490 MANN 



physiological parameters can be of value. Oxygen consumption rate 

 can be usefully employed as an index of physiological rate. In bivalve 

 molluscs, however, the respiratory response to temperature stress is 

 complex; both short-term acute and long-term acclimation responses 

 may be involved. These have been extensively described forAf. edulis 

 by Bayne and co-workers (Bayne, 1973; Widdows, 1973; see also a 

 comprehensive review in Bayne, 1976). Furthermore, these responses 

 can be superimposed on short-term fluctuations caused by feeding 

 activity (Bayne, 1973; Bayne and Scullard, 1977b). The nature of 

 such responses may vary somewhat between species, however; e.g., 

 Ansell and Sivadas (1973) stated that Donax uittatus does not 

 exhibit thermal acclimation of its respiratory processes. 



It is evident that the use of respiration data alone can provide 

 some distinct interpretive problems. Its use in conjunction with other 

 physiological parameters, however, can alleviate some of these 

 problems. The cycles of accumulation and depletion of carbohydrate 

 reserves in bivalves and the increasing use of protein as a respiratory 

 substrate after depletion have been briefly discussed earlier. In- 

 creased use of protein as a respiratory substrate inevitably results in 

 an increased rate of ammonia excretion. A combination of oxygen 

 consumption rate (O) and ammonia excretion rate (NH3 ) can be used 

 in the form of an O/NH3 ratio to assess what proportion of the 

 respiratory substrate is protein and, thus, to infer the status of an 

 animal's reserves: 



= Physiological index (9) 



NH 



The O/NH3 ratio was used previously in studies of isopods (Wieser, 

 1972) and the bivalves M. edulis (Bayne, 1973; Bayne and Scullard, 

 1977a) and Donax uittatus (Ansell and Sivadas, 1973). Note that the 

 ammonia excretion rate, not total nitrogen elimination, should be 

 used to evaluate the nature of the substrate being oxidized (see 

 Ansell and Sivadas, 1973). Total nitrogen includes a variable amount 

 of amino acids and urea (Potts, 1967; Hammen, 1968; Allen and 

 Garrett, 1971; Bayne, 1976) which may, in part, be connected with 

 osmoregulatory rather than excretory functions. The ratio of oxygen 

 consumed to total nitrogen excreted has been used in studies of 

 copepods (Conover and Corner, 1968; Corner and Cowey, 1968), 

 prawns (Snow and Williams, 1971), and M. edulis (Bayne and 

 Thompson, 1970). 



A seasonal variation in the O/NH3 ratio occurs in conjunction 

 with the natural gametogenic cycle, but using a ratio rather than the 

 absolute physiological rates can partially eliminate some of the 



