INDEXES OF CONDITION IN MARINE BIVALVE MOLLUSCS 487 



Ash-free dry-meat weight X 1000 ^ , 



— — — —rr = Index of condition (4) 



Shell length-* 



Using the weight of ash-free dry meat rather than that of 

 dry meat negates the effects of fluctuations in the ash content of the 

 meat, and using the cube of ihell length again assumes isodiametric 

 growth but neglects any potential changes in shell thickness. 



Indexes of condition (1 to 4) are all basically similar in that they 

 attempt to compare present meat content with its potential 

 maximum size using the shell as a reference material. This compari- 

 son is valid because, although meat content can either increase or 

 decrease, the shell can only increase or remain constant in weight and 

 volume (apart from some very small losses associated with abrasion). 

 All these indexes can be somewhat misleading with respect to meat, 

 however, because only the dry-meat indexes account for fluctuations 

 in water content. None of these indexes give data on the proportion- 

 ate quantities of available metabolic reserves, structural components, 

 and gonadal material in the soft tissues. 



Biochemical Indexes 



Within the seawater temperature range experienced annually by a 

 bivalve mollusc, there is generally a close relationship between the 

 absolute temperature and the cycles of storage and gametogenesis in 

 the animal (Masumoto, Masumoto, and Hibino, 1934; Loosanoff, 

 1936; Chipperfield, 1953; Walne, 1958; Millar, 1963; Porter, 1964; 

 Shaw, 1964; 1965; Holland and Chew, 1974; Dare and Edwards, 

 1975). A brief overview of this relationship is relevant here before we 

 discuss biochemical indexes per se. Obviously species differ in their 

 overall strategies of storage and gametogenesis, as do different 

 geographical populations of the same species (Seed, 1975). The four 

 phases of the complete gametogenic cycle which can usually be 

 identified are: (1) low metabolic activity, often coincident with cold 

 water temperatures in the winter months; (2) high metabolic activity, 

 often coincident with rising water temperatures in spring and with 

 phytoplankton blooms; (3) rapid gametogenesis or completion of 

 gametogenesis, coincident with depletion of carbohydrate reserves 

 and accumulation of lipid in yolky gametogenic products; and 

 (4) low levels of stored reserves. Spawoiing follows phase 3, and the 

 subsequent period of low reserve levels (phase 4) is a result of the 

 reserve's having been used in gonad proliferation and lost on 

 spawning and the high metabolic demands associated with high 

 water temperatures. Decreasing water temperatures in the fall 

 months are often accompanied by some replenishing of reserves and. 



