Gong et al.: Geographic variations in the distribution Dosidicus gigas 
55 
Table 4 
Function coefficients for 3 geographic populations of jumbo squid ( Dosi¬ 
dicus gigas ) sampled off the Costa Rica Dome (CRD), off the Peruvian 
exclusive economic zone (PE), and offshore waters of the central eastern 
Pacific (CEP) in 2009, 2013, and 2014. The morphometric features used 
for the examination are proostracum length (PL), conus length (CL), the 
maximum width of conus (CW), and the maximum width of prostracum 
(PW). 
CR 
PE 
CEP 
All individuals 
PL S 
19.56 
20.17 
23.78 
CL S 
12.02 
18.08 
25.72 
cw s 
31.57 
29.65 
32.15 
pw s 
-37.22 
-31.43 
-37.54 
(constant) 
-392.63 
-365.74 
-441.60 
Females 
PL S 
25.07 
25.63 
30.48 
CL S 
42.88 
47.98 
62.08 
CWg 
40.57 
38.26 
42.58 
PWg 
-26.65 
-18.64 
-25.87 
(constant) 
-532.33 
-498.69 
-629.86 
stored frozen because soft tissues can be deformed dur¬ 
ing the thawing process. In this study, positive correla¬ 
tions were found between the CL with FL and FW in 
all three population units. The conus, which is located 
in the middle of the tail fin, is the main morphological 
section of the gladius (Arkhipkin et al., 2012). With 
their anatomical analysis, Young and Vecchione (1996) 
found that fin cartilage connected the fin muscles and 
shell sac, which secretes the conus and cone flags. 
Therefore, the conus could be considered a supporting 
structure that helps the fin resist the dynamic pres¬ 
sures generated during swimming (Bizikov and Arkhip¬ 
kin, 1997; Arkhipkin et al., 2012). A longer conus might 
also provide appropriate support for the fin when the 
animal is experiencing high speed currents. 
Sexual size dimorphism 
Sexual size dimorphism is a common phenomenon that 
occurs in the hard structures of cephalopods (Fang et 
al., 2014; Liu et al., 2015a). Our results also showed 
that the shapes of gladii were different for the sex¬ 
es within two of the populations (i.e., off the PE and 
within the CEP). These results are in agreement with 
those from Liu et al. (2015a) who observed sexual di¬ 
morphism in another hard structure, the beak of D. 
gigas, from these same two populations which are 
found in the waters off Peru and Ecuador, termed PE 
and CEP, respectively in our study. This difference in 
shapes was despite the fact that both populations ex¬ 
perienced similar environmental conditions and food 
availability. However, sex-specific different nutritional 
requirements have recently been described for female 
and male squid and may explain the sexual dimor¬ 
phism we observed (e.g. Nototodarus gouldi, (Steer 
and Jackson, 2004), Sepioteuthis australis (Peel and 
Moltschaniwskyj, 2006) and Illex argentines (Lin et al., 
2015) ). In these cases, females and males within the 
population show divergent energetic needs for growth 
or reproduction, and one might expect this to result in 
squid with different growth-related parameters, which 
could have a potential impact on the gladius morpho¬ 
metric characteristics (van der Vyver et al., 2016). Ad¬ 
ditionally, there may be sexually related behavioral 
differences where females and males might forage on 
different prey items at different depths in a given habi¬ 
tat (Arkhipkin and Middleton, 2002). 
Geographic variations 
Dosidicus gigas is a species endemic to the eastern 
Pacific and has a complex population structure. Geo¬ 
graphic variations in age, growth, and reproduction of 
this species have often been observed (Sandoval-Cas- 
tellanos et al., 2007; Chen et al., 2013; Ibanez et al., 
2016) . As reported by Liu et al. (2015a), spatial varia¬ 
tions in the beak structure of D. gigas could be used 
to distinguish geographic populations. This difference 
was attributed to a phenotypic response resulting from 
distinct environmental conditions that a population 
experiences at a given geographic location (Liu et al., 
2015a; van der Vyver et al., 2016), but further research 
is needed to confirm the exact relationship between 
a species’ physiological response to variations in the 
environment. 
In this study, the SDA revealed a high rate of cor¬ 
rect classification of gladii shapes among geographic 
populations, even with the female data as an indepen¬ 
dent data set (Table 5B). When looking at the envi¬ 
ronmental conditions these populations experience, 
