larvae mechanically by interfering with swimming and feeding, and chemically 

 by production of toxic external metabolites. Some organisms have high food 

 value as foods for clam larvae, others cannot be utilized at all. Larval 

 growth rates vary with type of food. With some organisms, like Monas sp. , 

 unfed controls grew and survived better than experimental larvae. Many clam 

 larval cultures did well on dried and pulverized Vlva y but problems arose 

 because it was difficult to grind particles small enough, material settled to 

 the bottom, where it was unavailable, and decomposition fouled cultures. 

 Using dried Scenedesmus and Chlorella, agitating cultures, and controlling 

 bacteria, eliminated these problems. Overcrowding of clam larvae is not so 

 serious a problem as it is with other bivalves, but crowded clam larvae grow 

 more slowly. At densities as high as 250 eggs per ml most developed into 

 normal straight-hinge larvae. Metamorphosis is a gradual process for hard 

 clams. Advanced larvae have a functional velum and a foot, and alternatively 

 swim and crawl, perhaps for days. It is suggested that this may have survival 

 value by postponing metamorphosis if environmental conditions are unfavorable. 

 Hard clam larvae may metamorphose at about 175 y but usually not until 200-210 

 U long. Regardless of temp, larvae are always about the same size at setting. 

 Eggs and larvae of M. campechiensis grown at the same temp as M. mevcenaria do 

 not necessarily grow more slowly, as was formerly believed. A fungus, 

 Sirolpidium zoophthorum , can affect larvae of northern and southern hard clam 

 and their hybrids, often assuming epizootic proportions in M. mercenaria. 

 Larvae of all ages and juvenile clams up to 400 u long can be infected severely 

 and die. Bacterial infections can be controlled with antibiotics or 

 ultraviolet light. Routine use of certain chemicals also can improve larval 

 survival and control many fouling and competing organisms . Larvae of hybrids 

 between northern and southern hard clam often metamorphose at a higher rate 

 than parents, and growth of young hybrids may also show hybrid vigor. Larvae 

 of M. mercenaria are not too selective of substrate for metamorphosis, but 

 metamorphose in containers made of a variety of substances. Light or darkness 

 apparently has no effect on time of setting, and larvae showed no detectable 

 reaction to light. - J.L.M. 



1136 



Loosanoff, Victor L.,and Harry C. Davis. 1963. 



Shellfish hatcheries and their future. Coram. Fish. Rev. 25(1): 1-11. 



Shellfish hatcheries are needed because the supply of natural seed is 

 dwindling and also unpredictable. By selection, quality of product can be 

 improved for rapid growth, fatness of meats, and resistance to low salinity 

 and temperature. A good, clean water supply is a necessity. Methods 

 developed at Milford have made it possible to maintain ripe spawners 

 throughout the year. It is possible to grow a million clam larvae per sq 

 ft of table space every 15 days. This requires supplementary food, but at 

 lesser densities phytoplankton present in the water may be sufficient. Clam 

 larvae can be grown to metamorphosis on pulverized dried Viva or Laminaria. 

 One problem is that unutilized food settles to the bottom and develops rich 

 bacterial flora which affect clam larvae unfavorably. Later, larvae of 

 Mercenaria mevcenaria grew well on dried Scenedesmus , some reaching 

 metamorphosis in 10 days at 24°C. Hard clam larvae may develop from eggs 

 in the temp range 18 to 30°C. At the upper limit they may begin to set as 

 early as the 7th day after fertilization. Optimum temp range is between 25 

 and 30°C for larvae and about 25°C for juveniles. Temp may be important in 

 assimilation of certain foods by larvae. Optimal salinity for developing 

 eggs of hard clam from Long Island Sound is about 27°/°°, and normal larvae 

 will not develop at 17°/° ° or less. The staff at Milford is prepared to 

 offer suggestions to hatchery operators about handling certain problems, 

 such as control of disease, and control of competing organisms. Studies 

 required to improve shellfish hatchery operation include choice of location, 

 genetic studies, development of spawning stocks adapted to a variety of 

 environmental conditions, improved methods for obtaining set on a large 

 scale, control of undesirable pests and competitors, large scale trans- 

 plantations, development of better foods for larvae and young, methods of 

 protection against predators, especially for small clams, methods of 

 purifying contaminated clams, and improved mechanization of operations. 

 - J.L.M. 



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