326 THE BIOLOGY OF MARINE ANIMALS 



isms may be expected to operate in regulating the vertical migrations of 

 some pelagic marine animals (60#, 606). 



Adaptations to Weak and Bright Light. Adaptations of this kind 

 are best known in arthropods and vertebrates. Attention has already been 

 drawn to the migratory pigments found in crustacean and fish eyes, and to 

 the occlusible tapeta of certain nocturnal fishes (p. 314). Apposition eyes 

 of arthropods are adapted to function at high light intensities, superposi- 

 tion eyes at low intensities. On migration of the iris pigments the apposition 

 eye may function as a superposition eye. Retinomotor changes in the 

 fish eye act so as to screen the rods from strong light. An occlusible 

 tapetum is found in some nocturnal fish. The tapetum reflects light, which 

 has already passed through the retina, back upon the sensory cells; in 

 consequence, the sensitivity of the eye to weak light is enhanced, but at 

 the expense of visual acuity. 



The two types of photosensory cells, the rods and cones, of the verte- 

 brate eye function most effectively over different intensity ranges. The rods 

 function at low intensities : when exposed to bright light they become in- 

 sensitive ; on return to darkness they regain sensitivity over a period of an 

 hour in man. The cones are less sensitive and are able to function over a 

 range of higher intensities. 



Diurnal fish possess duplex retinae, containing both rods and cones. 

 Nocturnal fish and those from deep waters possess pure rod retinae. 

 Enhanced sensitivity to weak light is brought about by an increase in the 

 number of rods, and in the number of rods connected to each tertiary 

 neurone. The latter arrangement, allowing greater summation of photo- 

 receptor response, at the same time results in a decrease of visual acuity. 



Dark Adaptation. In general, animals become more sensitive to light 

 after having been in the dark for some time. Curves for two invertebrates, 

 showing recovery of sensitivity to light during the course of dark adapta- 

 tion, may be seen in Fig. 8.16, derived from the work of Hecht. 



In man and other animals with duplex retinae, vision, during the progress 

 of dark adaptation, is taken over by the rods. The spectral distribution of 

 sensitivity for the rods, based on visual purple, is different from that of the 

 cones (vide p. 329). As the eye becomes dark-adapted, the spectral sen- 

 sitivity curve shifts towards lower wave-lengths, the phenomenon known 

 as the Purkinje shift. There are, consequently, two spectral curves, with 

 maxima at 554 m/i and 507 m/u (in man). These give the distribution of 

 spectral sensitivity in the photopic (light-adapted) and scotopic (dark- 

 adapted) eye, respectively. Since light of longer wave-lengths is preferenti- 

 ally absorbed in penetrating sea water, it can be argued that the Purkinje 

 shift has biological meaning when read in terms of the light conditions 

 obtaining in an aquatic environment. 



Spectral Sensitivity. Animals are never equally sensitive to all regions 

 of a spectrum having an equal energy content, and for many reasons it is 

 important to know the spectral sensitivity of their photoreceptors. Deter- 

 minations of this factor have been carried out on many species by several 



