RESPIRATION 



185 



serves as a high-tension oxygen transport system only when the sand is 

 covered by the sea, and the oxygen tension of water in its burrow is high 



(91). 



In Caudina and Anadara (Area), with corpuscular haemoglobin, 2 

 becomes available only at low tensions (8-10 mm). In two closely related 

 holothurians, one, Cucumaria ebngata, possesses Hb, while another, 

 C. saxicola, lacks it. The former is a mud-dweller, the latter lives among 

 rocks, and the presence or absence of Hb appears to be correlated with 

 availability of oxygen. 



The possession of haemoglobin is exceptional in arthropods. Among 

 entomostracans it is sometimes limited to parasitic species (parasitic 



100 



.0 80 



? 



60 



Arenicola. Hb 



P H7-5,19°C Nephthys Vascular Hb 



pH7 4, 15°C 



£40 

 c: 



10 15 20 



Oxygen Tension (mm Hg) 



Fig. 4.19. Comparison of Oxygen Dissociation Curves of Polychaete 



Haemoglobins, namely Vascular and Coelomic Haemoglobin of Nephthys 



hombergi and Vascular Haemoglobin of Arenicola marina 



The vertical dotted lines above the abscissa show the levels of oxygen tension in the 

 interstitial water from the sand in which Nephthys lives, and in the residual water of 

 exposed Arenicola burrows (from Jones (91).) 



copepods, cirripedes). In the brine shrimp Artemia salina, haemoglobin 

 functions in oxygen transport when the animals are living in concentrated 

 brines (such media have low oxygen content, about one-third that of sea 

 water when saturated with air). Treatment with CO, leading to formation 

 of carboxyhaemoglobin, significantly reduces oxygen consumption 

 (external medium, salinity 195% , 2 2 c.c./L). Artemia is an example of a 

 species that gains or loses haemoglobin in response to low or high oxygen 

 concentrations of the surrounding medium. The haemoglobins of Chirono- 

 mus (tidal-pool and freshwater insects) have remarkably high oxygen 

 affinities (t u <\ mm Hg), and function in oxygen-deficient environments 

 (57, 66, 104). 



