452 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



3000 



4000 5000 



Wave- lengthy 



6000 



7000 



Figure 1. — Comparison of the spectral characteristics of luminescence and visual sensi- 

 tivity of small oceanic shrimplike creatures known as euphausiids. The interrupted line 

 shows the relative amounts of energy in the light emitted by a euphausiid Thysanoessa 

 raschii at different wavelengths. The continuous line is a difference curve for the photo- 

 sensitive pigment in the eye of another euphausiid, Euphausia pacifica. It is based on the 

 difference between the absorption of unbleached and bleached pigments, and is an indi- 

 cation of the spectral composition of the light absorbed by the eye; values for thiscurve 

 are shown to the right. Maximal light emission is at 4750 1; maximal visual sensitivity 

 is around 4600 A. 



light rays, and their spectral sensitivity curves have maxima from 

 4,750 A to 4,900 A. Inshore and surface fishes possess the visual 

 pigment rhodopsin, with an absorption maximum around 5,000 A. 

 In terms of visual stimulation, therefore, the lights of most marine 

 animals are very efficient, since all of their energy content lies in the 

 visual range to which the eyes of marine animals are most sensitive. 

 Figure 1 shows the close similarity existing between the emission spec- 

 trum of the light of euphausiids (small shrimps) and the absorption 

 spectrum of rhodopsm from the same animals. 



Animal lights are really very weak compared with, say, electric 

 lamps. The intensities of some of these lights have now been meas- 

 ured. They range from 1 X 10"® microwatt to 1 X 10'^ microwatt per 

 cm. square of surface receiving the light at a distance of 1 cm. At 

 the lower extreme are the weak flashes of single protozoans. The 

 strongest luminescence is that of pelagic siphonophores, comb-jellies 



