Clams regulated filtration rate according to quantity of food present, 

 filtering less water at highest cell densities. Numbers of cells filtered 

 per unit time increased with concentration of cells, approaching an 

 asymptote at higher cell densities. The asymptote was about 34x10** cells/hr 

 of Isoahvysis and 98x104 cells/hr of Phaeodaatylum. No clear asymptote was 

 reached with Dunaliella. Mean number of Dunaliella cells eaten/hr was 16.6x10^. 

 Lowest cell density tested was 17 cells/pl, which apparently was sufficient 

 for clams to get their maximum ration. Median cell volume of Dunaliella was 

 300 y3, about 5 times the cell volume of the other two species. The 

 relatively small size of the Isochvy sis ration as compared to Phaeodaatylum 

 probably was related to its high nutritive value. Isoahrysis is a much better 

 food for hard clam than the others. In flowing water, filtration rate .is 

 directly related to flow rate. Hard clams of mean shell length 0.8 cm and dry 

 meat weight 4.8 mg filtered about 10 ml/min at a flow rate of 300 ml/min. 

 Filtration rates were highly correlated with size of clam, especially if 

 calculated on the basis of dry meat weight. Filtration rate was inversely 

 correlated with water temp. Hard clams were particularly sensitive to 

 reduction in temp. Results of these experiments emphasize the importance of 

 standardizing rate of water movement in such studies. Experiments with 

 different concentrations of food suggest that adult bivalves can be as 

 sensitive to small changes in food concentration as larvae are. Siphonate 

 forms like Mevaenavia had lower filtration rates than oysters. - J.L.M. 



1930 



Walne, P. R. 1972. 



The importance of estuaries to commercial fisheries. In The Estuarine 

 Environment. R.S.K. Barnes and J. Green (eds.) . Applied Science Publishers, 

 Ltd., London: 107-118. 



Mevaenavia mevaenavia was found over 3 ha at a density of 206 g/m ^ in 

 Southampton Water, England. Establishment of Mevaenavia mevcenaria in 

 Southampton Water is due to warmer temperatures in shallow waters, reinforced 

 by the warm-water effluent of a nearby power station. - J.L.M. 



1931 



Walne, P. R. 1974. 



Culture of Bivalve Molluscs. 50 years experience at Conwy. Fishing News 

 (Books) Ltd., West Byfleet, Surrey, England, 173 p. 



Most of this book deals with the European flat oyster, Ostrea edulis. Many 

 techniques and methods, and accumulated experience, however, could be applied 

 to Mevaenavia mevcenaria. Chapter 6, "Other species of bivalves", contains 

 most information on hard clam. In the U.S., larger populations of quahog are 

 below low-water mark in soils varying from soft mud to shell and mud. Various 

 attempts have been made to establish the species in Britain and Europe, but 

 most were unsuccessful. Most seed placed in Essex rivers were severely 

 reduced by crab predation. At Conwy they grew very slowly. Later a 

 substantial population was found in Southampton Water in the upper part of 

 the estuary. Some beaches had 50 to 150 clams/m^ in a broad band from mid- 

 tide level to low-water mark of spring tides. This was traced to heavy 

 spatfalls in 1959-61, but older clams also were present. This demonstrated 

 that quahogs could thrive in Britain in a highly polluted and not very 

 promising area. In the laboratory larvae are reared in late winter and early 

 spring to gain max. advantage of summer growing period. Spat reared in July 

 and August will reach only 1-2 mm by end of fall and renewal of growth is 

 delayed until well into the following summer, and these spat may remain 

 permanently stunted. Quahogs of 4-7 cm are brought to the hatchery in 

 December and January and kept in 20-22°C running seawater. Over three years 

 the number of days to spawning varied from 29 to 52 days. Spawning is induced 

 by raising water temperature to 26°C and adding a suspension of male gonad. 

 After 24 hrs at 22°C embryos are swimming. On 2nd or 3rd day shell is well 

 enough developed to protect the larva, so that larvae can be collected on a 

 screen to change the water. Up to 1/2 million larvae can be reared in 75 

 liters of water. On the average about 42% metamorphosed. Average time to 

 metamorphosis was 21 days, at a shell length of about 250 u . At this stage 



537 



