164 THE BIOLOGY OF MARINE ANIMALS 



regulates down to about 50% saturation (80 mm Hg). Sea anemones do 

 not regulate, but jellyfishes, containing a small proportion of dry matter 

 (Pelagia, Geryonia), show relative independence of external 2 tensions. 

 Diffusion in the latter animals is adequate for the small amount of respiring 

 tissue present. Very active animals, because of high levels of oxygen 

 utilization, are more dependent upon high oxygen tensions (e.g. mackerel 

 compared with angler fish). At low temperatures the activity of poikilo- 

 therms is reduced, and they are able to tolerate lower 2 levels, i.e. critical 

 tensions are lowered with fall in temperature. 



Aquatic invertebrates differ greatly in their tolerance of abnormal 

 levels of 2 over long periods. Thus, Sobella pavonina, a tubicolous poly- 

 chaete, will tolerate 100, 21 and 10% dissolved 2 , but succumbs in 4% 

 dissolved 2 after 4 days. Young Arenicola marina, however, tolerates 

 21, 10 and 4% 2 , but 100% dissolved 2 is slowly toxic. Sabella lives in 

 well-aerated water, whereas Arenicola is a burrowing species that en- 

 counters periodic anoxia (62). 



Relative independence of external oxygen tensions may result from low 

 levels of metabolism, small size, short diffusion distance and so forth. But 

 many animals possess some degree of control over their respiratory 

 mechanisms, and in these forms low 2 tensions may stimulate respiratory 

 centres and evoke increased ventilation movements. Frequently, animals 

 with well-developed circulatory systems possess respiratory pigments 

 which, by virtue of their combining power with oxygen and their saturation 

 characteristics, continue to provide the tissues with adequate oxygen in 

 oxygen-deficient environments and confer relative independence of external 

 oxygen concentrations (11, 15, 30, 32, 109, 114, 150). 



RESPIRATORY RHYTHMS AND REGULATION 

 OF VENTILATION 



The ventilation patterns of aquatic animals show specific peculiarities and 

 vary in accordance with environmental conditions, including alterations in 

 temperature and levels of oxygen and carbon dioxide. These responses can 

 usually be interpreted as functional adaptations to respiratory stress. Since 

 there is much variation in ventilation patterns of different groups the sub- 

 ject can be reviewed conveniently on phyletic lines. Ventilation volumes 

 for various species are presented in Table 4.5, and should be consulted in 

 conjunction with the following account. 



Polychaetes. Tubicolous and burrowing polychaetes show certain 

 characteristic responses to respiratory stress. By an elegant method of 

 recording G. P. Wells has been able to study irrigation by these animals 

 under substantially natural conditions. The lugworm Arenicola regularly 

 pumps water headwards, occasionally tailwards, through its burrow. 

 Irrigation is rhythmical and occurs in bursts with definite pauses between 

 successive outbursts of pumping (Fig. 4.7). This rhythmicity is set by an 

 internal pacemaker located in the ventral nerve cord, and the timing of the 

 irrigation cycle is unaffected by reduction of the oxygen tension of the sea 



