486 MANN 



investigators since that date. This index is an improvement over the 

 volume-to-volume ratio index in that it accommodates the increase in 

 v^ater content of the soft tissues and the decrease in dry-meat weight 

 which occur on spawning (Masumoto, Masumoto, and Hibino, 1934; 

 Quayle, 1970) and, thus, remains a valid test during periods when 

 spawning is possible. However, Galtsoff (1964) stated that the 

 comparison is valid (for oysters at least) only if specimens of the 

 same species and age are used because oysters from over-crowded 

 natural reefs and young oysters are usually flat, with very httle inner 

 space between the valves. Consequently, this index of condition will 

 be relatively high because the soft tissues occupy almost the entire 

 shell cavity. 



Recently an alternative gravimetric index was used by Walne and 

 Mann (1975) and Mann and Ryther (1977): 



Dry weight of meat (g) X 1000 ^ , „ .... ._. 



fs ^-r-r — n . ,, , . = Index of condition (3) 



Dry weight of shell (g) ^ ' 



This allows both entities used in the calculation to be assayed by 

 the same method and, thus, facilitates the processing of a large 

 number of samples. The method does not account for possible 

 variations in shell-cavity volume caused by changes in overall shape 

 of an individual animal or in shell thickness, but it does incorporate 

 absolute shell size per se. 



Changes in shape of bivalve molluscs have been extensively 

 documented in the scientific literature. Ansell (1965), describing the 

 interrelationship of a number of parameters of growth in mature 

 Yenus (Mercenaha) mercenaria, concluded that, if only the length is 

 known for a given group of animals, the only transformations that 

 can be accurately made are estimates of total weight, total volume, 

 and shell-cavity volume. Thus, although his data indicate isodiametric 

 shell growth estimates of wet and dry flesh weights, total body 

 nitrogen and carbohydrate levels could not be estimated from length 

 data because of changes related to the gametogenic cycle. Ohba 

 (1959) suggested that growth in mature Tapes japonica was not 

 isodiametric since year classes could be separated by significant 

 differences in their breadth-to-length and breadth-to-height ratios. 

 This conclusion was not supported by the data of Nosho and Chew 

 (1972), however, who worked with the same species in Hood Canal, 

 Washington. Seed (1968) documented a number of factors influ- 

 encing shell shape in M. edulis. 



A fourth index of condition was used recently by de Wilde 

 (1975) in temperature and growth studies of Macoma balthica: 



