vation of natural and experimental populations of hard clam in Southampton 

 Water and Poole Harbour in southern England showed that spawning and 

 growth were related to condition of clams in each population. In V . 

 mereenaria , unlike oysters, little storage of food reserves occurs, and 

 development of gonadial material takes place as and when sufficient food 

 is available. This has an effect on commercial fisheries, because hard 

 clams reach peak condition (which is a measure of the state of the gonad) 

 immediately before and during the spawning season. Environmental factors 

 appear to affect condition, growth and spawning similarly, so that in areas 

 where growth is early, fattening and ripening of gonad will occur early 

 also. Animals from a population with condition index less than about 12 

 (mean wet weight of flesh as a percentage of total wet weight of animal in 

 shell) were unlikely to spawn even when subjected to spawning stimuli, 

 although work in the U.S. and in England has shown that spawning can be 

 induced in winter when condition index is lower than 12. It would be 

 interesting to know whether the induction process produces a short rise 

 in condition index, or changes the physiological state of gonad development 

 only. - J.L.M. 



101 



Ansell, A. D., and E. R. Trueman . 1967. 



Burrowing in Mereenaria mereenaria (L.) (Bivalvia, Veneridae) . J. Exper. 

 Biol. 46: 105-115. 



Studies were made with new techniques based on measures of impedance, 

 hydrostatic pressure and displacement, which offered minimum disturbance 

 to normal activities. Digging cycle includes extension of foot, closure 

 of siphons, adduction of shell valves, and retraction of foot, in that 

 order. Digging period includes many digging cycles, which bring the clam 

 to its final position in the substrate. Closure of siphons and apposition 

 of mantle margins temporarily seals the mantle cavity to form a pressure 

 chamber. Adduction dilates distal portion of foot, aiding anchorage, and 

 ejection of water from mantle cavity loosens sand. Downward movement is 

 by passive dropping of the heavy shell into the fluid cavity formed at 

 adduction, and by active movement caused by contraction of retractor 

 muscles pulling the shell downwards toward the anchored foot. Strength of 

 pedal retraction is 5-6 g in a clam of 21 g in water. When shell is 

 completely covered by sand the opening movement of the ligament is too 

 small to open valves completely, and pressures generated by withdrawal of 

 foot and siphons in a secondary phase of siphonal movements supplement 

 action of the ligament. Burrowing movements of Mereenaria are compared 

 with those of 5 other bivalves. Greatest contrast was with Ensis . 

 Mereenaria is protected from predators by the heavy shell. Its response 

 to disturbance, which involves stimulation of siphons, is to withdraw 

 siphons and close shell. Burrowing occurs normally only after gross 

 disturbance when clam has been exposed on the surface of the substrate, 

 or in response to continued unfavorable conditions in the overlying water. 

 Penetration is made with foot extended ventrally, by antero-posterior 

 rocking caused by successive contraction of the almost equally sized 

 anterior and posterior pedal retractor muscles. (Abstracter's note: 

 maximum depth to which Mereenaria may burrow is not stated, but according 

 to the illustrations this appears to be only about l/5th to l/6th the 

 height of the shell from ligament to valve edge.) - modified authors' 

 summary - J.L.M. 



102 



Ansell, A. D., F. A. Loosmore, and K. F. Lander. 1964. 



Studies on the hard-shell clam, Venus mereenaria , in British waters. II. 

 Seasonal cycle in condition and biochemical composition. J. Appl. Ecol. 

 1(1): 83-95. 



Total weight of each clam was determined after washing in fresh water and 

 air-drying. Wet flesh weight was measured after a standard draining period 

 of about 48 hrs . Usually, air-dried shells also were weighed. Ash content 

 of flesh was determined after ignition at 800°C. For chemical determin- 

 ations one or more meats were homogenized. Lipid was determined by Soxhlet 



29 



