CAMPANA: DAILY GROWTH INCREMENTS IN OTOLITHS OF PLAINKIN MIDSHIPMAN 



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environment. Daily increments are indicated, as is direction of sagittal growth (arrow). T = transfer check. Bar= 30ju.ni. 

 (A) 14L:10D/CTto24L/CT; (B) 24L/14T,:10T, to 24L/CT; (C) 24L/CT to 24L/CT. 



period of 2-4 wk. Therefore, photoperiod acted as a 

 zeitgeber for an endogenous rhythm during the early 

 larval stages, but became unnecessary with increas- 

 ing age. The nondaily increments produced after 

 hatch in this study (and that of Taubert and Coble 

 1977), probably comprised both daily and subdaily 

 increments. The combination resulted in the deposi- 

 tion of more than 1 increment/24 h. 



If a constant photoperiod was present at hatch, an 

 endogenous rhythm of increment deposition became 

 apparent after an acclimation period. Acclimation 

 also occurred when older fish were transferred from a 

 natural light cycle to constant light conditions. 

 However, the pattern of increment production during 

 acclimation differed at the two ages (Table 2). The 

 larval fish acclimation period may be analogous to 

 that of newborn rats transferred from a diel 

 photoperiod to constant conditions. An arhythmic 

 activity pattern continues for almost 2 wk in rats 

 before an endogenous circadian rhythm becomes 

 apparent (Davis 1981). 



The length of the acclimation period could not be 

 determined with accuracy. A shift in increment 

 appearance after transfer from a constant to a cyclic 

 environment generally occurred in 2-5 d. The reverse 

 transfer resulted in almost nonexistent increments 



Table 2. — Age effects on growth increment production in otoliths of 

 plainfin midshipman, Porichthys notatus, reared under three artifi- 

 cial environments. 



Larvae 



Juveniles 



Light important as zeitgeber 

 Daily & subdaily increments 

 similar during acclimation to 24L 



Long acclimation to 24L 

 Immature circadian rhythm 



Light not important as zeitgeber. 



Faint daily increments, but subdaily 

 increments dissimilar during acclima- 

 tion to 24L 



Short acclimation to 24L 



Mature circadian rhythm 



for a period of 5 d, but the visual contrast of the 

 growth patterns improved over the subsequent 10- 

 15 d. Therefore, the critical stage of the adaptation 

 process appears to have been completed in 2-5 d. 

 This result is consistent with that of Tanaka et al. 

 ( 1981), who observed a 6-d transitory period of incre- 

 ment formation when a 24-h light-dark cycle was sud- 

 denly reversed. 



Age-related changes in endogenous circadian 

 rhythms have not been examined in fishes. Mam- 

 malian studies indicate that endogenous rhythms 

 often appear after birth; once present, cycle 

 amplitude tends to increase with time until the 

 rhythm is "mature" (Davis 1981). Porichthys larvae 

 hatched under constant light appear to fit this pat- 

 tern. Daily and subdaily increments were not easily 



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