FISHERY BULLETIN: VOL. 73, NO. 1 



(Mansueti 1963). Unlike P. triacanthus , S . medu- 

 sophagus becomes neutrally buoyant or nearly so 

 (see below) and has a poorly ossified skeleton and 

 soft musculature (Bone and Brook 1973). Adult 

 S. medusophagus swim slowly and continuously 

 in near anguilliform manner and with only a 

 minor part of the propulsive force provided by 

 the small pectorals (pers. obs.). Because of the 

 fish's low density, little or no lift is required from 

 locomotor activity. 



Changes in the level of buoyancy and in the 

 nature of the buoyancy mechanism may coincide 

 with swim-bladder loss and other changes occur- 

 ring as stromateoids mature although the data are 

 as yet insufficient to permit conclusions to be 

 reached. Peprilus triacanthus and a closely re- 

 lated species, P. simillimus, are negatively 

 buoyant as adults (weight in water 1.4-2.3% of 

 weight in air) (unpubl. data). Juvenile .S. 

 medusophagus (85-200 mm SL) are slightly nega- 

 tively buoyant (Bone and Brook 1973) whereas a 

 larger (285 mm SL) specimen was found to be 

 neutrally buoyant in surface seawater (unpubl. 

 data). Large amounts of lipid have been found in 

 adults of both P. simillimus andS. medusophagus 

 especially in the skull and spine (Lee et al. in 

 press). Bone and Brook (1973) found relatively low 

 amounts of lipid in juvenile S. medusophagus , an 

 indication that lipid content may increase with 

 size in this species. Lipids may serve to partially 

 replace the swim bladder in a buoyancy function 

 as the organ regresses in P. simillimus and S. 

 medusophagus, two morphologically and ecologi- 

 cally dissimilar stromateoids. Peprilus simil- 

 limus, an active, continuous swimmer with long 

 pectoral fins, has a relatively well ossified skele- 

 ton, firm musculature, and is negatively buoyant, 

 whereas S. medusophagus, a slow moving, con- 

 tinuous swimmer with short pectoral fins, has 

 poorly ossified bones and soft, loosely packed 

 muscles, and approaches or attains neutral buoy- 

 ancy. 



Increased lipid content as a buoyancy replace- 

 ment for the swim bladder would be advantageous 

 for P. simillimus, S. medusophagus , and probably 

 other stromateoids that range over the upper sev- 

 eral hundred meters of the water column since the 

 low coefficient of compressibility of lipid compared 

 to gas reduces the stress of pressure changes with 

 depth. Nevenzel et al. (1969) pointed out the ad- 

 vantage of lipid for a vertically migrating mid- 

 water fish, and Butler and Pearcy (1972) discov- 

 ered that in two such species, the myctophids 



Stenobrachius leucopsarus and Diaphus theta, 

 swim bladder-to-body volumes were inversely re- 

 lated to body size and lipid content indicating that 

 lipids assume the primary buoyancy function as 

 the swim bladder regresses vdth age. An addi- 

 tional advantage of stored lipid, especially tri- 

 glycerides, may be as an energy source (Lee et al. 

 in press). Bone (1973) has suggested that verti- 

 cally migrating myctophids can be grouped into 

 functional types based on swim-bladder state, 

 lipid content, density, and size of the pectoral fins. 

 Stromateoids are not classed as a principal group 

 of vertical migrators partly because of their rela- 

 tive rarity, but many species do have a broad ver- 

 tical range. With more data, it may be possible to 

 divide stromateoids into functional groups accord- 

 ing to characteristics similar to those listed by 

 Bone (1973) for myctophids. 



ACKNOWLEDGMENTS 



A number of people have allowed me to examine 

 and in certain cases dissect specimens in their 

 care. My thanks go to N. B. Marshall and Alwyne 

 C. Wheeler, British Museum (Natural History); 

 Richard L. Haedrich, Woods Hole Oceanographic 

 Institution; Julian Badcock, Institute of Oceano- 

 graphic Sciences, England; Elbert H. Ahlstrom 

 and Elaine Sandknop, Southwest Fisheries 

 Center, National Marine Fisheries Service, 

 NOAA; J0rgen Nielsen, Zoological Museum, Uni- 

 versity of Copenhagen; Robert J. Lavenberg, 

 Natural History Museum of Los Angeles County; 

 and Richard H. Rosenblatt, Scripps Institution of 

 Oceanography. 



I sincerely appreciate the efforts of N. B. Mar- 

 shall (now at Queen Mary College, University of 

 London) who provided working space and 

 facilities at the British Museum (Natural His- 

 tory), passed along a great deal of information on 

 swim bladders, and read the manuscript. The re- 

 viewers gave valuable comments for improvement 

 of the manuscript. 



Paula K. McKenzie made the drawing of Fig- 

 ure 1. 



Financial support for this study was provided in 

 part by a NATO postdoctoral fellowship awarded 

 through the National Science Foundation and 

 held at the British Museum (Natural History) and 

 in part by a Sigma Xi Grant-in- Aid of Research 

 and by a Faculty Research Grant awarded by 

 California State University, Fullerton. 



108 



