elongation appears to be related to diel changes in water 

 availability and could not account for nocturnal growth in 

 submerged seaweeds. 



Proximate factors affecting nocturnal segment production 



Timing of segment production in Halimeda opuntia was 

 sensitive to changes in the timing of the light-dark cycle. 

 Plants that were transplanted from the large microcosm into the 

 small refuge tanks (Fig. 2) and subjected to reversed day/night 

 cycles produced new segments during the dark cycle even though 

 the water in the refuge tank would have had daytime chemical 

 characteristics since it was coming from the lighted microcosm. 

 This indicates that light levels are more important proximate 

 cues than diel changes in water chemistry. However, it takes 

 some time for plants to readjust to changing light cycles. When 

 plants were removed from the microcosm at the midpoint of their 

 normal light cycle, those placed in the dark did not produce new 

 segments in the next 24 hr while those held in constant light 

 produced new segments on their expected schedule even though it 

 was not dark at this time. These investigations need to be 

 expanded but these preliminary data suggest that onset of the 

 dark cycle is the primary proximate cue synchronizing segment 

 formation. If darkness comes too early (after 8 instead of 16 

 hr), no segments are produced. This suggests that some critical 

 amount of stored reserves may need to be accumulated before 

 segment production can be initiated regardless of other cues 

 received by the plant. 



Nocturnal growth of Halimeda could have arisen in response 

 to (1) physiological advantages associated with nocturnal water 

 chemistry, (2) advantages associated with a short term (10-12 hr) 

 escape from day-active herbivores, or (3) some unique and, as 

 yet, unrecognized physiological need of Halimeda . 



Experiments in the microcosm show that timing of segment 

 production responds to changes in the timing of the day-night 

 cycle even if plants in the dark cycle exposed to water 

 chemistry are not directly controlling the timing of segment 

 production. However, it is still possible that diel changes in 

 water chemistry could significantly affect the rate of growth as 

 opposed to the timing at which it occurs. Plants in the 

 microcosm were only recorded as producing or not producing new 

 segments at given time intervals. Potential differences in the 

 rate of production under different types of light cycles and 

 chemical conditions were not investigated. Although we cannot 

 rigorously exclude the possibility that diel changes in water 

 chemistry interact with Halimeda physiology to select for 

 nocturnal growth, we feel that other factors may have been more 

 important selective agents. 



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