INDEXES OF CONDITION IN MARINE BIVALVE MOLLUSCS 485 



examines the variation in a number of previously used indexes of 

 condition in relation to only one environmental stressor, tempera- 

 ture. Furthermore, discussion is limited to temperature ranges 

 comparable to those experienced by the species in their natural 

 habitats. The natural occurrence of a species in one particular 

 seav^^ater temperature range should not be taken to indicate that the 

 species is not luider temperature stress for at least some portion of 

 the year. For example, the southern geographical limit of Mytilus 

 edulis corresponds approximately to a mean summer water tempera- 

 ture of 26. 7° C (Wells and Gray, 1960). When water temperature is 

 held constant, however, this species is unable to acclimate its 

 metabolic and feeding rates to temperatures greater than 20 to 22°C 

 (Bayne, Widdows, and Worrall, 1977). Gonzalez and Yevich (1976) 

 made similar observations for populations of M. edulis in the effluent 

 canal of a steam generating plant. 



INDEXES OF CONDITION 



Volumetric and Gravimetric Meat-to-Shell Ratios 



Grave (1912) suggested expressing the "fatness" of an oyster as 

 the percentage of the shell-cavity volume occupied by meat. This can 

 be directly related to the ratio used by Baird (1958) to measure the 

 condition of mussels and oysters, i.e., 



X 100 = Index of condition (1) 



Shell-cavity volume 



Baird reported that for oysters this index is not affected by the size 

 of the specimen under examination but is size related for mussels. 

 Thus for mussels the index is valid only for comparing animals of the 

 same size (i.e., length). The index should not be used in cases where 

 spawning is in any degree possible, however. In natural populations 

 the spawning period may last for up to 6 months, and in populations 

 exposed to abiotic thermal stimulation it can be even longer. 

 Therefore, the index appears to be of limited value. 



Hopkins (in Higgins, 1938) modified the approach of Grave 

 (1912): 



Dry weight of meat (g) X 100 •, , . a*--.^^ (o\ 



— ^, — ^ — ^^ = — 1^-1^ — ^^ — = Index of condition (z) 



Shell-cavity volume (ml) 



Similar derivations subsequently made by Medcof and Needier 

 (1941) and Korringa (1955) have been used by a large number of 



