locus for 4 loci in M. mevcenavia. Selection favoring heterozygotes was 

 found at the lactate dehydrogenase locus, particularly in the Canadian 

 sample. Heterozygotes have 2 forms of lactate dehydrogenase. This 

 presumably aids metabolism in an environment requiring alternating aerobic 

 and anaerobic metabolism. - modified from Dissert. Abstr. Internatl. 33(2), 

 1972: 840B. - J.L.M. 



1462 



Pesch, Gerald. 1974. 



Protein polymorphisms in the hard clams Mevcenavia mevcenavia and Mevcenavia 

 eampechiensis . Biol. Bull. 146(3): 393-403. 



The northern hard clam is intertidal and in shallow waters from the Gulf of 

 St. Lawrence to the Gulf of Mexico. The southern species ranges from N.J. to 

 the Gulf of Mexico and West Indies. It is found offshore in the northern half 

 of its range, and offshore and inshore to the south. Adults of both species 

 are non-mobile and long-lived (up to 20 yrs) . They lie slightly buried in 

 sandy or muddy bottoms. During larval life they may be dispersed widely by 

 currents. Ample opportunity exists for genetic exchange between adjacent 

 populations. Electrophoretic mobility of an enzyme is determined by its 

 primary structure. Enzyme variants detected by electrophoresis are equated 

 with alleles at the locus controlling the primary structure of that enzyme. 

 With this technique, phenotypic differences reflect genotypic differences at 

 single loci. Such techniques were used to answer the questions: 1) how much 

 variation exists within the gene pool of northern and southern hard clam 

 populations? 2) what genetic differences exist among widely separated 

 populations of the 2 species? and 3) does the gene pool of the northern 

 species differ greatly from that of its southern congener? M. mevcenavia 

 was sampled in the Bideford River, P.E.I. , Canada; Boothbay Harbor, Me; 

 Narragansett Bay, R.I; and Wadmalaw Island, S.C. The southern species was 

 taken at Beaufort, N.C. and Tampa Bay, Fla. Electrophoretic patterns were 

 obtained for enzymes in gill, mantle, muscle, and whole animal homogenates. 

 Three enzymes were polymorphic in all 6 populations. An average of 2.6 

 alleles/locus was found in 4 populations of M. mevcenavia, 3.5 alleles/locus 

 for 2 populations of M. eampechiensis . Malate dehydrogenase-NAD form had a 

 single allele which predominated in all populations. A 2nd rare allele 

 appeared in the Me. and S.C. populations of northern clam. Polymorphism was 

 pronounced at an esterase locus, but no regular shift was seen with latitude. 

 North to south clines were observed at tetrazolium oxidase and lactate 

 dehydrogenase loci. Different patterns observed for each locus suggested a 

 complex of selective processes probably of differing adaptive significance. 

 In a portion of the gene pools the 2 species had many alleles in common 

 (12 of 16). Commonality of alleles, homology of chromosomes, ease of 

 hybridization, and intergrades in the field suggested that these species 

 have not yet reached reproductive isolation. (Abstracter's note: one 

 possibility not recognized in this study was the influence of frequent and 

 uncontrolled introductions of hard clam from one locality to another. In 

 some areas such introductions have been made for a long time, either 

 deliberately, in the common practice of bringing in spawners from colder 

 waters on the assumption that this will increase the chances of obtaining a 

 successful set, or inadvertently, mixed with oyster transplants. The 

 potential effects of such activities, which have not been inconsiderable, 

 should be recognized and examined.) - modified author's summary - J.L.M. 



1463 



Petch, T. 1907. 



The marine fauna of the Kumber district and the Holderness coast. Trans. 

 Hull Sci. Field Nat. Club 3: 40. 



According to Heppell (1961) Petch observed that live quahogs were being 

 taken in the Humber estuary by the Cleethorpes oyster dredgers. The colony 

 originated from accidental introduction with American oysters. - J.L.M. 



409 



