MULTIPLE-FACTOR AND SYNERGISTIC STRESSES 739 



and to the type of food available. The metabolic rate of the isopod 

 Idotea chelipes from Lake Veere, The Netherlands, is influenced by 

 temperature and salinity in such a manner as to suggest the need for 

 energy for osmoregulatory activities (Vlasblom, Graafsma, and 

 Verhoeven, 1977). Dimock and Groves (1976), working with 

 thermal— salinity effects on the respiration rate of the estuarine crab 

 Panopeus herbstii, concluded that these two factors showed signifi- 

 cant interaction. The amphipod Gammarus fossanim showed partial 

 metabolic regulation in waters of decreasing oxygen content. In the 

 microhabitat of this species, slowly running water is preferred, with a 

 maximal temperature of 22° C (Franke, 1977). 



A unique environment in the ocean is the oxygen minimum 

 layer, which may have oxygen levels less than 0.5 ml/liter. The 

 copepod Gaussia princeps exhibits a diurnal vertical migration but 

 may spend days below a depth of 400 m in the oxygen minimum 

 layer. Extending some of his earlier work on G. princeps, Childress 

 (1977) observed the effects of pressure, temperature, and oxygen 

 consumption of this midwater species. At all temperature— pressure 

 combinations, G. princeps displays a very low metabolic rate 

 compared to shallow-living copepods. A higher rate of oxygen 

 consumption was exhibited at the nighttime shallower depths and a 

 much lower rate (partially anaerobic) at deeper daytime depths. 

 Hydrostatic pressure significantly affected oxygen consumption at 

 pressures as low as 28 atm. These responses can be correlated with 

 the ecology of this species. Bivalve molluscs may also be exposed to 

 periods of anoxia. Bayne and Livingstone (1977) reported that 

 Mytilus edulis acclimated with respect to rates of oxygen consump- 

 tion after being maintained at reduced oxygen tensions for more 

 than 5 days. Temperatures of 10 to 22°C did not influence the 

 capacity to acclimate. 



Metabolism is also influenced by other environmental factors. 

 Buckingham and Freed (1976) reported that the metabolic rate of 

 Viviparus contectoides, a snail, is influenced by both pH, with two 

 optimums at 7.1 and 8.9, and temperature. The oxygen consumption 

 of the freshwater snail Indoplanorbis exustus was studied in detail in 

 relation to many factors, such as salinity, temperature, pH, oxygen 

 tension, body size, starvation, exposure to atmospheric air, and 

 diurnal rhythm (Nagabhushanam and Chintawar, 1977). The hemi- 

 chordate Ptychodera flaua responded to a lowered salinity by a 

 sudden rise in respiration rate. With respect to tidal changes, the 

 metabolic responses of P. flava fall into three classes: (1) a rhythm 

 synchronized with tidal phases, (2) a tidal rhythm with additional 



