Timmons and Bray: Age, growth, and sexual maturity of Rhinobatos productus 
353 
diameter, and number of opaque bands further em- 
phasized the strong relation between the three vari- 
ables for males and females combined (opaque bands 
and TL: r=0.92, n= 43, P=0.0001; centrum diameter 
and TL: r=0.99, n=60, P=0.0001; for opaque bands 
and centrum diameter see above). 
Growth zones formed at approximately the same 
time each year, as was evident from examination of 
centra of two captive guitarlish. One of these guitar- 
fish was injected with Terramycin in December 1989 
and in July 1990 and was held in captivity for 13 
months. The first Terramycin mark (closest to the 
focus of the centrum) was found at the peripheral 
edge of an opaque band (Fig. 6A). We do not know if 
the opaque band had formed prior to the injection or 
during the same month as the injection because the 
Terramycin may have diffused into the opaque band 
region. It was clear, however, that the mark was the 
same distance (0.07 cm) from the outer margin of 
the centrum as the peripheral edge of the opaque 
band. From the periphery of the opaque band to the 
outer margin of the centrum, a translucent band was 
present; and the outer margin of the centrum con- 
tained the other Terramycin mark. The predicted 
growth (0.029 cm) of this centrum (with the formula 
in Fig. 5) was lower than the actual growth of 0.07 
cm and shows that individuals probably vary in 
growth, especially in more optimal laboratory condi- 
tions. A second guitarfish was first injected in Octo- 
ber 1989 and again in July 1990. This guitarfish lived 
for 14 months and had completely formed one opaque 
band during this period (Fig. 6B). This opaque band 
was formed after the October injection, and a trans- 
lucent band was present beyond the opaque band to 
the outer margin of the centrum. The outer margin 
of the centrum showed the second injection mark at 
the periphery of the translucent zone at the time of 
death in January 1991. For this guitarfish, the pre- 
dictive equation (Fig. 5) estimated centrum growth 
to be 0.062 cm; it was actually 0.053 cm. In both gui- 
tarfish, opaque band formation occurred sometime 
between the months of October and December fol- 
