FISHERY BULLETIN: VOL. 73, NO. 1 



Figure 7. — Transverse section through the regressed swim bladder of Ariomma indica, 59.7 mm SL. 

 Arrow (1) points to regressed rete mirabile, arrow (2) to regressed gas gland. 



complex, multilayered arrangement as in Ariom- 

 ma bondi (Figure 2), or, less frequently, of small 

 cells arranged in circles or loops as in Peprilus 

 triacanthus (Figure 5). The functional significance 

 of cells of either different sizes or arrangements 

 is poorly understood. 



Rete Mirabile 



Retial length in stromateoids ranged from 0.4 to 

 2.0 mm (Table 2), similar to the 0.75 to 2.0-mm 

 range listed by Marshall (1972) for upper 

 mesopelagic (200-600 m) fishes. The ratio of retial 

 length to swim-bladder dimension (length x 

 width) (Table 3) as an approximation of relative 

 development is high in stromateoids and similar 

 to that of Marshall's (1960) deep-sea group which 

 includes some vertical migrators. The stromateoid 

 ratio is much higher than that of other epipelagic 

 fishes and demonstrates that the retia, as with the 

 gas gland, which together form the gas-secreting 

 complex, are relatively well developed in 

 stromateoids. In addition, the total length of the 

 retial capillaries (retial length x number of retial 

 capillaries) in relation to swdm-bladder volume is 

 similar to or exceeds that for the eel, Anguilla 

 anguilla, and certain deep-sea fishes (Table 3). 



Marshall (1972) pointed out that the only flex- 

 ible adaptation to increase the surface available 

 for countercurrent gas exchange is an increase 

 in length of the retial capillaries. An increase in 

 length will not only lead to increased gas ex- 

 change but also slow the rate of bloodflow and so 

 further enhance the efficiency of exchange (Mar- 

 shall 1960). Marshall (1972) showed that the 

 deeper the living space of a fish the greater the 

 absolute length of the retia. On the basis of the 

 pattern of retial length and depth of living de- 

 scribed by Marshall, the predicted depth zone for 

 larval and juvenile stromateoids would be the 

 upper mesopelagial. 



Besides length, the diameter of the retial capil- 

 laries of stromateoids shows a somewhat greater 

 similarity to that of deep-sea fishes than to other 

 epipelagic fishes. Stromateoids have capillary 

 bores of 4-10 iim (Table 2) whereas epipelagic 

 fishes listed by Marshall (1960) had diameters of 

 greater than 10 ^/m. Deep-sea fishes with large 

 erythrocytes have retial capillaries 7-18 jum in 

 diameter and those with small, nonnucleated 

 erythrocytes such as Maurolicus and Vinciguerria 

 have retial capillaries 2-10 /jm in diameter (Mar- 

 shall 1972). The smaller the diameter the greater 

 the efficiency of gaseous exchange (Marshall 



104 



