LUMINESCENCE IN MARINE ORGANISMS — NICOL 451 



occurs at two levels: illumination affects the excitory mechanism so 

 that the light cells are no longer excited, and it also decomposes the 

 luminescent substrate within the light cells. The amount of illumina- 

 tion necessary to inhibit luminescence is a function of light intensity 

 and duration of exposure, the product of the two being some constant 

 which varies with the species (from about 5X10^ to 60X10^ meter- 

 candle-minutes) . 



Luminescence in dinoflagellates (single-celled plants) is not only 

 inhibited by light, but shows true diurnal rhythmicity, disappearing 

 during the daytime and reappearing at night, even when the cells are 

 kept under constant conditions. Gonyaulax polyedra^ a species that 

 has been studied intensively by Hastings and Sweeney, maintains a 

 constant rhythm of luminescence for 14 days when cells are kept under 

 dim light. The cells, when stimulated, commence flashing in the eve- 

 ning, and the response fades away at dawn. Several factors are 

 responsible for this rhythmicity : some endogenous mechanism within 

 the cell controls the tuning of the rhythm ; periodical changes occur 

 in excitability of the cell ; and diurnal fluctuations take place in in- 

 tracellular levels of luciferin and luciferase, the amounts of which 

 increase at night. 



COLOR AND INTENSITY 



The lights of most marine animals appear blue or blue green to our 

 eyes. Jellyfish, siphonophores, various bristleworms, Pyrosoma, 

 many squids and fish, show blue luminescence. The lights of sea 

 pens, ctenophores or comb-jellies, polynoids, or scaleworms, and cer- 

 tain fish are blue green or green. Only rarely does the luminescence 

 appear yellow or red — for example, in a few squid and fish. One 

 deep-sea species of squid, Thaumatolampas diadema^ has light organs 

 which emit blue and red lights. 



More instructive than color, which is a subjective impression, are 

 relative spectral emission curves, which show the relative energy of 

 light of different wavelengths (fig. 1). These curves are fairly steep 

 and reveal that the emission is restricted to rather narrow spectral 

 regions in the visible region. For blue and blue-green lights, the 

 commonest ones observed, most of the energy is confined to narrow 

 wavelengths between about 4,200 A and 5,400 A. Usually the curves 

 are unimodal (i.e., possessing a single peak) ; occasionally there is a 

 small secondary peak, of unknovni significance, toward longer 

 wavelengths. 



These luminous emission spectra show reasonable similarity to the 

 spectral sensitivity curves of the eyes of marine animals. Eetinal 

 sensitivity curves and action spectra are unimodal, fairly steeply 

 peaked, with maxima in the blue-green or blue regions of the spec- 

 trum. Worms, shrimps, crabs, and squid are most sensitive to blue 



