changeover, the Apogon returned to their caves as the Chromis moved up 

 over the reef. 



Clues as to whether the activity patterns were basically endogenous or 

 exogenous were derived from a natural experiment that was arranged by 

 the weather on the morning of October 9. The combination of high 

 waves and a large amount of run off caused by heavy rainfall made the 

 water around the habitat very turbid. Also, the greater amount of 

 cloud cover and the greater surface reflection caused by the high 

 waves may have helped reduce the amount of available light. But both 

 these conditions had existed earlier and we believe that most of the 

 reduced amount of light on October 9 was due to the greatly increased 

 amount of sediment in the water. We were not permitted to leave the 

 habitat on SCUBA or rebreathers because the high waves prevented the 

 safety boat from remaining above us. We were, however, able to 

 observe the changeover on hookah from the habitat. The data from 

 this one morning is highly significant because changeover began 25 

 minutes later than the usual 0555 hours. 



To measure the effects of this storm-caused turbidity on the morning 

 changeover, comparisons were made between the usual time of appear- 

 ance of the first active individual of a species and the time of 

 appearance on October 9. Minimum delay is the difference between the 

 latest time of first appearance and the time on October 9. This 

 ranged from no delay to 55 minutes, mean 17 minutes. Mean delay was 

 calculated by subtracting the mean usual time of first appearance 

 from the time on October 9 for the three nocturnal and 17 diurnal 

 species that were observed that morning. This delay ranged from only 

 seven minutes in the case of Bodianus rufus to 72 minutes for 

 Thalassoma bifasciatum . Overall, the mean delay in changeover was 33 

 minutes for these 17 diurnal species. 



Another measure of the effect of the turbidity was the time it took 

 for the species of Chromis to reach their usual height of 2-3 m over 

 the reef. This process usually took 20-45 minutes, but on October 9 

 it took 55 minutes for C. cyanea and 65 minutes for _C. multilineata , 

 an increase of 15-20 minutes. 



As measured by the appearance of Thalassoma bifasciatum , a late- 

 rising species, changeover was not complete until 0730 hours, an hour 

 and 10 minutes after it began. 



Thus, in several different ways, changeover was delayed and extended 

 on the morning of October 9. We believe this delay was caused by 

 turbidity preventing sufficient light from reaching the fishes to 

 stimulate changeover. This indicates that most of these reef species 

 have a basically exogenous clock that requires the stimulus of light 

 to change from nocturnal to diurnal activities. Several species were 

 not delayed by the turbidity so perhaps they have a primarily endoge- 

 nous clock. Obviously, controlled experiments are necessary to test 

 these preliminary hypotheses. 



VI-259 



