Bioluminescence 673 



indicates that changes in the responsiveness of the excitatory mechanism are 

 probably also operative, at least to some degree, in the normal influence ol 

 light on the photogenic reflex mechanism. The inhibition bv light thus ap- 

 pears to act in two wavs, with the relati\'e importance of each method prob- 

 ably varying with the species. The direct eftect of light on the quantity of 

 photogenic material present may be called a direct eff^ect; and that eff^ect on 

 the excitatorv mechanisms for the photogenic cells or organs, an indirect one. 



The luminescent response of ctenophores has been shown to be readily 

 fatigable and to exhibit recovery in much the same fashion as other typical 

 sensory-neuro-eff^ector mechanisms. 



Functional Significance of Bioluminescence. The signiBcance of light pro- 

 duction in the lives of the numerous organisms possessing this capacity is 

 often by no means obvious. That some survival value is commonly associ- 

 ated with this function is, however, likely, inasmuch as most higher organ- 

 isms which luminesce have evolved specialized light cells or organs for the 

 purpose. Moreover, the flashing itself mav become incorporated into the gen- 

 eral response mechanism of the organism. 



Three general types of functions have been ascribed to animal lumi- 

 nescence. One of these is the luring of food, but as yet there has been no con- 

 clusive evidence for this. A second function ascribed to the organs is that 

 thev serve as protective devices. They may operate through warning or fright- 

 ening predators; by concealing the light-emitting animals, as in the ejec 

 tion of a luminous cloud by the deep-sea shrimp, Acanthephyra, and the 

 squid, Heteroteuthis; or through distracting predators. The pattern of ner- 

 vous control of luminescence in such an annelid as the scale-worm, Acholoe, 

 appears to suggest this last role. When the worm is transected, as by an at- 

 tacking predator, the posterior portion alone luminesces brightly, while 

 luminescence is inhibited in the anterior portion. This behavior could pos- 

 sibly result in a greater chance for the more viable anterior part to escape. 

 Suggestive also of such an emergency protective function of luminescence 

 is the fact that adrenalin induces bright and extensive luminescence in cer- 

 tain fishes. An interesting variation of the protective function of biolumi- 

 nescence in organisms has been advanced, namely that stimulation to lu- 

 minescence of one individual in a group might serve as a warning to its fel- 

 lows and permit their escape. 



The function of luminescent organs which appears to have best observa- 

 tional and experimental support up to the present is that of serving as sig- 

 nals for bringing together the two sexes in mating. The fireworm of Ber- 

 muda, Odontosyllis, provides a well established illustration of this role.^^ 

 The females, which exhibit a marked lunar periodicity, appear at the mating 

 periods at the surface of the sea where they swim in small circles, lumi- 

 nescing brilliantly. Males from deeper water swim directly toward the lumi- 

 nescing females and join in the mating "dance"; both sexes then liberate 

 their gametes into the water together. If, perchance, no male joins a lumi- 

 nescing female in a short time, her light gradually fades, but after a brief 

 period of rest her photogenic organs again become actixe and the luminescent 

 "dance" is repeated. Males moving directlv toward a luminescing female but 

 failing to reach her before the end of a luminous period have been observed 

 to cease their directed movement and wander aimlessly until the female again 



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