184 THE BIOLOGY OF MARINE ANIMALS 



reduces 2 consumption by 50 % at high 2 tensions (6-7 c.c./l.), and stops 

 2 consumption at a level of 3-4 c.c./l. Nereis, therefore, is dependent on 

 Hb to a high degree for oxygen transport under normal conditions 

 (92). 



Urechis caupo is a littoral echiuroid which inhabits a U-shaped burrow 

 through which sea water is circulated. Oxygen capacities lie between 3-7 

 vols %, comparable to those of marine fishes, and oxygen affinity of the 

 haemoglobin is fairly high (t u = 12 mm Hg). Urechis haemoglobin is little 

 affected by changes in pH over the physiological range (pH 6-6-7-5). 

 The critical tension, which in this species is 70 mm Hg (17°C), is probably 

 determined by the dissociation characteristics of the blood (t t — 80-90 

 mm Hg). The haemoglobin of Urechis is almost completely saturated 

 when well-aerated sea water is available, and only becomes operative in 

 oxygen transport under conditions of relative anoxia. When there is a 

 shortage of oxygen, it has been calculated that the oxygen requirements 

 of the animal can be met for about 14 min by oxygen dissolved in the 

 coelomic fluid and water of the hindgut (water-lung), whereas the oxygen- 

 ated blood pigment would permit normal metabolism for another 55 min. 

 This would be of value to the animal during rest periods between pumping, 

 periods which may last as long as an hour. In addition, the burrows of 

 Urechis are exposed during tidal ebb and the oxygen pressure in the sea 

 water in the burrow falls to 14 mm Hg (0-06 vols%). At this pressure the 

 haemoglobin of Urechis is 60% saturated, and functions effectively as an 

 oxygen carrier. 



In Urechis, then, blood Hb subserves 2 transport at low levels (to 

 14 mm Hg), and probably also provides an oxygen store during temporary 

 periods of oxygen lack. Somewhat similar considerations apply to 

 Arenicola marina. The Hb of Arenicola is completely saturated at 7 mm Hg 

 and shows high affinity for oxygen (t u about 2 mm Hg); there is likewise a 

 pronounced Bohr effect (Fig. 4.18). Oxygen stores in the blood are not 

 exhausted, therefore, until the positive pressure of 2 in the tissues falls 

 below t u level. During low tide the water in the animal's burrow contains 

 dissolved 2 at a tension of 13 mm Hg, at which level the Hb is saturated 

 (Fig. 4.19). The blood is depleted of 2 on passing through the tissues, and 

 on returning to the gills the pressure gradient of 2 between the external 

 medium and the venous blood permits the latter to be recharged with 2 . 

 The vascular Hb of Arenicola thus functions as an 2 -transporter during 

 periods of tidal exposure when the 2 supply in its burrow is depleted. 

 A storage function is probably negligible (2, 91). 



Nephthys is a burrowing polychaete with both coelomic and vascular 

 haemoglobins. The oxygen affinity of Nephthys Hb is low compared with 

 that of Arenicola, and the tension of half-saturation is below the oxygen 

 tension of interstitial water in its burrow during tidal ebb (Fig. 4.19). 

 The worm has an 2 -combining potential which would suffice its metabolic 

 needs for only some 10 min, and the Hb is thus inadequate to serve as an 

 oxygen store during exposure. It would seem that the pigment of Nephthys 



