Gong et al.: Geographic variations in the distribution Dosidicus gigas 
51 
ML 
Rostrum Conus Proostracum 
Figure t 
Morphometric measurements of the gladius and mantle recorded for jumbo squid (Dosidicus gigas) sampled 
off the Costa Rica Dome (CRD), off the Peruvian exclusive economic zone (PE), and in offshore waters of 
the central eastern Pacific (CEP) in 2009, 2013, and 2014 and examined in this study (measurements were 
taken on the basis of work by Arkhipkin et al., 2012). The measurements are mantle length (ML), conus 
length (CL), maximum width of conus (CW), proostracum length (PL), maximum width of prostracum (PW), 
length from anterior tip of conus to the widest point of proostracum (PWL), length from anterior tip of co¬ 
nus to proximal end of lateral plates (LPL), width between two anterior apices of lateral plates (LPW), fin 
length (FL), and fin width (FW). Adapted from Lorrain et al. (2011). 
turity have been identified (Nigmatullin et al. 2001). 
It has been assumed that there is a small group that 
occurs in the near-equatorial waters, a medium-size 
group that is distributed within the entire distribution 
range, and a large-size group that occurs at the north¬ 
ern and southern peripheries of its distribution range. 
However, a high degree of variability in the size of 
the groupings was observed along the eastern Pacific 
Ocean (Keyl et al., 2011; Morales-Bojorquez and Pa- 
checo-Bedoya, 2016); for example, the mantle length 
at first maturity was found to vary between 31.0 to 
77.0 cm off Mexico (Markaida, 2006). Moreover, little 
genetic diversity has been found among these three 
groups on the basis of microsatellite loci (Sanchez 
et al., 2016). In contrast to this hypothetical spatial 
distribution, other researchers have divided D. gigas 
into northern and southern populations on the basis 
of genetic structure (Sandoval-Castellanos et al., 2007) 
or elemental signatures in the statolith (Liu et al., 
2015b). In this study, not only D. gigas from north¬ 
ern and southern populations were examined, but also 
individuals from equatorial central eastern Pacific. 
For squid whose population structure is uncertain, a 
holistic management approach should always be con¬ 
sidered on the assumption that it is a single popula¬ 
tion. However, if it can be unequivocally shown that 
separate stocks are present in the D. gigas population, 
stock-specific harvest strategies may be more effective 
in providing a sustainable biomass. 
In D. gigas, the gladius, or pen, is a flexible inter¬ 
nal structure that grows from the tail fin toward the 
head (Fig. 1). Morphologically, the gladius consists of 
a feather-shaped plate, the proostracum that tapers 
toward the posterior end of the gladius, becoming a 
funnel-shaped conus with a smallish rostrum (Arkhip¬ 
kin et al, 2012). The gladius lies within the shell sac, 
which attaches to the fin cartilage in the vicinity of the 
conus and rostrum, whereas, the cartilage attaches to 
the fin muscles (Young and Vecchione, 1996; Arkhipkin 
et al, 2012). The gladius is a metabolically inert tissue 
that grows continuously throughout the lifetime of the 
species. Previous studies have shown that it is useful 
in age studies and can reveal ontogenetic patterns as¬ 
sociated with changes in diet and habitat (Perez et al., 
1996; Ruiz-Cooley et al., 2010; Li et al., 2017). None¬ 
theless, little attention has been paid to variations in 
gladius growth although the differences in age, growth, 
and population structure have often been observed 
(Sandoval-Castellanos et al., 2007; Chen et al., 2013; 
Ibanez et al., 2016). 
In this study, the morphometric characteristics of 
the gladius were analyzed. Our goal was to identify 
potential sexual dimorphism and spatial variations of 
gladius shape among population units within D. gi¬ 
gas. This study provides an alternative way to iden¬ 
tify the potential geographic populations of D. gigas 
and contributes new information on the gladius of 
Ommastrephidae. 
