566 THE BIOLOGY OF MARINE ANIMALS 



adapted animal are illuminated it is found that luminescence is inhibited 

 only in those exposed areas. The ability to transmit an excitatory wave 

 remains unaffected under these conditions, and the light has no direct 

 affect on the photogenic material. It seems, therefore, that illumination 

 in some way blocks excitability in the region of the photocytes (17, 47). 



The inhibition of luminescence in ctenophores by light has long been 

 known. Specimens of Bolina and Beroe exposed to daylight fail to lumin- 

 esce, but regain the ability to lighten after 15-30 min in the dark. The 

 inhibitory effect of light in these animals acts through two mechanisms, 

 namely, directly on the photogenic substance and also through the sensori- 

 nervous system. An extract obtained from animals exposed to daylight 

 does not luminesce; moreover luminescence in an active extract can be 

 reduced by exposing it to light, a process depending on the duration and 

 intensity of exposure. Illumination, therefore, causes a disappearance of 

 intracellular luminescent material. Although extracts of Mnemiopsis 

 lose their power to luminesce when exposed to strong light, the exposure 

 time required to abolish luminescence in this material is much greater than 

 that necessary to cause an intact Mnemiopsis to lose its luminescent ability. 



Photic inhibition in Mnemiopsis has been studied quantitatively by 

 Moore who found that inhibition of luminescence by illumination obeys 

 the Bunsen-Roscoe law, in which the time of exposure X the intensity of 

 illumination equals a constant K. Values of K for several ctenophores 

 (with tungsten-lamp light source) are: Beroe ovata, 57,285; Mnemiopsis, 

 4,776; Cestum veneris, 1,167 metre-candle min. (Fig. 13.24). The total 

 amount of light impinging on the animal, therefore, is the operating factor, 

 and the effect is photochemical. When half the surface of Mnemiopsis is 

 illuminated, luminescence is suppressed in that region alone. Special 

 nerves from photoreceptors are implicated, which establish purely local 

 connexions with the photogenic cells, and excitation of these pathways 

 effects destruction of the photogenic material. Mechanical stimulation of 

 an animal previously illuminated hastens the recovery of luminescence, 

 and this is interpreted as favouring the reconversion of a decomposition 

 product into the photogenic material (40). 



The scyphomedusan Pelagia noctiluca is one of the few animals which 

 show a diurnal rhythm of luminescence. Tactile stimulation during the day 

 is without effect, but early in the evening the ability to luminesce returns. 

 Exposure of a responsive specimen to strong illumination (62-5-1,000 

 m-c) acts on the nervous mechanism, and the effect follows the Bunsen- 

 Roscoe law. There is some doubt about the stability of these rhythms 

 (41). 



In general it appears that when luminescence is affected by exposure to 

 light, alterations in response are achieved in two ways, either by a direct 

 photochemical effect on the photogenic cells and their contained lumines- 

 cent material; or indirectly, by acting on sensori-neural pathways. Both 

 agencies can operate in the same animal and their relative importance 

 varies with the species. 



