.r<^ 



SKIN 



SEPTUM 



VERTEBRA 



KIDNEY 



GAS BLADDER 



RIB 



GONAD 



NTESTINE 

 STOMACH 



W<WW 



...ttT. iTirnnt, "■^°'-'-^'^"^'" cavity 



Figure l. — Gross anatomy of the gas bladder and surrounding region in the black scabbardfish, Aphanopus carbo. Left: Partial 

 sagittal section in region of the 14th-19th vertebrae. Muscle and other tissue have been removed from the region ventral to the 

 indicated septum. Right: Cross section in the region of the 14th vertebra. 



thicker. From that point posteriorly to the first 

 caudal vertebra, all the ribs, except the last two 

 to four, are of the same size, shape, amd relative 

 position. All the ribs extend laterally around the 

 kidneys but the last few extend farther ventrally 

 to encage the enlarged posterior portion of the 

 kidneys. Where the ribs contact the gas bladder 

 laterally, they turn abruptly posteriad and al- 

 most parallel the bladder while remaining in con- 

 tact with it. In doing so, they curl beneath the 

 bladder. Each rib extends posteriorly a distance 

 almost equal to two vertebrae (Figure 1). Each rib 

 appears to join a myocomma, then connect to the 

 ventrolateral wall of the bladder. The gas bladder 

 is thus surrounded by a "rib cage." 



The hypaxial muscles, in conjunction with the 

 ribs, surround the gas bladder almost completely. 

 The only gap is a narrow medial band of connec- 

 tive tissue to which the ribs attach, present be- 

 tween the peritoneal membrane and the gas 

 bladder (Figure 1). 



The unusual anatomy of A. carbo invites specu- 

 lation concerning its significance. The enclosure 

 of the gas bladder in a rib cage apparently rein- 

 forces the gas bladder wall, which Bone (1971) 

 has shown is composed of thick connective tissue. 

 The combination of a thick, tough wall reinforced 

 by muscle and bone seems likely to prevent the 

 expansion of the gas bladder when ambient pres- 



sure decreases more rapidly than the gas con- 

 tained in the bladder can be absorbed into the 

 bloodstream. This species is known to feed on 

 cephalopods (Zilanov and Shepel 1975). Possibly 

 the anatomical modifications of its gas bladder 

 and associated structures allow A. carbo indi- 

 viduals to pursue prey into significantly shal- 

 lower water without having to adjust bouyancy 

 and/or absorb gas. 



Acknowledgments 



For supplying us with specimens, we thank D. 

 Demory, B. Dixon, K. Martinson, and F. Sumner. 

 Robert Rofen kindly verified the identification of 

 P. atlantica. Four specimens of A. carbo collected 

 off Madeira, Portugal, were lent to us by G. E. 

 Maul, Museu Municipal do Funchal. Madeira. 

 The radiographs of A. carbo were gratuitously 

 done by G. Walters, Good Samaritan Hospital, 

 Corvallis. Jim Long and Tom Rasmussen ren- 

 dered various aids. Bonnie Hall drew the illus- 

 trations. Howard Horton and Carl Schreck re- 

 viewed the manuscript. The work of the second 

 author was supported in part by the Office of 

 Naval Research Contract #N00014-67-A-0369- 

 007 under Project NR083-102 and the work of the 

 first and third authors by the Agricultural Exper- 

 iment Station, Oregon State University. 



702 



