RESPIRATION 175 



Vanadium Chromogens. The presence of surprisingly high levels of 

 vanadium in ascidians has long presented an intriguing problem. Vanadium 

 content varies from 0-04% dry weight in Ciona intestinalis to 0186% in 

 Ascidia mentula; nine-tenths of the vanadium is in the blood. Present in 

 the circulation of certain ascidians are large numbers of cells, some of 

 which are green in colour and contain vanadium chromogen. These cells, 

 known as vanadocytes, make up 1-2% of total blood volume in some 

 species of Ascidiidae. Vanadium chromogens are confined to the order 

 Ascidiacea; they occur in the plasma of Cionidae and Diazonidae; and in 

 vanadocytes in several other families, including the Ascidiidae and Pero- 

 phoridae (Table 4.11). 



In view of the low vanadium content of sea water the ability of ascidians 

 to concentrate such large quantities of this element appears remarkable. 

 Experiments with Ciona and Ascidia using radioactive vanadium suggest 

 that the element may be largely taken up from sea water through the 

 alimentary tract, possibly in colloidal or adsorbed form on mucus sheets. 



Vanadium chromogen (native haemovanadin) contains vanadium and 

 hydrosulphuric acid bound together into a complex (disulphate-vanadium 

 acid), probably linked with protein. Vanadium forms about 10-15% of 

 the haemovanadin obtained from haemolysis of the blood cells. Inside 

 the corpuscles (vanadocytes), the vanadium is kept in reduced form by a 

 remarkably high concentration of H 2 S0 4 , reaching 9% (1-83 n). The 

 physiological characteristics of haemovanadin appear to exclude it as a 

 respiratory pigment: recent studies suggest it may have an oxidation- 

 reduction role in the animal (6, 8, 14, 23, 67, 156). 



The oxygen capacity of blood depends on the amount and nature of the 

 respiratory pigment present. In Table 4.9 information is summarized for 

 haemoglobin concentrations and erythrocyte contents of the blood or 

 body fluids of various animals. These figures show certain trends. There is 

 much variance in haemoglobin concentrations and oxygen capacities of 

 divers (birds and mammals); the erythrocytes of some diving mammals 

 show high oxygen capacities (seals, sea lions, porpoises, Table 4.13). 

 Haemoglobin concentrations are lower in poikilotherms, around 5% in 

 most fish, and less in the few invertebrates examined. Highly active 

 pelagic fish have large oxygen capacities and haemoglobin concentrations 

 (menhaden, scombrids), whereas sluggish bottom forms show low haemo- 

 globin values. A series of deep-sea fish examined proved to have average 

 haemoglobin values (Table 4.12) (22, 33, 141a). 



TRANSPORT OF OXYGEN 



Oxygen Capacity and the Dissociation Curve 



The effective transporting ability of blood is given by its oxygen capacity, 

 which is a measure of the maximal amount of 2 , in volumes per cent, 

 with which the blood will combine. Oxygen capacities of the blood of most 

 invertebrates are rather low, 3-18% in worms, 1-5% in molluscs and 

 1-3 % in arthropods ; levels are higher in fishes, from 5-1 5 %, and very high 



