Pickwell 



gelatinous pneumatophores themselves, but also their released bub- 

 bles. The potential significance to resonant sound scattering at var- 

 ious frequencies from these two sources is the subject of this report. 



METHODS 



Siphonophore floats were collected with the NEL Tucker net*' -^^ 

 towed through the DSL at 4 to 5 knots for periods of one-half to 2 hours. 

 Upon removal from the cod-end bucket by means of forceps, the floats 

 were placed in dishes for sorting. Those not used immediately were 

 placed in a refrigerator at 7° C, the approximate temperature at the 

 daytime depth of the DSL. ■'"^ 



Measurements of floats and expelled bubbles were accomplished 

 using an ocular micrometer in a dissecting microscope at 12 to 15X. 

 Volume of floats was calculated as a regular prolate spheroid. Vol- 

 ume of bubbles was calculated as a sphere, hemisphere or cylinder, 

 depending on whether the bubbles were measured freely in the sorting 

 dish, within a glass syringe where they adhered to the sides, or within 

 the tip of an analyzer pipette. 



Close-up photographs of pneumatophores in the act of voluntarily 

 extruding bubbles were obtained with a single-lens, reflex camera 

 equipped with lens extension tubes. Additional photos of bubble expul- 

 sion were obtained with a camera mounted on a compound microscope. 

 All observations and photographs were made at temperatures from 21 

 to 25° C. All work reported in this paper was performed aboard ship 

 with the exception of observations made from the Westinghouse deep 

 submersible vehicle, DEEPSTAR DS-4000. Five dives were made in 

 this vehicle for the primary purpose of observing and photographing 

 physonect siphonophores at close range. ■*"*' ^ ^ Additional observations 

 on their relative abundance within the DSL and at depths above and 

 below it were obtained during slow, controlled descents made with the 

 observation lights on continuously."'"^ Photographs of the siphonophores 

 were obtained with the DEEPSTAR 70-mm still camera and strobe 

 flash. 



RESULTS 



A typical sequence in the expulsion of a single bubble is shown in 

 Fig. 1. The time required for expulsion of an individual bubble from 

 the moment it first appears until it breaks free from the float may be 

 as little as 30 seconds. Bubble expulsion from the pneumatophore is 

 often associated with the appearance of bubbles of freshly-produced 

 gas in the basal gas-gland region of the float, but this is not invari- 

 ably the case.^°' ^^ 



Virtually no muscular contraction has been observed during pro- 

 duction of bubbles, but application of concentrated magnesium sulfate 

 to the sea water serves to "freeze" the extruding bubble in a partially 

 protruding condition as shown in Fig. 2. Once stopped in this manner 



384 



