558 THE BIOLOGY OF MARINE ANIMALS 



Luminescence due to Symbiotic Bacteria 



It is a peculiar fact that luminescence in certain cephalopods and 

 teleosts is due to symbiotic bacteria which these animals harbour. The 

 bacteria are housed in special sacs or organs, and emit a continuous light. 

 We shall consider the cephalopods first. In certain myopsid decapods, 

 including Loligo, Sepiola, Rondeletia and Euprymnd, light has been ob- 

 served in special glands which lie in the mantle cavity near the ink sac. It 

 appears, however, that not all individuals, even of one sex, produce light, 

 and in Sepiola rondeletii, for example, only some half the animals examined 

 proved to be luminous. 



The organs responsible for luminescence in these squid are special 

 accessory glands associated with the accessory nidamental gland (Fig. 

 13.21). According to the species the luminescent accessory glands are 



^j^^^rmm ^*^\_ Reflector 



Fig. 13.21. Transverse Section through the Light-gland and Associated 

 Structures of the Myopsid Squid Sepiola ligulata. (After Herfurth (31).) 



present in both sexes, or in the females only, and are paired or fused to- 

 gether to form a median organ. The light organs of Sepiola consist of 

 saccular invaginations of the mantle epithelium and open into the mantle 

 cavity through two papillae. Each organ is provided with a reflector and 

 lens-body, and the internal spaces are occupied by photogenic bacteria. 



Several workers have succeeded in isolating and culturing luminescent 

 bacteria from the light organs of these squid, and some have concluded 

 that they are true symbionts. The secretion of the accessory glands covers 

 the eggs with luminescent bacteria, but Herfurth (31) found no evidence 

 that these symbiotic bacteria actually penetrate into the egg. The accessory 

 glands of the embryo are bacteria-free, and it is only after hatching that 

 the glands become infected with bacteria from outside. Infection, conse- 

 quently, is not transmitted through the egg and must be resumed in each 

 generation. 



Another interesting form, Spirula, possesses a luminous organ in the 

 posterior dorsal region of the mantle, between the fins. This organ emits 

 a continuous yellow-green glow, suggesting bacterial light. The cells of the 

 organ contain small, presumably photogenic granules, which do not 



