Using DNA barcode to identify and genetic diversity of species is being researched and contributed significantly by scientists around the world. The short DNA sequencing used as a barcode for animals promises to provide an accurate, efficient species identification tool and identifying species with intact specimens, young specimens difficult to identify species by morphology (Kress et al. 2015).
Tautz et al. (2003) created the case for a DNA-based classification system (Tautz et al. 2003). Hebert et al. (2003) proposed that a single gene sequence would be sufficient to distinguish all, or at least the vast majority of animals, and proposed the use of mitochondrial I (cox1) DNA cytochrome oxidase subunits such as a global biological identification system for animals. Sequences are likened to a barcode, with species identified by a specific sequence or by a tight cluster of terribly similar sequences (Hebert et al. 2003), (Hebert and Barrett 2005). Once the global COI barcode database has been established for fish, this will be an invaluable tool for fisheries managers, will be an invaluable tool for fisheries managers, aquatic ecologists, and fish retailers, and for those who want to distribute develop micro fish identification. The scientific and practical benefits of fish barcodes are truly diverse (Ward et al. 2005). The results of our study analyzed the characteristics of a COI gene segment on eel flower isolated in Thua Thien Hue, Vietnam. This gene segment can be used as a genetic barcode to identify and evaluate genetic diversity for eels in Vietnam and the world.
Diversity and a close relationship with two individuals from GenBank were found in three different phylogenetic trees of the giant mottled eel Anguilla marmorata in Thua Thien Hue, Viet Nam can be attributed to several issues, most of which are based on life-history events of the species. First, during reproduction of adults and at the onset of larval migration, A. marmorata populations gain effective chances for mixing. Until now, spawning areas of A. marmorata suggested that occurred in the same oceanic areas as other anguillids, such as Anguilla japonica (Pous et al. 2010), (Tsukamoto et al. 2011). The spawning areas have not been well identified yet (Kuroki et al. 2008), (Pous et al. 2010), (Réveillac et al. 2008), (Robinet et al. 2008), except for the North Pacific population. So, all its collected samples have the same origin and belong to the same genetic pool. Second, being the catadromous migration and the long migration loop noticed in A. marmorata life (Arai 2016) could increase the probability of mixing of larvae during the migration from the Sargasso Sea to continental drift (Pujolar et al. 2009). It seems that the same process applies also to Thua Thien Hue, Viet Nam populations due to the close identity of all specimens in Thua Thien Hue with their counterparts in Taiwan and the Pacific Ocean (taken from GenBank). Third, before going up the rivers A. marmorata glass eels stay offshore for 3 months before moving into estuaries which might increase the mixing of individuals (Pujolar et al. 2009).
However, some determinant variables appear in their life and preclude the attaining of a real state “panmixia” of the populations (El-Nabi et al. 2017). Of these variables, spawning cohorts exhibit large variations in time of maturity, the latitude where the population exists and the characteristics of the population there, the onset of migration, and the arrival time to the Sargasso Sea. The yellow eel growth stage may be as short as two to three years in warm productive habitats, but about six to 20 years or more in more in different locations (Williamson and Boëtius 1993). A. marmorata, a catadromous eel, migrates upstream on nights, following the lunar cycle (Wang et al. 2014). The dramatic environmental changes between ocean and freshwater during particular phases of their life cycle shape their physiological features, e.g. visual sensitivity, olfactory ability, and salinity tolerance (Wang et al. 2014) all of which can manipulate the onset of the genetic signal. The time of arrival of spawners to the Sargasso Sea also seems to be variable. As a result, spawners from geographically separated areas could differ in the arrival time at the spawning sea areas. This isolating by the time of spawning groups that causes a restriction in gene flow, taking place between early and late spawners (Hendry and Day 2005). This trend leads to differences in the genetic distance of some individuals, such as HueDTL04, HueBL01, HueDTL02, HueND04, HuePD08, with the others.
The results of A. marmorata genetics research based on the COI gene segment showed that the Anguilla marmorata population in Thua Thien Hue, Vietnam can be divided into two groups indicated the genetic potentials of the species in the vast geographical range in which it lives. In the population structure of Anguilla marmorata, Watanabe et al. (2005) suggested at least four subpopulations (North Pacific, Micronesia, Indian Ocean, and South Pacific) with metapopulation structure evident in the Indian Ocean, and South Pacific. Ishikawa et al. (2004) found the differences among geographic samples that revealed the existence of five geographic subpopulations around North Pacific, Madagascar, Sumatra, Fiji, and Tahiti (Ishikawa et al. 2004). Minegishi et al. (2008) were more consistent with the molecular analyses proposing four genetically different subpopulations (North Pacific, South Pacific, Indian Ocean, Guam region), offering that the North Pacific population is fully panmictic with some meta-population structure in the South Pacific and the Indian Ocean populations (Minegishi et al. 2008).
Recently, Gagnaire et al. (2009) showed the existence of three genetically distinct, reproductive Anguilla marmorata populations in the North Pacific, South Pacific and Southwest India, showing partial gene isolation occurs due to reproduction, but for some inter-population gene lines it can occur during long-term migration of the species (Gagnaire et al. 2009). Genetic analysis of Donovan et al. (2012) on Anguilla marmorata eel on populations distributed in the Pacific Ocean recognition of two lineages distinctive for the eastern Caroline Islands and Guam, and the likelihood of an additional spawning area in the Indo-Pacific Ocean (Donovan et al. 2012). In the first study of phylogenetic relationships and genetic diversity from all Anguilla taxa inhabiting Indonesian waters, based on 1115 specimens belong to four Anguilla species. The results showed that A. marmorata was also split into two clades, supported by a high bootstrap value (Fahmi et al. 2015). This is necessary data for local management and conservation of this valuable resource in terms of both biodiversity and economic development (Fahmi et al. 2015).