Pickwell 



5 7 2 



where /Hq is said to be generally in the range of 10 to 1 dynes/ cm , 



and R is stated to, equal the radius of a sphere of volume equivalent to 



that of the swimbladder in question. These authors transform equation 



(3) to the approximate form 



f =1.5^^^^^-^ (4) 



r ?j V 



where f^ is in kc/ s, H is the depth in meters, and V, the volume of 

 the gas phase, in mm^. 



An important difference thus appears to arise in computing res- 

 onant frequencies since in equation (2) the radius, R, is the critical 

 dimension while in equation (4) the volume, V, is the critical dimen- 

 sion. Experiments have yet to be conducted to assess the relative 

 importance of these two approaches; particularly to assess the signi- 

 ficance of /io as it relates to resonance by siphonophore floats. 



However, the work of Strasberg''" indicates that the frequency of 

 vibration of a sphere and that of a spheriod of equal volume differs only 

 slightly until the ratio of major-to- minor axes of the spheroid becomes 

 greater than 4. This again suggests that the volume is the most im- 

 portant quantity relating to resonance by gas-bladders as well as by 

 free bubbles. 



Accordingly, both equations (2) and (4) have been employed in 

 Table 1 to determine the theoretical resonant frequency for the small- 

 est and largest pneumatophores measured and for an arbitrarily 

 chosen pneumatophore of "median" dimensions. Equation (2) is also 

 employed to determine fj, for a similar range in voluntarily expelled 

 bubbles. 



The data presented in Figs. 3 and 4 and Table 1 suggest that 

 siphonophores may be expected to contribute to resonant scattering to 

 some degree at any frequency from somewhat less than 10 kc/ s to 

 greater than 100 kc/ s. The contribution from expelled bubbles will 

 probably be amplified during upward migration since expanding gases 

 must be periodically released to avoid rupture of the pneumatophore. 

 However, observations by Jacobs^ ° indicate that intact nanomians 

 may regularly release bubbles and resecrete new gas on an approxi- 

 mately hourly basis. It seems possible that this type of behavior 

 at depth, as well as upward vertical migration, could lead to a con- 

 tinuous, though diffuse, screen of rising bubbles. Some of these 

 would pass through a size critical for resonance at a particular fre- 

 quency as they expanded or dissolved. 



386 



