undefined components were obtained from pooled ganglia and heart hemolymph 

 which had excitatory effects on Mercenaria heart. Mercenaria heart also has 

 been used to assay extracts of ganglia and muscle from other organisms. Two 

 questions arose: are any of the active components common to all mollusks? and 

 do the active substances in extracts play a physiological role in the living 

 mollusk? Studies beginning with extracts of M. mercenaria ganglia have 

 helped to answer these questions. Extracts of molluscan ganglion and nerve 

 may augment or depress beat of isolated heart preparations, and induce 

 rhythmical activity in quiescent hearts and other muscles of mollusks. Seven 

 cardioactive substances, including ACh and 5KT, have been found. 5HT has 

 long been known to induce rhythmical activity, but peak C, another 

 cardioexcitor component, also has oscillogenic activity, and it is often 

 effective when 5HT is not. An enzyme in molluscan ganglion extracts abolishes 

 peak C activity. Peak C, perhaps in concert with 5HT and other substances, 

 apparently is responsible for oscillogenic action of ganglion extracts. Its 

 physiological role might be long-term regulation of rhythmical activity of 

 cardiac and visceral muscle in mollusks. It is suggested that these substances 

 act by modifying binding of Ca+ + to cell membrane. - J.L.M. 



711 



Greene, Gregory T. 1975. 



Incremental shell growth patterns as affected by environment in Mercenaria 

 mercenaria. Unpublished B.A. thesis, Princeton University, 77 p. 



Daily growth markings, and winter and summer slowdowns or breaks in growth, 

 were studied from microscopic markings and other characteristics of 

 longitudinal sections of valves of Mercenaria mercenaria. The winter growth 

 period usually begins in December, and summer slowdowns usually in July and 

 August, in Great South Bay, N.Y. Highest growth rates were in spring (April- 

 May) and fall (October-November) . Clams in sand grew 58% faster, on the 

 average, than clams of the same age in mud. Mud clams showed numerous growth 

 breaks and generally slower year-round growth. Clams from shallow, still 

 waters varied the most in growth. Daily and seasonal growth patterns were 

 more subdued in clams from deep water. Spawning marks were seen on only a 

 few shells. No tidal patterns in growth were observed. - from author's 

 summary - J.L.M. 



712 



Greene, Gregory Trevor. 1978. 



Population structure, growth and mortality of hard clams at selected loca- 

 tions in Great South Bay, New York. A thesis presented to the graduate 

 school in partial fulfillment of the requirements for the degree of Master 

 of Science in the Marine Environmental Sciences Program, State Univ. of 

 N.Y. at Stony Brook, xi + 199 p. 



Growth rates of clams (Mercenaria mercenaria) in the Bay are highest in 

 spring and fall. Reductions in growth occur during summer and winter, 

 apparently in response to unfavorable environmental conditions. On the 

 average clams reach legal harvestable size in 3 to 3 1/2 yrs, but individuals 

 take as little as 2 1/2 yrs or as long as 4 yrs or more. Significant 

 variations in growth occur at different locations in the Bay. Year classes 

 are detectable in size frequency distributions and can be used to deter- 

 mine size of clams in the 3 or 4 most recent yearly sets at particular 

 locations. In heavily clammed areas most clams are under 4 yrs of age and 

 harvests from such areas consist mainly of clams from a single year class, 

 in which clams are just reaching harvestable size. Unclammed areas contain 

 much larger and older clams, and relatively few seed and small littlenecks. 

 Whelks and moonsnails are the most important predators at the present time. 

 These are most abundant relative to numbers of clams in areas of high 

 salinity, and least abundant in areas of low salinity, especially near river 

 mouths. They eat an average of about 3.5 clams each per month at 20 °C. In 

 high salinity areas these predators are abundant enough to have substantial 

 impact on adult clam populations. Total yearly natural mortality of large 

 seed clams (41 mm long) averaged 11.4 percent. - J.L.M. 



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