CAMPANA: DAILY GROWTH INCREMENTS IN OTOLITHS OF PLAINFEM MIDSHIPMAN 



PN = peripheral nucleus. (A) Hatched under a diel light cycle; (B) hatched under a diel temperature cycle; (C) hatched under a constant 

 environment. 



irregular in width (Fig. 5C). In contrast, increments of 

 fish transferred to the constant environment as 

 juveniles were visually faint, becoming more promi- 

 nent after 2-3 wk. Juveniles transferred from a cons- 

 tant environment to a cyclic regime deposited 

 similar-appearing increments before and after 

 transfer. However, posttransfer increments tended 

 to be more regular in width than in constant environ- 

 ment fish; the change generally became apparent 2-4 

 d after transfer. Visual contrast of daily increments 

 may have increased in the fluctuating temperature 

 regime, but the change was not consistent among all 

 otoliths. No such change was evident among the post- 

 transfer increments of fish shifted from 24L/CT to 

 14L:10D/CT, although the incidence of subdaily 

 increments appeared to decrease. Fish transferred 

 from the constant environment to either of the cyclic 

 regimes produced daily increments after transfer; 

 high increment counts (Table 1) were derived from 

 the irregular, pretransfer region of the otolith. 



DISCUSSION 



Daily growth increments were deposited on the 

 otoliths of plainfin midshipman under a variety of 

 environmental conditions. My results indicated that 



light, temperature, age, and an endogenous circadian 

 rhythm may all influence the production and/or 

 appearance of daily and subdaily increments. 

 However, some of the variables tested interacted to a 

 large degree, and their influence on increment pro- 

 duction was subject to alteration through time. 



A cyclic light regime influenced increment produc- 

 tion in larval fish more than any other variable tested. 

 Under a natural photoperiod, daily increments were 

 produced from the time of hatch. In contrast, con- 

 stant light conditions disrupted the production of 

 posthatch increments, resulting in a high incidence of 

 prominent nondaily increments (> 1 increment/24 h) 

 and irregular increment widths. My observations are 

 consistent with those of Taubert and Coble (1977), 

 who observed numerous, nondaily increments in lar- 

 val Tilapia hatched under constant light conditions. 

 Those authors concluded that light acted as a 

 zeitgeber for an endogenous rhythm and that without 

 a cyclic photoperiod, daily increment production was 

 not possible. My results only partially support their 

 conclusion. Photoperiod entrained daily increment 

 production in newly hatched midshipman. However, 

 in the absence of cyclic light or temperature stimuli, 

 an endogenous circadian rhythm of increment 

 deposition became apparent after an acclimation 



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