HORN: SWIM-BLADDER STATE AND STRUCTURE 



Table 4. — Size ranges during which swim-bladder regression occurs in nine species of stromateoids and during which the same species 

 have been observed in association with animate (mainly coelenterate) or inanimate floating objects. Former ranges derived from data of 

 present study and latter from sources listed. 



'SL = standard lengtfi, FL = fork lengtfi. TL = total length. 

 ^MCZ = Museum of Comparative Zoology, Harvard University. 



ZMC = Zoological Museum, Copentiagen. 



BMNH = British Museum (Natural History), London. 



(Capen 1967; Kleckner and Gibbs 1972) and even 

 a relatively small gas-filled sac provides some de- 

 gree of buoyancy which may be significant de- 

 pending upon what other lift or buoyancy devices 

 are utilized. Larval and juvenile stromateoids, the 

 stages which have the organ, have a different 

 mode of life (see below) and are in some species at 

 least probably less dense than adults. Only a 1% 

 reduction in specific gravity of a fish lowers the 

 required percentage volume for neutral buoyancy 

 from 3.1%, the lower value in Alexander's (1966) 

 calculated range (which was based upon specific 

 gravities of adults), to 2.2% (Horn 1970a). Thus, 

 even a small swim bladder would be an important 

 contribution to buoyancy. Data on specific gravi- 

 ties of young stromateoids which might help to ex- 

 plain the range of percentage volumes found with- 

 in the group are not yet available. 



Gas Gland 



The area of the gas gland relative to swim-blad- 

 der volume is similar to that in a number of deep- 

 sea fishes and much greater than that of a series of 

 epipelagic or shallow-sea ones (Table 3). Marshall 

 (1960) stated that the large gas gland of deep-sea 

 fishes, especially vertical migrators, may be an 

 adaptation for rapid gas secretion as the fish de- 



scends. Even though juvenile stromateoids occur 

 only in the epipelagial, the adaptive significance 

 of a large gas gland would be the same for them as 

 for deep vertical migrators since stromateoids 

 range over depths in the upper 100-150 m where 

 pressure changes are greatest (e.g., the pressure 

 at 10 m is 2 X that at the surface). Maintaining 

 association with animate floating objects as many 

 stromateoids do requires that fishes range, even if 

 slowly, over depths in the surface layers and in so 

 doing secrete gas during descents if the hydrostat- 

 ic advantage of the swim bladder is to be effected. 

 Thus, the main selective value of the large gas 

 gland may be for making fine adjustments to 

 buoyancy. At least some of the epipelagic fishes 

 listed in Table 3 have a narrow vertical range near 

 the surface and would not require as large a gas 

 gland. 



The size and structure of the gas gland cells vary 

 widely among stromateoids, a common situation 

 in both shallow- water (Woodland 1911; Fange 

 1953) and deep-sea fishes (Marshall 1960). Cells 

 measured in stromateoids ranged from 6 to 46 /j m 

 (Table 2), although some other cells in a few 

 species appeared to be multinucleate and syncy- 

 tial or similar to the giant cells (50-150 /jm) de- 

 scribed by Fange (1953) and Marshall (1960). The 

 gland consisted of relatively large cells in a 



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