ILLUMINATING THE OCEANS 



These tiny squids (which are among the plankton) are 

 often caught up into surface waters by currents and 

 sometimes cast ashore. It is a squid which produces hght 

 by secretion. A gland near the ink-sac stores the sub- 

 stance in abundance, in a reservoir from which it is 

 extruded by muscular contraction whenever the creature 

 wills it. The animal usually produces no light until dis- 

 turbed, but immediately it is touched it shoots out a 

 stream of mucus, which is known as luciferine. This, as 

 it meets oxygen in the water, creates a chain of brilliant 

 bluish-green points of light — rod-shaped light-particles 

 which glow brightly for some minutes. 



There are at least two other squids which produce 

 light by secretion. Cousteau and Houot were down 3,500 

 feet in the French Navy's bathyscaphe F.N.R.S.3, in 

 1953, when a squid about one and a half feet long 

 appeared in the field of their searchlight. It shot out a 

 blob of what appeared to be white ink, but when the 

 searchlight was switched off the extruded secretion 

 glowed with a phosphorescent light. As the men watched 

 they saw two other squids discharge similar luminescent 

 clouds. 



The third way in which squids produce light is by 

 microscopic organs called photophores — organs which 

 are covered with a layer of chromatophores. These are 

 normally expanded so that they completely cover the 

 photophore, but when they are contracted at the will of 

 the squid then light is emitted. The number of photo- 

 phores possessed by squids varies with the species — some 

 have less than twenty. There is one species, Nemato- 

 lampas regalis, which measures only a few inches across, 

 and has nearly a hundred photophores : five on each 

 eye, ten within its mantle, and seventy on its arms and 

 tentacles. 



One Mediterranean squid has nearly two hundred 

 photophores. Some squids have them only on their eyes 

 and on some of their arm-tips. Others have them over 



263 



