swimming speeds up to 4-5 BL/s, these are the only 

 kind of potentials recorded; no larger potentials 

 are observed. Because herring are delicate fish, 

 velocity/endurance experiments in respirometers 

 or water tunnels are likely to underestimate their 

 real capabilities, for it is probable that they slowly 

 deteriorate during their sojourn under experimen- 

 tal conditions. Our limited series of measurements 

 of sustained swimming speeds (Figure 2) showed 

 that juvenile herring were able to maintain speeds 

 around 4 BL/s for periods of at least 5 h, a per- 

 formance about double that previously observed 

 by Boyar ( 19611, but similar to that seen in large 

 circular tanks by Hempel (in Blaxter 1969). 



Boyar's study was very much more extensive 

 than ours; some of his results are plotted in Figure 

 2 for comparison, where it can be seen that the 

 form of the velocityendurance curves we obtained 

 is similar to those found for other fish ( e.g. , Hunter 

 1971 ). It seems probable that 4-5 BL/s represents a 

 sensible upper value for continuous sustained 

 cruising by herring of this size. 



If the speed of flow in the respirometer is in- 

 creased above this speed, or if the fish becomes 

 progressively exhausted, it intersperses periods of 

 steady swimming, as before, (during which it 

 slowly falls back to the downstream electrified 

 grid) with a few rapid tail beats, which drive it 

 upstream, and the cycle is repeated. During these 

 rapid beats (Figure ID), large potentials around 1 

 mV are observed. Similar potentials are seen 

 when the fish is struggling, and there can be no 

 doubt that (as in dogfish. Bone 1966) the electrodes 

 in the red fibre layer pick up these large potentials 

 from the underlying white fibres. White fibres in 



o 



5 - 



tn 4 — 



I 

 t- 



3 - 



o 

 o 

 m 



5 10 



MINUTES 



30 



1 2 5 



HOURS 



FIGL'RE 2. — Swimming speeds (in body lengths per second) plot- 

 ted against the time that the speeds were sustainable (abscissa). 

 Dots: present observations; open circles: data plotted for compar- 

 able fish from Boyar (1961). Note the different forms of the 

 velocity/endurance curves given by the two sets offish, probably 

 the consequence of damage to Boyar's fish in his apparatus. 



herring are similar to those of dogfish in that they 

 are focal ly innervated (Bone 1964) and they must 

 therefore propagate action potentials. The white 

 fibre system in herring was rapidly exhausted, for 

 the fish could not swim at velocities above 5 BL/s 

 for more than 1-2 min (as indicated in Figure 2). 

 Thus there is good accord between our elec- 

 tromyographic observations and the values ob- 

 tained for maximum sustained swimming vel- 

 ocities: in herring only red muscle fibres are 

 employed during sustained cruising. 



Histologically, the red and white fibres are dif- 

 ferent from each other. The red fibres are of more 

 or less uniform diameter, are multiply innervated, 

 and lipid and succinic dehydrogenase (SDH) posi- 

 tive. In contrast, the white zone of the myotome 

 contains both large fibres, and much smaller fibres 

 arrayed around them in a sort of lattice. Both 

 types contain little lipid, are SDH negative, and 

 there are no intermediate fibres either in the 

 juvenile herring which we examined in the res- 

 pirometer, or in adults. These histological ar- 

 rangements are summarized in Figure 3A. 



Carp 



The carp used were much more robust and 

 larger fish than the Pacific herring and it proved 

 possible to make simultaneous recordings of activ- 

 ity within white and red portions of the myotomes. 

 The results obtained were entirely different from 

 those seen in the herring. At speeds between 0.5 

 BL/s (the lowest speed at which the fish would 

 swim reliably) and the maximum speed used, 

 around 4 BL/s, electrical activity was always de- 

 tectable from both sets of electrodes in red and 

 white zones of the myotomes (Figure 4). As speed 

 increases from the lowest values, the bursts of 

 activity from each zone became more synchronous 

 and shorter and their amplitude increased. Occa- 

 sional spikes of greater amplitude were observed 

 from the white muscle zone (Figure 4B), these 

 were faster events than those composing the re- 

 mainder of the motor bursts. When the fish was 

 swimming near the maximum speed sustainable 

 in the respirometer ( Figure 4C ), these rapid poten- 

 tials formed the larger part of the motor bursts and 

 were always seen on both red and white record- 

 ings, though smaller from the former. Presuma- 

 bly, they represent spikelike activity from the 

 white zone of the myotome, picked up (as in her- 

 ring) by electrodes in the red zone. Since the red 

 and white electrodes did not lie in the same 



693 



