viable than those from mollusks conditioned gradually. Obtaining spawn from 

 hard clam is relatively simple in summer. Merely raise water temp a few 

 degrees and add sperm suspension. But hard clam larvae can be raised year- 

 round by proper manipulation. The entire conditioning period for clams in 

 winter takes about 2 to 3 wks; less in spring. In Long Island Sound hard 

 clams may continue to spawn until late August or mid-September. They do not 

 completely recover until end of November. Many complex physiological 

 processes lead to accumulation of reserve materials, of which glycogen is 

 most important. Clams cannot be conditioned to spawn during this period. 

 But to supply ripe clams from late August to late November it is possible to 

 delay gonadal development until late fall by reducing water temp. One way 

 of doing this was to take clams early in the season, e.g., late May, and 

 transplant to Maine, where summer water temp averages about 7°C lower. 

 M. mevaenavia does not resorb undischarged gonad material in fall as oysters 

 do. In Maine waters Long Island Sound clams retain sperm or eggs through 

 summer and can be induced to spawn in fall, winter or spring, always producing 

 gametes which develop into normal larvae. Hard clam also can spawn several 

 times per year provided that changes in ecological conditions, especially 

 temp, are controlled so that they recover rapidly from spawning. Method of 

 spawning ripe clams in summer is: ripe clams are placed in glass spawning 

 dishes with about 1 liter of seawater of the same temp at which they were 

 conditioned. Dish is partly immersed in a large tray which is filled with 

 hot water, thus quickly raising temp in dish to desired level. Attempts to 

 fertilize eggs stripped from females usually fail because germinal vesicles 

 remain intact. This can be avoided by breaking down vesicles in a weak 

 solution of NH4OH, but the percentage of normally developing larvae is 

 relatively low. In a single spawning a female clam may release as many as 

 24.3 million eggs. There is no significant difference in viability of spawn 

 produced by hard clams of different ages, ranging in length from 37 to 110 mm. 

 The egg proper of hard clam measures only 70-73 u, but total diameter of egg 

 and surrounding gelatinous membrane is about 17 u. Abnormal development of 

 eggs and larvae may occur. Cultures in which less than 50% of eggs develop 

 into normal straight-hinge larvae were discarded. Sometimes abnormality is 

 associated with kind of food, e.g., some hard clam larvae will cease to feed 

 in presence of Chlamy domonas sp. , become emaciated, and die. Others in the 

 same culture will feed and grow normally. Clam larvae that are abnormal from 

 overcrowding, low temp, or high turbidity often grow to metamorphosis if 

 returned to favorable conditions. Dense algal blooms may also cause abnormal 

 development. Plankton samples collected during or immediately after heavy 

 algal blooms are often devoid or almost devoid of bivalve larvae. Heavy 

 blooms of dinof lagellates also may cause abnormal development. New vessels 

 of soft glass may contain substances toxic to clam eggs or larvae. Growth 

 rate of veligers from straight-hinge to metamorphosis is affected principally 

 by food and temp, especially temp. Hard clam larvae kept at near 30°C began 

 to set as early as 7 days after . fertilization, at 18°C only after 16 days. 

 Clam larvae from .the same parents, under identical conditions, varied 

 considerably in growth and time to metamorphosis. Clam larvae can endure and 

 even grow in very turbid water. Clams will grow from egg to metamorphosis at 

 temps from 18 to 30 C C. At temps much higher or lower, metamorphosis is 

 inhibited. Early cleavage eggs have a narrower temp tolerance than more 

 advanced stages. In cultures at constant temp of 24 °C and good food, like 

 Isoehrysis galbana, hard clams begin to set 12 days after fertilization. 

 They can not tolerate low salinities as oysters can. Optimal salinity for 

 development of eggs was about 27.5°/°°. Normal larvae did not develop at 

 17.5°/°° or lower. Upper limit was 35°/°° but few normal larvae were 

 produced. Straight-hinge larvae grew reasonably well at 17. 5°/°°/ but at 

 15°/oo none metamorphosed. Hard clam larvae seem to be able to utilize most 

 microorganisms small enough to be ingested. Larvae can grow and metamorphose 

 on a restricted diet, even a single algal species like Chlorella or some other 

 species. There was no evidence that organic detritus is utilized. Hard clam 

 larvae can select certain foods from a mixture. They can regulate food intake 

 and thus survive in heavy concentrations of food, but over a long time this 

 regulatory capacity is impaired and larvae may become choked with food and 

 die. optimal concentrations of food cells depended on their size. Large food 

 cells must be in lower concentrations per unit volume of water than smaller 

 cells. Dense concentrations of certain foods like Chlorella affect clam 



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