the intertidal experiment valves were open almost exclusively in high water 

 and darkness. Several days of one opening per 24 hrs were followed by several 

 days of 2 openings, controlled by interaction of light cycle and tidal cycle. 

 In absence of a tidal cycle all clams had a 24 hr rhythm, sometimes in phase 

 with valves open in darkness, sometimes with light. Opening at first appeared 

 to be in phase with lights-on at 6 am, but rephasing occurred after a few days 

 or weeks and may have been an artifact. Some clams in the non-tidal regime had 

 a second rhythm of 22-23.2 hrs, some had a lunar-related second rhythm somewhat 

 longer than 24 hrs. Shells grown intertidally record solar and lunar (tidal) 

 cycles. Growth occurs mainly at night. Valves are closed in daylight, tissue 

 pH drops, and conchiolin concentrates on the growing edge, forming the dark 

 band of the increment; or concentration of conchiolin will come about from 

 contact of the inner side of the accreting periostracum, where conchiolin has 

 not yet entirely polymerized, with the growing shell edge, as the mantle is 

 withdrawn. Night activity would appear to be adaptive in shallow water, where 

 major predators like birds are light-active and locate prey visually. 

 Addition of a tidal cycle to a solar-day rhythm of shell growth could account 

 for formation of semi-monthly clusters of lines. Just as open intervals in 

 experiments were interrupted periodically by low tides, growth of nightly 

 increments should be interrupted at certain times of the semi-month, 

 producing a narrower band, or leaving a dark band similar to the light- 

 deposited dark band. If high tides occur when the moon is in upper and lower 

 transit, then low tide will occur at midnight during first and third lunar 

 quarters. Deposition of complex increments will occur around lunar quarters. 

 In non-tidal experiments, which simulated some aspects of subtidal habitats, a 

 general pattern of response was more difficult to find. But most clams showed 

 two independent but interacting rhythms in which the number of open-intervals 

 per 24 hr period alternated from 1 to 2 . Failure to establish some permanent 

 and consistent rhythmic pattern showed that light cycles alone were poor 

 entrainers of bivalve rhythms. Under constant experimental conditions, light 

 cycles, tidal cycles, and temp cycles were absent, but gravitational and 

 electromagnetic cycles were still present. These are similar to abyssal 

 conditions, where growth increments occur, but no tidal clustering. It is not 

 certain at present whether these increments measure geophysical cycles, or 

 if so, which. Even in a constant-conditions regime, bioclocks apparently are 

 not completely independent variables, but interact with lunar and solar events. 

 In M. mercenaria it was concluded that valve movement and increment deposition 

 are under control of a biological clock, and valve opening under "constant 

 conditions" appeared to be responsive to actual solar-day and lunar-day cycles. 

 - J.L.M. 



1798 



Thompson, Ida L., and Franklin H. Barnwell. 1970. 



Biological clock control and shell growth in the bivalve Mercenaria mercenaria. 

 Geol. Soc. Am., Abstracts with Programs 2(1): 704. 



Opening and closing of valves of hard clam were recorded continuously in 

 artificial sea water aquariums for 8 weeks. Under constant illumination 

 persistent overt rhythms of activity were measured, with periods deviating 

 somewhat from 24 hr. Under cycles of 12 hr light and 12 hr darkness, distinct 

 daily rhythms were noted, with large individual differences in pattern. 

 Microscopic examination of shell growth patterns showed that both experimental 

 lots laid down periodic layers. Comparison of detailed patterns with opening 

 and closing activity was under way. - J.L.M. 



1799 



Thompson, Mary H. 19 64. 



Cholesterol content of various species of shellfish. I. Method of analysis and 

 preliminary survey of variables. U.S. Dept. Interior, 3u. Comm. Fish., Fish. 

 Industr. Res. 2: 11-15. 



Mercenaries, mereenaria taken in Nov 1962 from upper Chesapeake Bay contained 

 about 77 to 86 mg/100 g total cholesterol. Oil content was 2%, moisture 

 content 8 6.6%. Cholesterol content was somewhat higher than oysters, sea 

 scallop, and Dungeness crab (35-65) but lower than blue crab and peneid 

 shrimp (73-165). Samples were taken at various times of year, but seasonal 

 variations were mot measured. - J.L.M. 



497 



