features involve inherent rates of gas production and the 

 energy cost of resisting high hydrostatic pressures. 



It was recognized that any predictions based on the 

 acoustic behavior of siphonophores would require investiga- 

 tion into their metabolism, the rates of gas secretion and 

 diffusive loss across the pneumatophore walls, and the 

 volumes of gas involved. Results of such studies would 

 permit estimates of the energy equivalent to the physical 

 work required to reinflate the float until it is again an 

 effective sonic reflector. It further becomes possible, 

 from volume studies, to more clearly determine the ability 

 of the float to withstand pressures from within due to ex- 

 panding gases during upward migration, and then to estimate 

 the number of gas bubbles, constituting secondary acoustic 

 targets, which will be voided by the rising siphonophores. 

 The volumes of these extruded bubbles are an indication of 

 their possible resonance at sonar frequencies. 



Subsequent sections will describe a program of 

 investigation into the physiological dynamics of siphono- 

 phores and the resulting data which contribute to under- 

 standing them as sonic scatterers. 



EXPERIMENTAL METHODS AND RESULTS 



The data to be presented here were obtained on 

 cruises aboard USS MARYSVILLE (EPCER 857) in 

 November 1964, and July and August 1965, and one cruise 

 aboard the Scripps Institution of Oceanography R/V T-441 

 in May 196 5, in the region of the San Diego Trough to the 

 west and south of San Diego. 



On the latter trip, a siphonophore not previously 

 encountered by us was taken from depths ranging from 

 approximately 40 to 80 meters in an area over the 

 San Diego Trough about 15 to 20 miles west of San Diego. 

 This species, which also possesses a gas -filled float, has 

 been tentatively identified as Bhtzophysa sp. (figs. 2-4). 

 Data obtained from this organism provide an interesting 

 comparison with N. bijuga, and show similar physiological 

 responses. The specimens were captured whole and rela- 

 tively undamaged, as compared to N. bijuga which invariably 

 fragments when taken in nets. 



