CHAPTER 20 



. Bioluminescence 



I HE ABILITY to producc light is very widely distributed among bacteria, 

 B fungi, and animals/" Among animals, luminous species have been 

 B reported in the major portion of the recognized phyla from the 

 Protozoa through the Chordata. There appears to be little or no general pat- 

 tern in the distribution of the capacity among or within the animal groups; 

 its occurrence is quite sporadic. It may occur in one species of a genus and 

 be absent in another. Practically all of the known luminous species are ma- 

 rine or terrestrial. Manv luminescent marine species are found among the 

 abyssal and planktonic faunas. Luminescent littoral marine species are also 

 numerous. The only luminescent animal occurring in fresh water is an 

 aquatic glowworm. Luminescent species have never been described for cave 

 faunas. 



Luminescence in animals is the result of chemiluminescent reactions in 

 which a substrate is oxidized. The chemiluminescent reaction is usually as- 

 sociated with the presence of special granules in the cytoplasm of the lu- 

 minous tissue. In many organisms the light-producing reactants are expelled to 

 the exterior, where the actual reactions in production of light occur. This 

 type of light production is known ^s extracellular luminescence, in contrast 

 with intracellular luminescence, in which the light-yielding reaction pro- 

 ceeds within cells. In animals with extracellular luminescence the light-pro- 

 ducing organs take the form of unicellular or multicellular glands which se- 

 crete to the exterior. Sometimes a differentiation of two types of secretory 

 cells are observed in multicellular luminous glands. Both appear to con- 

 tribute to light production. 



In higher animals with intracellular luminescence there is a general tend- 

 ency toward an evolution of specialized photogenic organs. Whereas in the 

 protozoans the luminescing granules are dispersed in the cytoplasm, in many 

 higher animals such as cephalopods, insects, and fishes thev are located 

 within light-producing cells, which form only a portion of organs which may 

 possess, in addition to these cells, light-absorbing and light-reflecting layers, 

 light filters, refractive bodies, and nerve supply. Such organs superficially 

 resemble photoreceptors. 



It is by no means always an easv matter to determine whether any par- 

 ticular luminescent animal possesses of itself the ability to generate light. 

 Such luminescence may result from the presence of luminescent bacteria in 

 or on the organism in question. One criterion for distinguishing bacterial 

 luminescence from that originating within animal cells is that in the former 

 the light appears to be continuous, whereas in the latter it is usually pro- 



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