planting was important. Seed stocked at 750/m 2 grew 0.6 to 0.7 mm/month, but 

 at 100 to 500/m 2 growth was uniformly good, 1.4 to 1.7 mm/month. Fences were 

 necessary to exclude predators . Without protection none survived more than a 

 few months. Principal predator was blue crab, Callinectes sapidus. Mortality 

 in fenced plots averaged 10%. The only other important predator was Busyoon 

 contvarium. In Va. predation has been controlled by preparing beds with 

 crushed oyster shell, crushed stone or pea gravel. At Milford quahog larvae 

 grew best in 5 ppm lindane. The authors express hope that this chemical will 

 not be adopted unthinkingly by clam growers. Fences used to exclude predators 

 reduced water circulation, and a layer of soft sediment, presumably generated 

 by clams, developed. This layer did not appear to be detrimental, but it was 

 noted that siltation can cause 100% mortality under certain circumstances. 

 Expense of keeping seed in the hatchery until it is resistant to enemies is 

 recognized as an economic constraint. In Fla. quahogs were grown from less 

 than 5 mm long to 10-15 mm at concentrations up to 5,000/m 2 in sand-filled, 

 mesh-covered boxes. This may be too expensive for commercial application. 

 Quahog seed from Milford have been shipped to other places along the 

 Atlantic and Gulf of Mexico coasts, and to Japan, Europe, and the U.K. In 

 Ireland these were planted on bottoms where the favorite cockle will not 

 grow. At Poole Harbor, Dorset, England, hard clams have been cultured using 

 waste heat, sewage, and flue gas, as a means of reducing pollution and 

 producing food. Clams grew more rapidly at all stages and spawned earlier 

 and more prolif ically . In N. America and Europe, where clams are a luxury 

 food, clam culture may be economically feasible. Genetic studies should be 

 continued. A method for growing clams of uniform size also is needed. 

 - J.L.M. 



134 . , , ,„^„ 



Barker, Richard M. 1964. 



Microtextural variation in pelecypod shells. Malacologia 2(1): 69-83. 



Studies were based on small numbers of 4 bivalve species, including lots of 

 15 Mercenaria mercenaria from localities from Prince Edward Island, Canada 

 to Florida. Dendritic crystallization of CaC03 plays an essential role in 

 producing great textural variation. Growth layers fall into 5 cyclic 

 groupings. One first-order layer contains 2 second order layers, 24 third 

 order, about 365 fourth order, and about 1,460 fifth order layers. These 

 consist of 3 ultra-fine elemental layers: 1) cryptocrystalline CaCC^; 2) 

 opaque or semi-opaque conchiolin; and 3) conchiolin dispersed in a micro- 

 crystalline aggregate of aragonite. A 5th order layer is a simple alter- 

 nation of conchiolin with one of the carbonate elements. The 4th order 

 layer is about 30 p thick and has exactly 8 elemental layers. The 3rd 

 order layer has an average thickness of about 0.5 mm and is created by a 

 cyclic variation in thickness of the 4th order layer with a period of 15 

 layers. The 3rd order layer also has a dark phase of relatively thick 

 composite and conchiolin layers alternating with a light phase in which 

 the cryptocrystalline layers are almost as thick as the others. The 2nd 

 order layer has a normal phase in which numbers of conchiolin and composite 

 elements exceed those of cryptocrystalline CaC03, and a carbonate phase in 

 which cryptocrystalline CaC03 elements are more numerous. This layer is 

 about 6.5 mm thick, and the normal phase makes up about 6 mm of it. The 

 1st order layer has an average thickness of about 13 mm and is characterized 

 by thickening and thinning of its 2 component layers. If these layers re- 

 flect environmental periodicities, the 1st order layer represents annual 

 change of temp and salinity, the 2nd order equinoctial tides and storms, 

 the 3rd the fortnightly tidal cycle, the 4th day and night, and the 5th 

 order layer the daily tidal rhythm. These growth layers are superimposed 

 upon 2 main layers parallel to shell surfaces: 1) an outer main layer in 

 which crystals are small and growth layers thick; 2) inner or middle main 

 layer in which crystals are large and growth layers thin. Microscopic 

 measurements show that crystal size is related inversely to salinity and 

 thickness of growth layers directly to temp. There was no microscopic 

 evidence of complete cessation of growth, although summer layers may be 

 ten times as thick as winter layers. Mathematical evaluation by more 

 rigorous and extensive analyses are recommended. Conclusions should be 

 tested by laboratory experiments and further ecological studies. - J.L.M. 



38 



