Matsumoto et al.: Sexual maturation in a captive Rhincodon typus based on observations made over 20 years 
83 
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Figure 5 
Relationships between the ratio of the clasper outer length to pelvic inner length (CLO:P2I) 
and concentrations of (A) testosterone (T) and (B) progesterone (P4) from May 2008 through 
November 2013 for a male whale shark (Rhincodon typus) at the Okinawa Churaumi Aquar¬ 
ium in Japan. 
The age of live elasmobranchs can be estimated 
only by using growth models. Two studies have report¬ 
ed growth curves for the whale shark. Wintner (2000) 
estimated the ages of 15 dead whale sharks by count¬ 
ing the number of growth bands in vertebrae, assum¬ 
ing that a single pair of growth bands corresponds to 
1 year, and generated a growth curve of this species. 
By using that growth curve, the age of our shark at 
capture (at a size of 460 cm TL) was estimated to be 
10 or 14 years old. More recently, Hsu et al. (2014) 
provided new growth curves using a greater sample 
size (h= 92) of species collected from the population 
in the northwestern Pacific Ocean. They estimated 2 
growth curves on the basis of growth bands being de¬ 
posited either once or twice a year, and they conclud¬ 
ed that the band pairs were formed twice a year. The 
growth curve provided by Hsu et al. (2014) is likely 
applicable to estimate the age of our shark at capture 
because the shark was taken from the same region. 
Based on the growth curve of Hsu et al. (2014), the 
age of our shark at the time of capture was estimated 
to be 7.6 years. Consequently, the estimated age at the 
time that clasper elongation ceased was 24.9 years. This 
result indicates that sexual maturation of our shark oc¬ 
curred later than the age at maturity estimated for wild 
individuals in the Indo-Pacific region (17 years old; Hsu 
et al., 2014). Meanwhile, Perry et al. (2018) estimated 
that males in the Maldives mature at 25 years, following 
the methods described by Hsu et al. (2014). This estima¬ 
tion is comparable to our result. The timing of sexual 
maturity of the whale shark may be associated more 
closely with size than with chronological age, a notion 
that is consistent with previous findings (Gelsleichter et 
al., 2002); however, further investigations are needed. 
