SWIM-BLADDER STATE AND STRUCTURE IN RELATION TO 

 BEHAVIOR AND MODE OF LIFE IN STROMATEOID FISHES 



Michael H. Horn^ 



ABSTRACT 



Fourteen of the 15 genera of stromateoid fishes possess a relatively small (0.6-3.4% of body volume), 

 euphysoclistous swim bladder which forms early in life (3-5 mm standard length) and regresses in all 

 genera except pwssiblyA'^omeMs before maturity (150-200 mm standard length) is reached. The organ is 

 thus an almost exclusive characteristic of the juveniles which occupy surface layers (upper 100-150 m) 

 in coastal and oceanic waters. 



The gas gland of the swim bladder consists of cuboidal to irregularly shaped cells 6-46 m m in 

 greatest dimension. The retia mirabilia range in length from 0.4 to 2.0 mm and in diameter of 

 individual capillaries from 4 to 10 fi m. The area of the gas gland and the length of the retia relative to 

 the size of the swim-bladder lumen are great compared to the same in other epipelagic fishes and are 

 similar to those of deeper living, mesopelagic fishes. The relatively large gas-secreting complex is 

 considered to be an adaptation for rapid and efficient gas secretion in maintaining hydrostatic 

 equilibrium as the juvenile fishes swim in the surface layers, frequently in association with floating 

 objects, where pressure changes are greatest with depth. 



Swim-bladder loss accompanies changes in behavior and mode of life and is part of the transition 

 from the juvenile to the adult stage of life. Hovering and high maneuverability as principal components 

 of locomotor behavior in juveniles give way to continuous swimming in adults which are generally 

 independent of floating objects and occupy a greater depth range. The relative length of the paired fins 

 changes with age and varies among the species. Peprilus triacanthus and P. simillimus , negatively 

 buoyant, active swimmers, have long pectoral fins as adults whereas Schedophilus medusophagus , a 

 neutrally or nearly neutrally buoyant, slow-moving fish, has short pectoral fins. Both P. simillimus 

 and S. medusophagus have high levels of lipid which may serve to replace the swim bladder in a 

 buoyancy function when the fishes are adults. 



The swim bladder (or gas bladder), a gas-filled 

 organ unique to bony fishes, has its greatest func- 

 tional significance as a hydrostatic device, i.e., one 

 that provides neutral or nearly neutral buoyancy 

 to the fish. It is one of the most plastic of vertebrate 

 organs (Marshall 1960) and occurs in a great di- 

 versity of fishes from a variety of habitats. The 

 swim bladder is not necessary for life as it is ab- 

 sent in many fishes, but according to Fange ( 1 966) 

 about one-half of the 20,000 existing species have 

 it as adults and even more as larvae or juveniles. 

 The organ, owing to its diversity of form and wide- 

 spread occurrence, should reflect in its presence or 

 absence and structure the behavior and mode of 

 life of the fishes possessing it. In stromateoid 

 fishes, the swim bladder regresses in 13, possibly 

 14, of the 15 genera, and the regression seems 

 to be associated with other morphological changes 

 and changes in mode of life (Horn 1970a). 



'Department of Biology, California State University, Fuller- 

 ton, CA 92634. 



Manuscript accepted Meirch 1974. 



FISHERY BULLETIN: VOL. 73, NO. 1, 1975. 



The swim bladder of stromateoid fishes has re- 

 ceived little mention in the literature partly due to 

 its absence or reduced state in adults. Goode and 

 Bean (1895) and Jordan and Evermann (1896) 

 stated that the organ was "usually absent" in the 

 Stromateidae, and the former as well as Grey 

 (1955) reported its absence in the Tetragonuridae. 

 Fowler (1936) in his treatment of several 

 stromateoid genera indicated that the swim blad- 

 der was "present or absent." Goode and Bean 

 (1895) stated that it was present in Nomeus as did 

 Haedrich (1967) for Ariomma. Based upon an ex- 

 amination of approximately one-half of the species 

 in the group, I have found the organ to be present 

 at some stage (larval and/or juvenile) in the life of 

 all stromateoid genera except Pampus . Even in 

 Pampus it may be present at an early stage since 

 larvae or small juveniles (< 20 mm SL, standard 

 length) were not studied. 



The perciform suborder Stromateoidei con- 

 sists of 6 families, 15 genera, and about 60 species 

 (Haedrich and Horn 1972) and is characterized by 

 toothed saccular outgrowths in the gullet and by 



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