54 A BIOLOGY OF CRUSTACEA 



also be colourless, but this is not so. Many deep sea Crustacea, 

 particularly the bathypelagic decapods (see p. 131) are a uniform 

 brilliant red in colour; at least that is how they are coloured when 

 brought to the surface. There is the possibility that when they are 

 in the depths they are not so coloured, but maintain the translucent 

 blue that Hip poly te varians assumes at night. Nevertheless, it can be 

 argued that red is the best concealing colour for Crustacea in the 

 middle depths, where red light has all been absorbed. In the dim 

 light of moderate depths a red crustacean would appear black and 

 so be quite inconspicuous. But inconspicuousness does not appear to 

 be the aim of many of these Crustacea; they compensate for their 

 apparent blackness by having luminescent organs. 



Luminescent organs may be divided into two types : those which 

 discharge a luminescent fluid into the water, and those which 

 produce a light in a distinct position on the body of the animal. 

 The two types probably serve different functions. Those which 

 discharge a luminous fluid usually do so when the crustacean is 

 agitated, so that they may serve to produce a luminescent patch 

 which would act as a diversion when the crustacean is attacked. 

 The non-discharging lights may well act as signals to other members 

 of the same species, and serve as part of a mechanism for producing 

 swarms in which the chances of meeting a sexual partner in the 

 breeding season are increased. It has been calculated that the 

 luminescent organs of a Euphausia may be visible to another 

 Euphausia at a distance of a hundred feet, so that if the lights really 

 are a stimulus to swarming they could operate over a wide area. 



Most of the luminescence produced by Crustacea is produced by 

 chemicals made in the bodies of the crustaceans themselves. But 

 there are some luminous bacteria which infect Crustacea and give 

 the whole animal a glow. In Lake Suwa, Japan, there is a fresh- 

 water shrimp, Xiphocaridina compressa which is frequently infected 

 with the bacterium Microspira phosphorenm. On hot summer 

 nights the shrimps swarm at the surface of the lake, and the lumin- 

 escence provided by the bacteria is of such beauty that the shrimp 

 is protected by a government order. Bacteria are also responsible 

 for the known cases of luminosity among isopods and amphipods. 



The luminescence discharged into the water by Crustacea has 

 proved to be most suitable for the study of the mechanism of light 

 production. The ostracod Cypridina hilgendorfi in particular has 

 been subjected to much study, and has been shown to possess one 

 of the simplest systems of light production. Two substances are 



