COLBY ET AL.: PRESSURE SENSITIVITY OF ATLANTIC HERRING 



corded its position at 15-s intervals over the 2.5- 

 min period of a given AP and then averaged 

 those 10 values. Thus, unless a larva either over- 

 compensated or fully compensated during the 

 first 15 s, its average position during the 2.5-min 

 test would necessarily not be at the level of com- 

 pensation. 



The vertical and horizontal limits of the appa- 

 ratus also probably impeded vertical progress of 

 larvae in some instances, because the position of 

 the larva within the apparatus at the initiation of 

 a change in pressure determined the potential 

 vertical distance the larva could move to com- 

 pensate; this would be most important as a 

 source of bias at the larger AP's. 



Still another possible explanation for the in- 

 complete nature of the compensation may lie in 

 the artificiality of the experiment. Fish are not 

 normally subjected to abrupt hydrostatic pres- 

 sure changes as they swim with a vertical com- 

 ponent. It is difficult to design an experiment to 

 show a hydrostatic pressure sense in a free swim- 

 ming vertically moving fish larva. Gibson 5 has 

 shown, however, that the activity of juvenile 

 plaice (which lack a swim bladder) varies regu- 

 larly during sinusoidal changes of hydrostatic 

 pressure of amplitudes of about 50 cm H2O re- 

 peated over a 4-h period, thus demonstrating 

 sensitivity to slow changes of pressure in a fish 

 without a swim bladder. 



The site of pressure sensitivity in the herring 

 larvae has not been identified, but it seems to be 

 related to the bulla because sensitivity is en- 

 hanced when the bulla is full of gas. It is possible 

 that abrupt changes of pressure applied to the 

 top of a column of water might generate particle 

 displacements in the water that could be per- 

 ceived by neuromast organs. We do not believe 

 this is a likely explanation of the observed pres- 

 sure sensitivity in stage-I and stage-II larvae, 

 however, because in some experiments the pres- 

 sure change was applied over about 5 s, which 

 reduced any resonant effects in the apparatus, 

 but was equally successful in causing correct re- 

 sponses. 



Because the swim bladder serves as a gas reser- 

 voir for the bulla, the bulla cannot provide per- 

 ception of absolute pressure for a juvenile or an 

 adult herring. However, in the larva the develop- 

 ment of the gas-filled bulla precedes that of the 



5 R. N. Gibson, Principal Scientific Officer, Dunstaffnage 

 Marine Research Laboratory, P.O. Box No. 3, Oban, PA34 

 4AD, Argyll, Scotland, pers. commun. March 1979. 



swim bladder and therefore the bulla may tem- 

 porarily serve as a depth indicator ( Blaxter et al. 

 1981), permitting a larva to limit the maximum 

 depth reached during vertical movements ini- 

 tiated by changes in light intensity. Having a 

 mechanism to limit the maximum depth of verti- 

 cal migration may enable a larva to maintain its 

 position in the water column. This could be of 

 adaptive value similar to that described for an- 

 chovy by Hunter and Sanchez (1976), in that it 

 may serve to keep larvae together and facilitate 

 the development of schooling. A depth indicator 

 might also serve as an energy-saving mechanism 

 if it enables a larva to maintain its position in 

 that portion of the water column where food is 

 most abundant. 



In conclusion, we have found that herring lar- 

 vae display pressure sensitivity both before and 

 after the bulla system has developed, although it 

 is enhanced in larvae with a gas-filled bulla. The 

 threshold of sensitivity was not determined but 

 lies below 13 cm H2O (1 cm Hg). For a herring 

 larva near the sea surface this observation im- 

 plies that pressure sensitivity is <1.3% of the 

 ambient pressure. Prior treatment of larvae to 5 

 atm pressure did not significantly impair sensi- 

 tivity. 



LITERATURE CITED 



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