SIPHONOPHORES AND THEIR RELEASED BUBBLES 



AS ACOUSTIC TARGETS WITHIN THE DEEP SCATTERING LAYER* 



G. V. Pickwell 

 U. S. Navy Electronics Laboratory 

 San Diego, California 



INTRODUCTION 



In studies on the Deep Scattering Layer (DSL) a central concept, 

 long favored, has been the resonant bubble theory. ■*■' ^ This theory 

 holds that return signals from many scattering layers in the sea are 

 obtained largely from gas-filled cavities resonating in response to the 

 particular sonic frequency being used. Until recently the swimblad- 

 ders of various mesopelagic fishes were thought to be the main, if 

 not the sole, contributors to resonant sound scattering.^'*' ^' ^ 



Since the observations by Barham from various deep submersi- 

 ble vehicles it has become clear that in addition to fishes with swim- 

 bladders, certain types of siphonophores residing at DSL depths also 

 possess gas-filled structures.''' ^'^ These colonial coelenterates, 

 particularly of the suborder, Physonectae, maintain inflated floats, 

 or pneumatophores, at the apex of the colony by production of carbon 

 monoxide gas from specialized gas glands within the pneumatophore.''"'' 



Species of the physonectae have been observed to congregate in 

 comparatively large numbers at the daytime depths of the DSL and to 

 migrate vertically with the DSL on a twice- daily basis. ^' ®' ^ 

 Freshly captured specimens of the pneumatophores of one physonect 

 species, Nanomia bijuga (Delle Chiaje), are obtainable from net 

 hauls through the DSL over the San Diego Trough. These isolated 

 individuals, although broken off from the remainder of the colony, 

 have been observed to manufacture fresh gas''" ■"■ and to voluntarily 

 extrude bubbles of this gas through a pore in the float tip. ^^ 



The possibilities for resonant gas-filled cavities arising from 

 siphonophores in the DSL are thus increased to include not only the 



*The material presented in this paper is extracted from a larger body 

 of data which appears in a forthcoming NEL Research Report (see 

 reference 12). 



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