VISUAL OBSERVATIONS IN MIDWATERS 



Bioluminescence 



Beebe^ (1934) using the bathyscaph and Monod* using 

 the French bathyscaph "FNRS- 3 observed bioluminescence 

 down to the greatest depths reached (1400 meters). Monod 

 found that the bioluminescence was much less near the 

 bottom. 



In Project NEKTON I, the bathyscaph descended to 

 the maximum known depth in the ocean (35, 800 feet in the 

 Challenger Deep). During this descent and others made to 

 the bottom at shallower depths, bioluminescence was found 

 to be present at all depths, but was not necessarily contin- 

 uous from surface to bottom. However, the greatest abun- 

 dance of bioluminescence was observed between the base of 

 the sunlit zone (the depth near the surface where the inten- 

 sity of daylight masks out the weak light generated by 

 marine organisms) and 10, 000 feet. 



The common pelagic and bathypelagic sources of bio- 

 luminescence are found among the protozoans, coelenter- 

 ates, ctenophores, euphausiids, decapod crustaceans, 

 salps, and fishes. Both intermittent and steady lumines- 

 cence can be found among these animal groups. 



Observations made by lowering bathyphotometers and 

 by direct visual observations reveal that deep-sea faunas 

 predominantly display intermittent flashing. Until the work 

 of Clarke and Backus^ very little was known about the 

 absolute magnitude of the luminescent flashes of oceanic 

 animals under natural conditions. Relatively few laboratory 

 studies have been conducted on the subject other than those 

 by Nichols'^ (1924) and by Clarke and Backus® (1956). In 

 both laboratory and sea conditions, the intensity of a lum- 

 inescent flash at 50 centimeters was found to be approxi- 

 mately 0. 00005 to 0. 000075 microwatt per cm^ . 



An inherent difficulty in the system employed by Clarke 

 and Backus, or in any cable- lowered photometer, is that it 

 is difficult to determine the distance of the luminous source 

 from the photometer. This problem can be partially re- 

 solved by having bathyscaph observers conduct direct visual 

 observations along with estimates of the intensity. Intensity 

 of a luminescent flash can be qualitatively estimated using 

 the scale employed by astronomers in describing star 



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