94 DISCOVERY REPORTS 



Finally the ' frictional resistance ' must be very small in the resorbent areas of these fishes. The 

 capillary bed is very close to the gases of the swimbladder (Text-fig. 40), the intervening connective 

 tissue being no more than 1/1 in thickness (judging from transverse sections). 



Considering only the fishes that are known to cross the near-surface thermocline, the preceding 

 discussion could well indicate that their rate of gas resorption is 20-30 times that of the perch. They 

 have a factor of 2-5 in their favour in resorbent surface volume of swimbladder ratio, and a factor 

 many times this related to the concentration gradient of gas. The unknown factor is the amount of 

 blood the resorbent surface can handle in a given time. However, judging by the ascent of deep- 

 scattering layers (Clarke and Backus, 1956), the time taken to reach the surface (including the slow 

 rise before sunset) is at least 2 hr. 



Text-fig. 39. Venous part of the circulation to the oval in Myctophum punctatum ( x 15). Note the size of the oval veins 

 relative to the retial vein, ov, oval ; ra, retial artery ; rv, retial vein ; rm, rete mirabile ; vov, veins to oval. 



lumen of swimbladder 



Text-fig. 40. Part of the resorbent capillary network of the swimbladder of Argyropelecus aculeatus. Note the very 

 thin layer of tissue separating the capillaries from the lumen of the swimbladder. 



Partial migrators 



Gas secretion 



During their vertical migrations, fishes that do not penetrate beyond the upper thermocline will be 

 subjected to lesser physical stress than those that go on to the surface mixed layer. A hatchet fish 

 migrating from a depth of 150 to 400 m. is not faced with providing as much oxygen for the swim- 

 bladder as a lantern fish moving from 20 to 400 m. However, as we have seen (pp. 74-75) the rete 

 and gas-gland of a hatchet fish swimbladder are highly developed, seemingly as well as those of a 

 lantern fish. In view of the earlier discussion concerning gas-production in thermocline-crossers, 

 this aspect need not be pursued at greater length. 



