tern, but this process cannot be long continued: in 

 the respirometer flow velocities which overload 

 the red system and involve occasional activity 

 from the white system soon exhaust the fish. Pre- 

 sumably this artificial situation, where the fish 

 are forced to swim at such speeds, is not found in 

 nature. 



The taxonomic position of clupeids is not yet 

 agreed upon (see Greenwood etal. 1966), but in the 

 organization of their myotomal motor system they 

 show the primitive pattern of focal innervation of 

 the white fibres (Bone 1970) found also in elas- 

 mobranches, Agnatha, and dipnoi, but in few 

 other teleosts. 



We may surmise that in all fish where the white 

 motor system is innervated in this way, sustained 

 swimming will be the responsibility of the red 

 system alone, as it is in herring and dogfish. It is 

 important to notice that this is not to say that 

 gradation may not take place separately within 

 either system. For example, there are five fibre 

 types in the dogfish myotome (three slow and two 

 fast) distinguishable by histochemical and ultra- 

 structural criteria, and it is entirely reasonable to 

 suppose that the two fast fibre types are recruited 

 for movements of different rapidity as Kry vi and 

 Totland (1977) have suggested. At present, our 

 preliminary ultrastructural and histochemical 

 investigations of young and adult herring 

 myotomal fibres have only shown one type of red 

 fibre and two types of white fibre. The two white 

 fibre types may operate at different stages during 

 rapid swimming, but there is no direct evidence for 

 this assumption, and it may be more reasonable to 

 interpret the smaller white fibres as growth stages 

 in the development of the larger (see Bone in 

 press). 



In carp, the situation during sustained swim- 

 ming at all speeds is entirely different. There is 

 inevitably some ambiguity in the interpretation of 

 electromyographic records since the position of the 

 electrode tip may not be certainly known, and the 

 records obtained may be from nearby small elec- 

 trical events or from distant larger ones, but it 

 certainly does not seem probable that the small 

 events recorded from the carp white muscle at 

 slow sustained swimming speeds can have been 

 picked up from the distant red muscle system. To 

 judge from our records taken deep within the 

 white muscle, as far as possible from the lateral 

 red strip, some fibres within the white zone are 

 active even at the slowest sustained speeds, and 

 this activity increases as the fish increases its 



swimming speed. This kind of electrical activity at 

 the slower sustained speeds is very similar to that 

 of the red motor system, and presumably repre- 

 sents the activity of fibres which are not propagat- 

 ing muscle action potentials. Such records could 

 not, naturally, be obtained from the white system 

 of fish where the white fibres are focally inner- 

 vated, and in fact are not seen in herring or 

 dogfish. At higher sustained speeds, or when the 

 carp is disturbed, much larger rapid potentials are 

 observed from the electrode within the white zone. 

 Plainly, two alternative explanations are possible 

 for the variety of electrical response from a single 

 recording site within the white muscle. Either the 

 electrode tip lies close to fibres of two different 

 types, one of which is capable of propagating mus- 

 cle spikes and the other is not. In this situation, 

 the potentials observed simply reflect the fact that 

 the former system is only activated at higher 

 speeds, the latter operating during slow swim- 

 ming and so resembling the red motor system. In 

 other words, in the carp myotome, the arrange- 

 ment is essentially a mosaic one, in which red 

 fibres are intermingled with the usual fast fibres of 

 the white zone. Or, alternatively, the white zone 

 contains only a single muscle fibre type, which is 

 capable of local contractions not involving muscle 

 action potentials, but can also be stimulated to 

 twitch rapidly and, in this state, propagates mus- 

 cle action potentials. As pointed out earlier (Bone 

 1975) this would be an ingenious way of ensuring 

 for a single muscle fibre that it always operated at 

 the flattened upper part of the power curve, con- 

 tracting at very different rates whilst swimming 

 slowly and rapidly. 



Our electromyographic records do not allow us 

 to distinguish between these two alternatives but 

 there is no evidence from the histochemical 

 studies by Patterson et al. (1975), or the recent 

 excellent paper by Johnston ( 1977), that there are 

 "red" fibres in the white zone of the carp myotome. 

 These authors have demonstrated clearly, how- 

 ever, that there is a zone of intermediate fibres 

 between the lateral red and deep white fibres of 

 the carp myotome. They have also shown that 

 these three fibre types are active at different 

 swimming speeds. At 1 BL/s only red fibres were 

 found to be active; at 1.3-1.5 BL/s both red and 

 pink fibres were active, whereas at 2.0 BL/s and 

 above, electrical activity appeared from the white 

 zone of the myotome. These results clearly indi- 

 cated the sort of recruitment of intermediate fibres 

 at intermediate sustained swimming speeds 



697 



