSTERS AT LOW TEMPERATURES

1. Olympia oysters, kept throughout the winter in Milford Harbor, showed 100 per cent mortality.2. The mortality of these oysters increased progressively as the temperature at which they were kept was progressively decreased.3. Death usually occurred after a month or more of exposure to low temperatures.4. Mortality did not appear to be due to tissue starvation, since many of the dying oysters still contained a good reserve of glycogen.5. Although our breeding stock was selected, each year, after the least resistant oysters had already succumbed, Olympia oysters, even after five generations at Milford, had still not become acclimated to our prolonged period of low temperatures.