1291 



Menzel, Winston. 1977. 



Selection and hybridization in quahog clams (Mercenaria spp.). In Proc. 

 8th Ann. Meeting, World Mariculture Soc, La. State Univ., Div. Continuing 

 Educ: 507-521. 



From 1972 to 1975 northern (Mercenaria mercenaria) and southern (M. 

 campechiensis ) clams, original parents from Maine to Louisiana, including 

 Fi and F2 hybrids and backcrosses, were reared to determine which crosses 

 gave the best growth rates. Forty-six crosses were reared to 50 mm long. 

 The following conclusions and recommendations were made. 1. Genetic 

 manipulations and hybridization, along with selection, will establish 

 superior commercial traits for growing in tropical and semitropical areas. 

 2. Hybrids should have at least half of the northern species in the 

 pedigree for satisfactory keeping quality. 3. Best growth rates were 

 obtained in tropical and semitropical areas. 4. If clams are to be grown 

 in colder areas, selected clams from Delaware Bay should be used for best 

 growth. 5. Clams from Delaware Bay should be used in hybridization with 

 southern species for culture in tropical and semitropical areas. - J.L.M. 



1292 



Menzel, R. Winston, and Margaret Y. Menzel. 1965. 



Chromosomes of two species of quahog clams and their hybrids. Biol. Bull. 

 129(1): 181-188. 



Although the two American species df Mercenaria usually are thought to be 

 easy to recognize, diagnostic characteristics are sufficiently variable to 

 cause uncertainty at times. The two species hybridize readily in the 

 laboratory and may also do so in nature. Live specimens of both species 

 were obtained from several localities within their range, from Conn, to 

 west coast of Fla. Both species have 19 pairs of chromosomes at metaphase 

 I and 38 at embryonic mitoses. At metaphase I, chromosome pairs are small 

 and slender and the chromatid split can often be seen. The metaphase I 

 bivalents of M. mercenaria are similar to those of M. campechiensis but 

 tended to be more compact and less easily analyzed for chiasma frequency 

 and position. The F^ hybrid was intermediate in this regard, and all 

 chromosomes were paired regularly as 19 homomorphic bivalents at metaphase 

 I. At anaphase I in both species the hybrid chromosomes at one spindle pole 

 were commonly more compact and darkly stained than at the other. Mitotic 

 chromosomes of first and second cleavage divisions were rather long and 

 slender but tended to get shorter and more compact in later divisions, at 

 least at metaphase. High chromosome number and small cells made it 

 impractical to count chromosomes of individual nuclei after second cleavage 

 metaphase. Individual chromosomes ranged widely in relative lengths and 

 arm-length ratios. The ease with which hybrids can be made experimentally, 

 and existence in nature of forms which appear to be intermediate suggest 

 that some gene flow may occur. There appears to be no gross chromosomal 

 barrier to such interchange. This, of course, does not mean that such 

 interchange actually does occur in nature, but whether or not it does 

 happen, hybrids could be an important source of variation for selection of 

 improved strains for commercial production. - J.L.M. 



1293 



Menzel, R. W., and H. W. Sims. 1964. 



Experimental farming of hard clams, Mercenaria mercenaria, in Florida. Proc. 

 Natl. Shellf. Assn. 53: 103-109. 



Hard clams 33-44 mm long were planted in fenced plots in Alligator Harbor, 

 Florida. At concentrations of 7 5 clams/ft 2 clams grew 0.6-0.7 mm/month, but 

 at concentrations ranging from 10-50 clams/ft 2 growth was uniformly good, 

 1.4-1.7 mm/month. Mortality was 5-18% in fenced plots and 100% in unfenced. 

 Blue crab was the major predator. Planted clams less than 10 mm had 100% 

 mortality even in fenced plots, probably caused by shipping from Connecticut. 

 About 80% of the 37-44 mm clams reached market size (50 mm) in 8-10 months. 

 Average length at harvest was 52-56 mm. - D.L. 



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