400 THE BIOLOGY OF MARINE ANIMALS 



of ions, sodium moving into the cell and potassium escaping to the ex- 

 terior. Electric organs, like muscle, contain stores of phosphocreatine. 

 Presumably the phosphagen provides energy for restoring the resting 

 condition of the membrane after activity. 



In Electrophorus (and other teleosts) the electroplate is regarded as a 

 transformed muscle fibre, the innervated surface of which bears a modified 

 muscle membrane. When excited (normally by a chemical transmitter), 

 the surface is depolarized and develops a muscle action potential. In sela- 

 chians, on the other hand, present evidence strongly suggests that the 

 electroplate is a highly developed motor end-plate, the innervated surface 

 of which develops an end-plate potential. This would explain why the 

 organ of Torpedo is not directly excitable, and why excitability is lost after 

 nerve degeneration or curarization. The resting potential in a Torpedo 

 electroplate is about 50 mV. In Raja clavata the peak potential developed 

 by a discharging electroplate is about 60 mV, with little or no reversal of 

 potential across the nervous face at the peak (22a, 40, 66a). 



The nerves to the electric organ are cholinergic. The organs of Torpedo 

 contain appreciable quantities of acetylcholine and cholinesterase. 

 Curare renders the organ inexcitable, and by perfusing with eserinized 

 fluid it has been possible to demonstrate the release of acetylcholine during 

 activity (Torpedo, Narcine, Raja). Injection of acetylcholine into the per- 

 fused organ produces potential changes which vary with the dose em- 

 ployed. These results support the theory that acetylcholine is the transmitter 

 at the junction of nerve and electric organ. 



Electric rays and torpedoes are sluggish in habits. They employ their 

 electric organs for stunning prey, and they also discharge when irritated 

 (p. 244). Freshwater electric fish produce small pulses which are used in 

 direction-finding, but a comparable function has not been established for 

 marine species. Electric rays are immune to their own shocks and to those 

 of other electric fish in their neighbourhood, but it is not known how the 

 animal protects itself and insulates its nervous system against its own 

 electrical discharges (4, 5, 6, 7, 8, 9, 42, 43, 50, 66, 66a, 67, 70, 71, lla, 

 101, 110). 



FLOTATION AND GAS-BLADDERS 



Invertebrates. Animals living in mid-water or surface regions of the ocean 

 counteract gravity in diverse ways. When the densities of their bodies 

 exceed that of sea water, constant locomotory activity is required to 

 maintain height. The data assembled in Table 9.4 show the densities of 

 some marine animals and the loads which various species would carry 

 when maintaining themselves off the bottom. Some of the teleosts in this 

 table have densities equal to sea water; these animals have a hydrostatic 

 organ or swim-bladder. 



Many pelagic invertebrates have densities which are not much different 

 from those of sea water. This condition may be achieved by accumulation 

 of ions of low specific gravity, as in Noctiluca, although the exact mechan- 



