Genetic structure of Sthenoteuthis oualaniensis population

 in the east Indian Ocean and South China Sea

ZHAO Yan1, WANG Congcong1,2,3,4,5, LIU Bilin1,2,3,4,5, LIN Longshan6, LI Yuan6

(1.College of Marine Sciences, Shanghai Ocean University, Shanghai201306, China; 2. Key Laboratory 

of Sustainable Exploitation of Oceanic Fisheries Resources, Ministry of Education, Shanghai201306, China;

 3. National Distantwater Fisheries Engineering Research Center, Shanghai Ocean University, Shanghai201306,

 China; 4. Key Laboratory of Oceanic Fisheries Exploration, Ministry of Agriculture and Rural Affairs, Shanghai201306, 

China; 5. Scientific Observing and Experimental Station of Oceanic Fishery Resources, Ministry of Agriculture and 

Rural Affairs, Shanghai201306, China; 6. Third Institute of Oceanography, Ministry of 

Natural Resources, Xiamen Fujian361005, China)

Abstract: Purpleback flying squid, Sthenoteuthis oualaniensis, widely distributed in tropical and subtropical waters such as the equator in the Indian and Pacific oceans. The species possesses fast growth rate, strong reproductive capacity and short life cycle, occuping an important position in the marine ecosystems.  Sthenoteuthis oualaniensis is not only the predator of zooplankton, lanternfish and other organisms, but also the food source for many marine predators, therefore, the changes of its resources have a profound impact on the entire ecosystem. At the same time, it has a relatively high resource abundance.  Sthenoteuthis oualaniensis possesses a large resource in the Northwest Indian Ocean and the South China Sea, with high potential commercial value. It is gradually becoming one of the important fishery species. As the basic unit of fishery management, researches on the population structure of Sthenoteuthis oualaniensis are crucial to the rational evaluation and management of fishery resources, and it is also important for the scientific management and rational maintenance of Sthenoteuthis oualaniensis fishery resources and genetic resources reference. Due to the complex current changes in the equatorial waters of the East Indian Ocean, researches on the genetic structure of Sthenoteuthis oualaniensis have not yet been reported. Whether there are genetic differences between groups still needs further researches. Population genetics are helpful in understanding the population structures and genetic variations in this sea area. The South China Sea is an important economic sea area in China, and there are important fishery resources available for exploitation, among which Sthenoteuthis oualaniensis resources are the most abundant species. It should be further developed and utilized. Further understanding of the genetic structure of Sthenoteuthis oualaniensis population in the South China Sea is extremely important for its’ sustainable development and utilization in the South China Sea. Whether there are genetic differences between Sthenoteuthis oualaniensis of the East Indian Ocean equatorial waters and the South China Sea, and whether genetic differences are significant also require further study. Therefore, the mitochondrial cytochrome b gene (Cytb) marker was selected in this paper to analyze the genetic structures, genetic differences, genetic diversities, and population historical dynamics among four Sthenoteuthis oualaniensis populations in the East Indian Ocean and South China Sea. This would provide scientific basis for the management and utilization of Sthenoteuthis oualaniensis fishery resources, as well as the protection of genetic diversities.

This research was conducted to understand the population genetic structure of Sthenoteuthis oualaniensis from the East Indian Ocean and the South China Sea, and to rationally manage the resources of squid fishery. The cytochrome b (Cytb) sequences of 120 mitochondrial DNA from four subpopulations of Sthenoteuthis oualaniensis were measured. These samples were collected from the north and south of the East Indian Ocean and the South China Sea. Cytb gene marker primers were used for the PCR amplification and sequencing, and a total of 120 Sthenoteuthis oualaniensis samples were genetically analyzed in four populations of the eastern Indian Ocean in the northern equator, southern and northern South China Sea. The results showed that the total diversities of haplotype and nucleotide were 0.273 32 and 0.981, respectively. The highest haplotype diversity of the group from the north of the South China Sea was 0.989, while that of the group from the south of the South China Sea was only 0.306 60. The results of the haplotype network diagram and phylogenetic tree showed that the genetic differentiation between populations of the East Indian Ocean and the South China Sea was significant. The analysis result of molecule variance indicated that 54.90% of population variation came from different groups, and the genetic differentiation was significant (P<0.01). The genetic variation rate within the population was 45.10%, and the genetic variation caused by geographical isolation was slightly higher than the variation of individuals within the population.The analysis of Fst value showed that Fst value between two groups from the north and south of the East Indian Ocean was 0.085 88, and Fst value between two groups from the north and south of the South China Sea was 0.025 36. These results indicated that the indistinctive genetic differentiation existed between two groups from north and south of the South China Sea, while the differences of the genetic differentiation between two groups from north and south of the East Indian Ocean were small. The frequent ocean activities of Sthenoteuthis oualaniensis in both sides of the East Indian Ocean were the main factor leading to the genetic differentiation in the north and south of the East Indian Ocean. While the geographical isolations between the East Indian Ocean and the South China Sea might be the main reason causing the prominent genetic differentiation between two geographical groups. The neutrality test results of the population in the northern Eastern Indian Ocean showed a very significant negative value (Fs=-26.568 09, P<0.01), and the nucleotide unpaired distribution map showed a significant undirected peak, which speculated that the population might experience a historical population expansion event . In conclusion, the two groups in the East Indian Ocean and the South China Sea were independent, therefore, they should be regarded as two independent units.

Through the study of Cytb gene of 120 individuals in the north and south equatorial waters of the East Indian Ocean, the southern and northern waters of the South China Sea, this study revealed that there was a significant genetic differentiation between the East Indian Ocean equatorial sea area and the South China Sea area, which should be regarded as two independent groups in fishery management. The populations in both sides of the equator in the East Indian Ocean has moderate genetic differentiation, and the addition of Sthenoteuthis oualaniensis samples in the waters of the Strait of Malacca could preferably verify the speculation in this paper. The Cytb gene fragments used in this study could only reflect the genetic differentiation characteristics of the female parent, and there were certain differences in the evolution rate of different gene fragments and the phylogenetic information displayed. In the future, it is necessary to integrate other gene fragments or genomic DNA data more comprehensively. Moreover, in the future, morphological data information could be added, and combined with the population structure information reflected by the morphology, a more indepth understanding of the population genetic structure of Sthenoteuthis oualaniensis could be obtained.

Keywords: East Indian Ocean; South China Sea; Sthenoteuthis oualaniensis; cytochrome b gene; population genetic structure