which was implied by their accompanying 

 biochemical studies. Interestingly enough, 

 Johnston et al. ( 1977) observed the same kind of 

 electrical activity from the white zone of the 

 myotome that we observed at low speeds, and it 

 seems therefore extremely probable that such ac- 

 tivity (around 75 fxV in their records at 2.0 BL/s) 

 is indeed generated by muscle fibres in the white 

 zone. They did not observe spikelike activity from 

 the white zone, presumably because their fish 

 were not swimming sufficiently fast, i.e., they in- 

 vestigated only the lower sustained swimming 

 speed range. 



It is then still an open question whether indi- 

 vidual fibres in the white zone can sometimes op- 

 erate producing only local potentials, at other 

 times generating muscle action potentials; or 

 whether there are two different fibre types in the 

 white zone, as yet not distinguishable histochemi- 

 cally. We incline to the former opinion, but to 

 settle the matter evidence from intracellular 

 studies will be essential. 



In rainbow trout, our results were again differ- 

 ent. We obtained no evidence for activity of the 

 mosaic zone of the myotome during sustained ac- 

 tivity even at 4.5 BL/s (the maximum speed at 

 which we could swim the smaller fish). Consider- 

 ing Hudson's (1973) electromyographic evidence 

 from the same species, where he observed activity 

 from the mosaic zone at speeds above 3.0BL/s, this 

 seemed at first rather surprising. 



However, the fish Hudson used came from a 

 stock of notoriously poor swimming performance 

 (see Webb 1971), and it is therefore quite possible 

 that we never attained the critical speed at which 

 the mosaic muscle became active in our fish. The 

 main muscle mass in rainbow trout consists of a 

 mosaic of small reddish fibres scattered amongst 

 larger pale fibres (Johnston et al. (1975) have 

 studied them histochemically), and it is thus un- 

 clear whether the low-level electrical activity which 

 Hudson (1973) recorded from this region (similar 

 to that which we found in carp white muscle) 

 comes from the same fibres as those generating 

 muscle action potentials during burst swimming. 

 In other words, the two kinds of electrical re- 

 sponses from the rainbow trout mosaic muscle 

 may result from the activity of two different kinds 

 of muscle fibres. 



Fish are so diverse, and their patterns of life so 

 varied, that it is hardly surprising that there 

 should be differences on their locomotor muscula- 

 ture. We perhaps ought rather to be surprised at 



698 



the general uniformity of design of the locomotor 

 system imposed by the aquatic medium. It seems 

 probable, from the distribution of patterns of in- 

 nervation amongst different fish groups, and in- 

 deed amongst the teleosts alone, that focally in- 

 nervated, twitch fibres operating by anaerobic 

 glycolysis for short bursts of swimming represent 

 the primitive arrangement of the aquatic fast 

 motor system (see Bone 1970). 



This fast-motor system contrasts markedly with 

 the universally found multiply innervated 

 nontwitch red fibre system for sustained move- 

 ment that operates aerobically. However, his- 

 tological and biochemical investigations of the 

 white myotomal zones of some specialized teleosts 

 such as tuna (Guppy et al. in press) or carp 

 (Johnston et al. 1977) have shown a definite 

 aerobic capacity in the white fibre system, and the 

 original simple dichotomy between anaerobic 

 white fibres and aerobic red fibres rather naively 

 suggested from elasmobranch studies (Bone 1966) 

 is plainly not a good description of the operation of 

 the myotome in all teleosts. 



On the whole, it seems reasonable to assume 

 that in most teleosts where the white portion of the 

 myotome is multiply innervated, there will be 

 aerobic intermediate fibres for use during fast sus- 

 tained cruising, and that at the maximum cruis- 

 ing speed at least some fibres in the white zone of 

 the myotome will also be active aerobically. This 

 seems to be the situation in rainbow trout, and it 

 probably also obtains in most scombrids. 



The situation in carp is less clear. The work of 

 Smitetal. ( 1971) has shown that goldfish (close to 

 carp) are able to sustain high speeds in a res- 

 pirometer apparently using the white muscle 

 system anaerobically. In line with this, Driedzic 

 and Hochachka ( 1975) were unable to detect other 

 energy sources than anaerobic glycolysis in carp 

 white muscle, and Johnston et al. (1977) only 

 found low values of aerobic enzymes in this sys- 

 tem. We have provided clear evidence that the 

 white motor system is operating over a wide speed 

 range, from the lowest speed at which the fish will 

 swim in the respirometer, and it seems bizarre 

 that a relatively inefficient anaerobic metabolism 

 should drive sustained activity. At low sustained 

 swimming speeds carp might keep in overall 

 aerobic balance by transferring lactate from the 

 white zone to other regions of the body, where 

 lactate could be completely metabolized (Bone 

 1975). Driedzic and Hochachka found only low 

 lactate levels in the white zone after severe 



