RESPIRATION 155 



which attends greater activity, for example during periods of active swim- 

 ming as contrasted with periods of rest. Likewise in teleosts swimming 

 activity results in an increase in 2 consumption of from 35-82%. 



Among phyletically related species it is instructive to observe the 

 increased oxygen consumption which attends more vigorous behaviour 

 and habits, for example between active swimming lamellibranchs (Pectini- 

 dae) and sedentary or burrowing forms (Mya, Venus, Ostrea, etc.); or 

 between active pelagic and sluggish fish (mackerel versus puffer fish, for 

 example). These inter-specific differences are also reflected in respiratory 

 rates of isolated tissues, brains of marine teleosts having been used for 

 such comparative experiments. In certain invertebrates it has been found 

 that brackish-water species have higher metabolic rates than strictly 

 marine species, and that transferring individuals of an osmoregulatory 

 species to anisosmotic media increases oxygen consumption. In animals 

 capable of some degree of osmotic regulation, e.g. amphipods, this may 

 be the result of increased osmotic work (cf. Chapter 2). In strictly marine 

 stenohaline animals, however, such as Calanus and Asterias, reduced 

 salinity may depress oxygen consumption (52, 80, 104a, 109, 114, 147, 

 152). 



Diurnal, seasonal and other periodic changes in metabolism are prob- 

 ably widespread. In some animals there are regular diurnal variations in 

 2 consumption, the maxima of which coincide with periods of heightened 

 muscular activity, e.g. sea pen Cavernularia obesa, starfish Astropecten 

 polyacanthus. Persistent endogenous rhythms are well marked in certain 

 littoral animals (oyster drill Urosalpinx, periwinkle Littorina and fiddler 

 crab Uca). The oyster drill shows diurnal cyles of 2 consumption with 

 peaks between 4.30-6.30 a.m. and 7.30-9.30 p.m. (Woods Hole). In addi- 

 tion to diurnal rhythms, there may be persistent tidal and lunar cycles in 

 metabolism, which are maintained for long periods under constant labora- 

 tory conditions. These rhythms, although endogenous in nature, must be 

 preset in phase with external conditions, after which they maintain phasic 

 timing for considerable periods without further reinforcement. We shall 

 refer, in a later section, to the existence of seasonal acclimatization in 

 animals dwelling in temperate regions. In the mussel (Mytilus edulis), which 

 has been studied through an annual cycle, metabolism shows a pronounced 

 fall during spawning (May-June), and a steady rise during the intervening 

 period (July-March) when reproductive reserves are being built up (17, 

 15a, 115, 116, 139). 



Possible endocrinal control of metabolism in lower animals, in a manner 

 analogous to thyroid regulation in mammals, has been a postulate inviting 

 much research. Contrary to expectation, no firm evidence linking thyroid 

 function with respiration has been discovered in fish. Complete thyroidec- 

 tomy in the dogfish Scyliorlunus canicula is without effect on 2 consump- 

 tion, although the existence of alternative sites for the synthesis of thyroid 

 hormone has not been excluded. Administration of thyroid hormone has 

 no influence on respiration in the toadfish (Opsanus) but increases ac- 



