RADTKK and DKAN: INCREMENT FORMATION OF OTOLITHS OF MUMMICHOO 



15 20 



LENGTH ( MM I 



Figure 7.— A regression plot of the diameters of all of the 24° 

 and 30° Fundulus heteroclitus sagitta plotted against the stan- 

 dard length of the fish. 



mode of 23 mm (Fig. 8a). However, otolith incre- 

 ment-frequency histograms of the sample en- 

 abled us to differentiate cohorts (Fig. 8b). 



A statistical analysis of the data from the field 

 population showed that the relationship between 

 the length of the fish and otolith diameter was 

 linear (y = 0.01 + 0.027a, r = 0.90). The relation- 

 ship of increment number and otolith diameter 

 was curvilinear (y = 0.7601- 0.02 17.r + 0.000rx 2 , 

 r = 0.92). Thus, the diameter of the otolith in- 

 creased as the fish grew; the width of the incre- 

 ment was wider in younger, smaller fish than in 

 older, larger fish; and the number of increments 

 increased as the length of the fish increased. 



When the time of hatching was estimated, 

 using increment counts (Fig. 9a), groups were 

 found that correlated with the occurrence of new 

 and full moons. We observed that the increments 

 tended to be more distinct in larvae collected 

 from the field than in laboratory-reared larvae. 

 When ages were adjusted for the two or three 

 prehatching increments, the relationship was 

 even more obvious (Fig. 9b). Incremental data 

 indicated that the fish collected hatched at the 

 new and full moon spring tides. 



DISCUSSION 



Embryological Formation of 

 Otoliths 



Otoliths (sagittae) are the first calcified tissues 

 to form in developing F. heteroclitus embryos, 

 and although they are prominent and easily 

 observed features that have been presented in 

 numerous developmental studies, their forma- 

 tion is not discussed. Long and Ballard (1976) 

 clearly showed otoliths that formed at stage 20 in 



LENGTH { MM 



jll^ 



INCREMENTS 



Figure 8.— a) A length-frequency histogram of all fish col- 

 lected in the sample, b) A histogram showing the frequency 

 of the increment number of the sagitta of same fish as in 8a. 



JUNE 

 DATE (1977) 



JUNE 

 DATE (19771 



Figure 9.— Fundulus heteroclitus larvae collected on 9 June 

 1977. a) Estimated hatching dates are determined from num- 

 bers of increments in the sagitta; b) estimated hatching times 

 are adjusted for the two increments formed prior to hatching. 

 Also represented are diurnal high tides (upper lines) and low 

 tides (lower lines) and lunar phase (open circles = full moon, 

 closed circles = new moon). 



embryos of the white sucker, and Armstrong and 

 Child (1965) showed otoliths in mummichog em- 

 bryos at stage 23 with calcification at stage 24, 



211 



