254 BONY FISHES ix. 7- 



develop appreciable voltages in the surrounding fluid, up to 550 volts 

 in Electrophorus, the electric eel of the Amazon. These voltages are 

 achieved by series summation of the electromotive forces generated 

 by the individual cells. 



The columnar array of several hundreds or thousands of electro- 

 plaques in series in the strongly electric fish, Electrophorus, Malapterurus 

 and Torpedo are paralleled so that the electric organs of these fish can 

 generate considerable current at high voltage. A maximum peak power 

 of up to 600 watts has been observed in T. nobiliana. The electric 

 organs form the major part of the body of the strongly electric fish. 

 The discharges are used for offence and defence. The weakly 

 electric fish (Gymnotidae, Mormyridae) have only a few columns 

 of series arrays, and relatively few electroplaques in each column. 

 However, many of the species emit pulses of low voltages more 

 or less continuously and regularly (60-400/sec). These pulses prob- 

 ably serve as the power components in an electrical guidance 

 system. All species of the continuously emitting fish are sensitive to 

 changes in the conductance of the water. Presumably the fish sense 

 the altered electric field of their discharges; although the receptors 

 have not yet been identified specialized lateral line organs (mormyro- 

 masts) are often present. 



8. Luminous organs 



Fishes of many different families live at great depths and 95 per 

 cent of individuals caught below 100 fathoms are luminescent. The 

 development of luminescent organs is therefore a further example of 

 parallel evolution. The organs usually show as rows of shining beads 

 of various colours on the sides and ventral surface of the fish. 



In many species the light is due to organs containing luminous 

 bacteria, whose appearance may be controlled by the movement of a 

 fold of skin, or of the whole organ, or of chromatophores. Some 

 teleosts, however, have self-luminous photophores and these are also 

 found in Spinax and a few other Squalidae. They are formed from 

 modified mucous glands, and may be provided with reflectors and 

 even lenses. They can be flashed on and off, probably by sympathetic 

 stimulation. 



The luminous organs probably often serve for recognition of the 

 sexes and often show distinctive patterns. They may serve to startle 

 attackers and in a few cases to illuminate the prey. In the deep-sea 

 anglers (Ceratias) the luminous tip of the fin is used as a lure. 



