INCREMENT FORMATION IN THE OTOLITHS OF 



EMBRYOS, LARVAE, AND JUVENILES OF 

 THE MUMMICHOG, FUNDULUS HETEROCHTUS 1 



R. L. Radtke 2 and J. M. Dean 3 



ABSTRACT 



The formation of otoliths and the effect of light cycles on increment formation were studied in 

 embryos, larvae, and juvenile mummichogs, Fundulus heteroditus. We found that increments in the 

 sagitta of mummichogs were a reliable indicator of the daily age of the fish. Calcification of the 

 sagitta was initiated in the core, after matrix formation, at stage 24 of embryological development. 

 The sagitta was the first calcified tissue to develop and there were two or three increments formed 

 before hatching. Daily increment formation in the sagitta was initiated by light and controlled by a 

 24-hour photoperiod. When embryos were subjected to a 24-hour dark or <24-hour (6L:6D) photo- 

 period, daily increment formation was disrupted. Laboratory experiments at24°C and 30°C con- 

 firmed that there was one increment formed each day, which was independent of growth rate and 

 which validated the age of fish in field collections. Wild populations reproduce in the intertidal zone, 

 a physically stressed environment and, judging by the age, which was estimated from incremental 

 data, reproduction is synchronized with tidal cycles. 



Interpretation of increments in the hard tissues 

 of fish has long been utilized as a method to esti- 

 mate age composition of adult populations. Most 

 of the interpretive emphasis has been placed on 

 otoliths and scales. However, the process of age 

 determination isnotasimpleone(Bagenal 1974). 

 Otoliths are especially useful for determining 

 the age of fishes, such as larval forms, which lack 

 scales or have very small ones. 



The otoliths of teleosts consist of deposits of cal- 

 cium carbonate in the form of aragonite (Irie 

 1955; Degens et al. 1969). The morphology of 

 these structures is so specific it can be used as a 

 taxonomic character (Messieh 1972; Hecht 

 1978). Three structures (the sagitta, lapillus, and 

 the asteriscus) are found in the membranous 

 labyrinth of inner ear on each side of the brain 

 cavity (Lowenstein 1971; Popper and Coombs 

 1980). The sagitta is often the largest and is most 

 often used for age determinations and, unless 

 otherwise stated, was the otolith used in the 

 present study. 



■This is contribution 390 of the Belle W. Baruch Institute for 

 Marine Biology and Coastal Research, University of South 

 Carolina, Columbia, SC 29208. 



2 Belle W. Baruch Institute for Marine Biology and Coastal 

 Research and the Department of Biology, University of South 

 Carolina, Columbia, SC; present address: The Pacific Game- 

 fish Foundation, P.O. Box 25115, Honolulu, HI 96825. 



3 Belle W. Baruch Institute for Marine Biology and Coastal 

 Research and the Department of Biology, University of South 

 Carolina, Columbia, SC 96825. 



Manuscript accepted November 1981. 

 FISHERY BULLETIN: VOL. 80, NO. 2, 1982. 



Pannella(1971, 1974) postulated that daily in- 

 crements are found in otoliths of adult fishes, and 

 Brothers et al. (1976) showed that such incre- 

 ments can indeed be found in otoliths of young 

 fishes and be used for age estimation. Struhsaker 

 and Uchiyama (1976) postulated that back calcu- 

 lation of daily increment data from otoliths could 

 be used to age the nehu, a tropical marine fish, 

 and Ralston (1976) obtained similar results with 

 a tropical butterfly fish. Taubert and Coble 

 (1977) did direct age observations of otoliths in 

 juvenile freshwater fish and Barkman (1978) 

 was equally successful with the young of a tem- 

 perate estuarine species Menidia menidia. A 

 more accurate daily journal is available in the 

 otoliths of most young fishes than can be found in 

 their scales, since scales are often absent in the 

 early stages of development (Bagenal 1974), and 

 scale metabolism is dynamic (Yamada and 

 Watabe 1979). 



The discovery of daily increments in otoliths 

 increases the resolution and precision of age de- 

 termination and promises to provide fishery 

 biologists with new levels of information. The 

 deposition of the increments in a rhythmic fash- 

 ion could be a mark of a daily event, and possibly 

 a measure of growth, but the full extent of the 

 influence of external and internal factors on the 

 formation of otolith increments has not been de- 

 termined. 



There is need of knowledge about the age com- 



201 



