INDEXES OF CONDITION IN MARINE BIVALVE MOLLUSCS 489 



basis, summer levels were similar in all groups but winter levels 

 increased with increasing age. 



Since carbohydrate and protein are the major biochemical 

 components of adult bivalve soft tissues (Masumoto, Masumoto, and 

 Hibino, 1934; Dare and Edwards, 1975; Walne and Mann, 1975), an 

 increase in the percentage content of one is usually reflected in a 

 decrease in percentage content of the other. Since the depletion of 

 carbohydrate is accompanied by an increased use of protein as a 

 respiratory substrate, the ratio of these two components can also be 

 used as an index of condition: 



^^j^ = Biochemical index (6) 



The recent development of analytical instrumentation to both 

 carbon and nitrogen contents simultaneously stimulated Ansell and 

 Sivadas (1973) to suggest that the carbon-to-nitrogen ratio may be 

 useful as an index of condition in the bivalve Donax vittatus (da 

 Costa). This ratio gives a measure of the total organic content of the 

 soft tissues in relation to protein content and thus, by inference, a 

 measure of nonprotein organic material available for use as a 

 respiratory substrate. Thus 



Q 



t;7 = Biochemical index (7) 



As in other gross biochemical components, there is a seasonal 

 temperature-related cycle in the ash (inorganic) content of soft 

 tissues (ash is defined as the material remaining after ignition at 

 500° C or higher for a period of 12 hr). In the oysters Crassostrea 

 gigas and Ostrea edulis, percentage ash content is usually highest in 

 the winter months and lowest in summer (Walne and Mann, 1975). 

 The decrease in relative organic content in winter pai'tially reflects 

 the utilization of stored reserves during this period and, thus, can be 

 considered a further index of condition: 



% Organic (100-ash) content = Biochemical index (8) 



Physiological Indexes: Ammonia Excretion and the Oxygen-to-Ammonia Ratio 



All the indexes discussed previously are based on quantitative 

 measurements of a potentially utilizable reserve material, and all 

 suffer from inadequacy because they do not account for rates of 

 accumulation and depletion of such material. In this context certain 



