FISHERY BULLETIN: VOL. 80. NO. 2 



which agreed with this study, but their ontogeny 

 is not well known. The importance and function- 

 al nature of the early otolith calcification has not 

 yet been determined. 



Two or three increments were easily visible in 

 the mummichog otolith at the time of hatching. 

 Accurate age determination of field samples 

 could be affected until the number of increments 

 formed at the time of hatching is considered. 

 Brothers et al. (1976) studied increment forma- 

 tion in several fish species and found that the 

 California grunion, Leuresthes tenuis, had two 

 increments at hatching. Some species, such as 

 the northern anchovy, Engraulis mordax, had no 

 increment formation until the time of yolk-sac 

 absorption, 6 d after hatching (Methot and 

 Kramer 1979). Taubert and Coble (1977) found 

 that three species of Lepomis began increment 

 formation at swim up. Scott (1973) studied the 

 otolith structure in larvae of the northern sand 

 lance, Ammodytes dubius, and suggested that 

 otoliths first formed in the postlarvae at a mean 

 total length of 2.4 cm. However, his interpreta- 

 tion was a result of back calculations, not direct 

 observations of otol iths from known age or larval 

 stages of the fish. 



We have found that multiple spherules in the 

 core of the sagitta, followed by numerous incre- 

 ments, are formed prior to hatching in the Asiatic 

 salmon or masou, Oncorhynchus masou; chum 

 salmon, 0. keta; pink salmon, 0. gorbuscha; 

 Arctic char, Salvelinus alpinus; brook trout, S. 

 fontinalis; rainbow trout, Salmo gairdneri; and 

 the sculpin, Cottus nozawa. The juveniles of the 

 live bearing guppy, Lebistes reticulatus, and 

 mosquitofish, Gambusia affinis, form a large 

 number of increments prior to being spawned 

 (Radtke and Dean unpubl. data). Mummichogs, 

 California grunion, and the Atlantic silverside, 

 Menidia menidia, have tidally correlated incu- 

 bation periods of about 10 to 14 d and the salmo- 

 nids incubation period can exceed 50 d. In con- 

 trast, the northern anchovy and spot, Leiostomus 

 xardhurus, have short incubation periods of <2 

 d. This indicates that embryos which have longer 

 incubation periods and large yolk sacs may form 

 several increments before hatching, while em- 

 bryos that have short incubation periods might 

 not start increment formation until hatching or 

 after yolk-sac absorption (Brothers et al. 1976; 

 Methot and Kramer 1979). Much work remains 

 to be done on a range of species before we can 

 attempt to interpret the functional significance 

 of increment formation in embryos. 



The Effect of Light on 



Increment Formation in 



Embryos and Larvae 



The increments observed in otoliths in this and 

 other studies (Pannella 1971; Brothers et al. 

 1976; Struhsaker and Uchiyama 1976; Taubert 

 and Coble 1977; Barkman 1978; Methot and 

 Kramer 1979) appear to be indicators of daily 

 biological events. Rhythmic physiological activi- 

 ties, such as the occurrence of rhythmic mineral 

 deposition in coral (Wells 1963), crayfish gastro- 

 liths (Scudamore 1947), and marine bivalves 

 (Clark 1968; Pannella and MacClintock 1968), 

 are controlled to a large extent by environmental 

 changes synchronized to the diurnal astronomi- 

 cal cycle. 



The only examination of the effect of endogen- 

 ous daily biological rhythms on fish otoliths was 

 by Taubert and Coble (1977), who studied the 

 effect of environmental factors on daily incre- 

 ment formation of Tilapia mossambica larvae 

 hatched in constant light. Their different experi- 

 mental groups all showed increment formation 

 but it was not always daily. They found normal 

 increment formation in all experimental groups 

 with a 24-h periodicity and any other cycle other 

 than 24-h period disrupted increment formation. 

 Since daily cycles are known to occur in blood 

 chemistry of fish (Garcia and Meier 1973), those 

 daily chemical changes could be reflected in the 

 daily increments of the otoliths. Mugiya (1966) 

 found monthly changes in total and diffusible 

 calcium in the endolymph of the semicircular 

 canals of the rainbow trout and the flatfish, 

 Kareius bicolaratus, and he related his finding to 

 the formation of the opaque and translucent 

 zones found in adult otoliths. Daily changes in 

 the calcium metabolism of the fish also occur 

 (Mugiya et al. 1980) which are reflected in the 

 formation of the I and D. 



Daily increments were formed in F. heterocli- 

 tus larvae kept in a L12:D12 cycle, but were 

 absent when the developing embryos were kept 

 in constant darkness (Fig. 5). Light had a defi- 

 nite effect on increment formation, as embryos 

 kept in constant light showed increment forma- 

 tion and otolith diameters that were comparable 

 with the L12:D12 group. An insight into this dis- 

 crepancy was gained in the analysis of the group 

 which initiated increment formation after a 

 light stimulus on day 10 after fertilization. The 

 possibility that light is a synchronizing stimulus 

 at the cellular level was demonstrated by Pitten- 



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