168 THE BIOLOGY OF MARINE ANIMALS 



Ventilation affects utilization since the faster the animal ventilates, the 

 shorter will be the period of contact of the water with the gill filaments. 

 With the same oxygen content a greater ventilation volume results in 

 decreased utilization. In the eel utilization falls when the 2 content of the 

 inspired water drops below 2-1-5 c.c./l., but down to a level of about 

 1 c.c./l. the animal still obtains sufficient oxygen for its needs. Increased 

 respiratory activity causes a rise in metabolism, amounting to 40% in the 

 eel, and 70% in the trout; this is due to increased work by the respiratory 

 muscles. In Spheroides, utilization remains approximately the same over a 

 fivefold decrease in tension (at 46% for 2 levels of from 4-7-1 c.c./l.), 

 and is not altered over a temperature range of from 12-22°C (11, 30, 76, 

 100). 



Conclusions 



In slow-moving and sedentary animals oxygen consumption and 

 utilization are low. Oxygen withdrawal from the respiratory current is of 

 the order of 20% or less in sponges, lamellibranchs and tunicates. These 

 are all sedentary filter feeders. The magnitude of ventilation currents in 

 such animals is determined primarily by nutritive requirements, and there 

 is usually a very large margin for respiration. Absolute values show very 

 great variation, depending on a complex of environmental and intrinsic 

 factors. Greater regularity is obtained by expressing ventilation volumes 

 in terms of dry weight or amino-N (Tables 4.5 and 4.6). Oxygen utiliza- 

 tion in worms with muscular pumping mechanisms stands at a higher level 

 than in filter feeders, ranging from 30-75% in different polychaetes and 

 echiuroids. Utilization depends upon the oxygen content of the inspiratory 

 current, the rate and amplitude of pumping, and the metabolic condition 

 of the animal. 



Active cephalopods and fish have high levels of oxygen consumption 

 and utilization. Oxygen lack is generally a more effective respiratory stimu- 

 lus than rise in C0 2 tension. The C0 2 tension of sea water is very low 

 (free C0 2 around 0-23 mm Hg) and relatively constant, and changes in 

 this factor are probably outside the physiological experience of most 

 marine animals. Responses to temporary oxygen deficiency take a variety 

 of forms: increased ventilation (fish, cephalopods, Crustacea, Chaetop- 

 terus), decrease in activity and lowering of metabolism (sponges, various 

 polychaetes, Limulus), utilization of oxygen stores (Arenicola), incurrence 

 of oxygen debt (lamellibranchs) and escape responses (fish). In all aquatic 

 poikilotherms (invertebrates, fish) the ventilation rate is directly propor- 

 tional to the circumambient temperature over normal tolerable ranges 

 (11, 25, 28a, 29, 67, 76, 87, 88, 89, 167). 



RESPIRATION IN DIVING VERTEBRATES 



Three main factors are involved in respiration among air-breathing diving 

 vertebrates, namely accessory aquatic respiration (marine reptiles), 

 adaptation to prolonged submergence in absence of oxygen renewal 



