50 Abstract—The jumbo flying squid (Dosidicus gigas) is widely distribut- Geographic variations of jumbo squid (Dosidicus ed in the eastern Pacific Ocean and its population structure is complex. gigas) based on gladius morphology Morphometric analysis of hard struc- tures of cephalopods is frequently Yi Gong1 employed to explore the geographic 1,2,3,4 variation within discreet population Xinjun Chen units. In this study, 8 morphometric Yunkai Li (contact author)1,2,3,4 characteristics of the gladius, or pen, Zhou Fang1 were measured and compared from 562 jumbo squid specimens collected Email address for contact author: [email protected] off Peru, the Costa Rica Dome, and from the equatorial central eastern 1 College of Marine Sciences Pacific. The growth rate of the gla- Shanghai Ocean University dius was evaluated by comparing 999 Huchenghuan Road it with the ages determined from Shanghai, 201306, China statoliths. Results showed signifi- cant differences in gladii morphol- 2 The Key Laboratory of Sustainable Exploitation of Oceanic Fisheries Resources ogy between sexes and geographic Ministry of Education populations. Stepwise discriminant 999 Huchenghuan Road analysis indicated that gladius mor- Shanghai 201306, China phometric characteristics were suit- 3 National Engineering Research Centre for Oceanic Fisheries able discriminatory variables with Shanghai Ocean University an overall correct classification rate 999 Huchenghuan Road of 78.6% for distinguishing different Shanghai 201306, China populations. Higher daily growth 4 National Demonstration Center for Experimental Fisheries Science Education rates were found in the proostracum Shanghai Ocean University than in the conus, possibly because 999 Huchenghuan Road of the different biological functions Shanghai 201306, China of these anatomical parts. These re- sults support our general hypothesis that gladius morphology provides an alternative way to distinguish potential geographic populations of D. gigas and it adds new informa- tion regarding the gladius of the Morphometric analysis has been an from Alaska (60°N) to Chile (46°S) Ommastrephidae. accepted method for studying the and stretching to the west (125°– geographic variation of population 140°W) at the Equator (Nigmatul- units within species of cephalopods lin et al., 2001; Ibáñez and Cubil- (Nesis, 1993; Crespi-Abril et al., los, 2007). As a pelagic cephalopod, 2010; Liu et al., 2015a; van der Vyver D. gigas has an important ecological et al., 2016). Cephalopods show high function in marine ecosystems, not phenotypic plasticity in both soft and only as a voracious predator but also hard structures owing to their bio- as a valuable source of prey (Field et logical characteristics, such as rapid al., 2007; Alegre et al., 2014). It also growth, short life span, and highly supports an important commercial migratory activity (Crespi-Abril et fishery, and in 2014 the annual catch Manuscript submitted 3 March 2017. al., 2010; Arkhipkin et al., 2015; van exceeded 1,000,000 metric tons (FAO, Manuscript accepted 7 November 2017. der Vyver et al., 2016). Therefore, 2016). The main fishing regions are Fish. Bull. 116:50–59 (2018). morphometric analysis may be a use- located off the coasts of Chile and Online publication date: 7 December 2017. ful approach for studying geographic Peru, off the Costa Rica Dome and doi: 10.7755/FB.116.1.5 variations in pelagic cephalopods within the Gulf of California. The views and opinions expressed or that have an extensive distribution Throughout its geographic range, implied in this article are those of the range. the population structure of D. gigas author (or authors) and do not necessarily The jumbo flying squid Dosidi( - is complicated and often debated. reflect the position of the National cus gigas) is widely distributed in In the past, three distinguishable Marine Fisheries Service, NOAA. the eastern Pacific Ocean, extending groups with a different size-at-ma- Gong et al.: Geographic variations in the distribution Dosidicus gigas 51 Eye Head Rostrum Conus Proostracum Figure 1 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). In D. gigas, the gladius, or pen, is a flexible inter- It has been assumed that there is a small group that nal structure that grows from the tail fin toward the occurs in the near-equatorial waters, a medium-size head (Fig. 1). Morphologically, the gladius consists of group that is distributed within the entire distribution a feather-shaped plate, the proostracum that tapers range, and a large-size group that occurs at the north- toward the posterior end of the gladius, becoming a ern and southern peripheries of its distribution range. funnel-shaped conus with a smallish rostrum (Arkhip- However, a high degree of variability in the size of kin et al, 2012). The gladius lies within the shell sac, the groupings was observed along the eastern Pacific which attaches to the fin cartilage in the vicinity of the Ocean (Keyl et al., 2011; Morales-Bojórquez and Pa- conus and rostrum, whereas, the cartilage attaches to checo-Bedoya, 2016); for example, the mantle length the fin muscles (Young and Vecchione, 1996; Arkhipkin at first maturity was found to vary between 31.0 to et al, 2012). The gladius is a metabolically inert tissue 77.0 cm off Mexico (Markaida, 2006). Moreover, little that grows continuously throughout the lifetime of the genetic diversity has been found among these three species. Previous studies have shown that it is useful groups on the basis of microsatellite loci (Sanchez in age studies and can reveal ontogenetic patterns as- et al., 2016). In contrast to this hypothetical spatial sociated with changes in diet and habitat (Perez et al., distribution, other researchers have divided D. gigas 1996; Ruiz-Cooley et al., 2010; Li et al., 2017). None- into northern and southern populations on the basis theless, little attention has been paid to variations in of genetic structure (Sandoval-Castellanos et al., 2007) gladius growth although the differences in age, growth, or elemental signatures in the statolith (Liu et al., and population structure have often been observed 2015b). In this study, not only D. gigas from north- (Sandoval-Castellanos et al., 2007; Chen et al., 2013; ern and southern populations were examined, but also Ibáñez et al., 2016). individuals from equatorial central eastern Pacific. In this study, the morphometric characteristics of For squid whose population structure is uncertain, a the gladius were analyzed. Our goal was to identify holistic management approach should always be con- potential sexual dimorphism and spatial variations of sidered on the assumption that it is a single popula- gladius shape among population units within D. gi- tion. However, if it can be unequivocally shown that gas. This study provides an alternative way to iden- separate stocks are present in the D. gigas population, tify the potential geographic populations of D. gigas stock-specific harvest strategies may be more effective and contributes new information on the gladius of in providing a sustainable biomass. Ommastrephidae. 52 Fishery Bulletin 116(1) Materials and methods Sampling and preparation D. gigas samples were collected during various commercial jigging vessels oper- ating in 2009, 2013, and 2014 in the wa- ters of northern (off the Costa Rica Dome, [CRD]) and southern hemisphere (off the Peru Exclusive Economic Zone, [PE]) and equatorial Pacific Ocean (offshore waters of the central eastern Pacific, [CEP]) (Fig. 2). All squid were frozen on board and then transported to the Key Laboratory of Sus- tainable Exploitation of Oceanic Fisheries Resources at Shanghai Ocean University where they were defrosted under normal room temperature. Dorsal mantle length Pacific Ocean (ML), fin length (FL), and fin width (FW) were recorded to the nearest 1 mm, and body weight (BW) was measured to the nearest 1 g. Sex and maturity stage were Figure 2 determined on the basis of visual evalu- Map of the major surface currents (adapted from Anderson and ation of the gonad morphometric charac- Rodhouse, 2001) and the locations where jumbo squid (Dosidicus teristics defined by Lipin;ski and Underhill gigas) were sampled in 2009, 2013, and 2014 off the Costa Rica (1995). The gladius was extracted from its Dome (CRD). Sampling locations off the CRD are indicated with dorsomedial site in the mantle cavity and open circles, sampling locations off the Peruvian exclusive eco- cleaned in an ultrasonic cleaner with dis- nomic zone (PE), are indicated with black triangles, and those in tilled water. Gladii which appeared to be offshore waters of the central eastern Pacific (CEP) are indicated damaged were eliminated. Statoliths were with black squares. Morphological features were used to distin- extracted for age determination and the guish between populations in different geographic zones number of growth increments were counted under the assumption that the increment is deposited daily (Jackson and Forsythe, 2002). b ⎛ ML ⎞ Y * = Y 0 , (1) i i ⎜ ML ⎟ Morphometric measurements ⎝ i ⎠ where Y = one of the morphometric variables of the A total of 562 gladii from the three regions were used gladius; in the subsequent analyses, and detailed information * Y = the standardized value for the individual I; on the samples can be seen in Table 1.