RESPIRATION 151 



volves measuring the heat produced (direct calorimetry). In small organ- 

 isms metabolism is usually measured by determining the oxygen con- 

 sumption. 



Oxygen Consumption 



The metabolic rates of different animals show enormous variation, 

 depending on a complex of interacting intrinsic and extrinsic factors. In 

 aerobic species oxygen available to the cells is delimited by the oxygen 

 content of the surrounding medium, and the rate at which it can be sup- 

 plied to the tissues. Anaerobes display specialized metabolic economies, 

 and are treated in some detail by Brand (15). 



Various factors influencing oxygen consumption include: specific 

 metabolic characteristics of the tissues; intrinsic regulatory mechanisms 

 (nervous and hormonal control); age, size, sex, activity, nutrition, season 

 and temperature. The interrelations of these factors are extremely complex, 

 and some of the data are only suggestive. Our theme will be metabolism 

 and levels of oxygen consumption in relation to the marine environment. 



Factors influencing Oxygen Consumption. Specific Differences. As 

 a rough generalization it may be stated that particular rates of oxygen 

 consumption are characteristic of each species. To be strictly comparable, 

 measurements have to be made under uniform conditions, usually with 

 the animal at rest, and at some temperature of reference. Some selected 

 data for oxygen consumption in marine poikilotherms under approxi- 

 mately normal conditions are presented in Table 4.3. Absolute values 

 available often show great variation, even when secured by one investigator 

 on the same species. Although not strictly comparable, owing to the differ- 

 ent temperatures at which the determinations were made and variations 

 in the activity of the animals measured, they give a rough index of oxygen 

 uptake. The figures show no phylogenetic trend but certain conclusions 

 are justified. Very small animals (protozoans, small metazoans) generally 

 have high rates of oxygen consumption. Inactive forms tend to show low 

 metabolic rates — namely sponges, lamellibranchs and ascidians — com- 

 pared with other groups such as decapod crustaceans, cephalopods and 

 teleosts. The former are sedentary ciliary feeders displaying little sustained 

 muscular activity. Data of this kind are amenable to treatment in other 

 ways, which bring out certain fundamental relations. 



When compared on the basis of wet weights, the oxygen consumption 

 of species from widely separated groups shows much disparity. Animals 

 differ greatly in water content and therefore in the amount of active proto- 

 plasm. Transparent pelagic invertebrates containing much water — e.g. 

 medusae, ctenophores, heteropods and salps — have low rates of oxygen 

 consumption, around 5-15% of those of active solid animals such as 

 octopods and bony fish. When expressed in terms of dry weights, meta- 

 bolic rates show greater agreement. Respiratory rates in tissues of a wide 

 variety of marine invertebrates, per unit dry weight, approximate those of 

 vertebrates at the same temperature, according to Robbie (133). 



