A Current Update on the Distribution, Morphological Features, and Genetic Identity of the Southeast Asian Mahseers, Tor Species

Simple Summary

The mahseer, particularly the Tor species, is of significant cultural and economic importance as a high-value freshwater food fish in the Southeast (SE) Asian region. However, overexploitation of natural stocks because of high demand and their deteriorating habitat has resulted in a marked decline of Tor species in the wild. There are 13 Tor species that inhabit SE Asian rivers. All these species share distinct morphology, which is the presence of the median lobe. The unique characteristics, including body color, mouth position, and number of lateral scales, distinguish one species from another. Nonetheless, the taxonomy of the Tor species remains unstable and confusing, with contradictory data presented by different authors from different countries for a single Tor species. Therefore, in this review, we have gathered data for the SE Asian Tor species, outlining their distribution, morphology, and genetic identification. In addition, the present review also proposes a list of valid Tor species in the SE Asian region. The proposed list will serve as a standard and template for improving SE Asia’s Tor taxonomy, enhancing the study’s continuity, and a better understanding of specific Tor species.

Abstract

The king of rivers or mahseer comprises three genera: Tor, Neolissochilus, and Naziritor, under the Cyprinidae family. The Tor genus has been classified as the true mahseer due to the presence of a median lobe among the three genera. The Tor species are widely distributed across Southeast (SE) Asia, and 13 Tor species have been reported previously: Tor ater, Tor dongnaiensis, Tor douronensis, Tor laterivittatus, Tor mosal, Tor mekongensis, Tor putitora, Tor sinensis, Tor soro, Tor tambra, Tor tambroides, Tor tor and Tor yingjiangensis. However, the exact number of valid Tor species remains debatable. Different and unstandardized approaches of applying genetic markers in taxonomic identification and morphology variation within the same species have further widened the gap and ameliorated the instability of Tor species taxonomy. Therefore, synchronized and strategized research among Tor species researchers is urgently required to improve and fill the knowledge gap. This review is a current update of SE Asia’s Tor species, outlining their distribution, morphology, and genetic identification. In addition, the present review proposes that there are ten valid Tor species in the SE Asian region. This list will serve as a template and standard to improve the taxonomy of the SE Asian Tor species, which could serve as a basis to open new directions in Tor research.

1. Introduction

Mahseers, known as the king of rivers, are amongst the largest scale carp and a valuable group of freshwater fish in Asia classified into three genera: Tor, Neolissochilus, and Naziritor under the Cyprinidae family and Cyprininae subfamily [1]. Among these three genus, only Tor species are valid mahseers [1,2]. Tor species are attractive for sport fishing, commercially valuable as highly esteemed food, and have increasing demand for aquaculture [3]. In Southeast Asia (SE Asia), Tor species are among the major species captured and produced by aquaculture [4]. Unfortunately, degradation of the natural habitat caused by the construction of dams in the rivers, deforestation, agricultural development, overfishing, pollution, and a lack of policy for Tor species fisheries has led to near extinction of these species [3,5,6,7].

This king of the rivers is endemic to Asia, and originated from Southwest China; it is widely distributed in West Asia across the trans-Himalayan region encompassing the rivers in Pakistan, India, Sri Lanka, Bangladesh, and Myanmar. The Tor species’ habitats further extend to most SE Asian nations across Thailand, Laos, Vietnam, Malaysia, and Indonesia [1]. Tor species mainly thrive in fast-flowing large rivers and lakes, and they migrate upstream in rivers with clear water streams and pristine and rocky bottoms for breeding [8]. The role of Tor species in the river ecosystem remains elusive due to limited studies. Nevertheless, studies have shown that Tor species migrate for spawning, maintaining the nutrient balance between the river or lake and stream [9,10]. For the past five years, only a few studies have become available that describe the distribution and population density, which further contributes to the lack of information about the Tor species in SE Asia [11].

The word mahseer has its origins in Bengal, the two root word Maha means greatness and Seer means mouth or head [2]; literally, it means having a strong and large head. Generally, Tor species are characterized as large-sized freshwater fish with a compressed and elongated body [12], a strong and large cycloid scale, a large mouth, and a large tail in addition to strong muscles and fins [2]. In the previous two decades, the morphology of Tor species has been used for speciation. Nonetheless, due to locality, environment adaptation, and genetic variation, Tor species have undergone phenotypic changes [2,13]. In SE Asia alone, 13 Tor species have been reported [1,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29]. Phenotypic variation has caused instability and confusion for Tor species speciation in the SE Asian region [13]. As a result, some Tor species have been re-classified into other genera or re-assigned into other Tor species [3]. This unstable taxonomy problem has further added the difficulties to conserve Tor species.

In the last ten years, DNA sequences, particularly mitochondrial DNA, have been used for Tor species identification and speciation as well as using morphology alone [1]. Three common mitochondrial gene sequences have been used for identification and speciation purposes [13,15,30,31]. Nonetheless, the unstandardized approach and genetic variation also create instability and confusion for Tor species speciation in the SE Asian region [13].

There are no reviews that collectively summarize distribution, locality, morphology, and genetic identification specifically for SE Asia’s Tor species. Therefore, this review aims to provide a comprehensive focus and discussion on the current updates on the distribution, locality, morphology, genetic identification, and taxonomy of SE Asia’s Tor. The information in this review will provide another point for the related agencies or biologists to develop strategies to conserve wild Tor species, maintain Tor aquaculture’s productivity, and preserve their habitats and surrounding biodiversity.

2. The Genus Tor

Among the three genera of mahseers, the genus Tor (Figure 1) has been considered the “true mahseers” based on the morphological structure of the median lobe present in this genus but not in the other two genera [1,3]. Although 24 Tor species were recorded previously [1,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29], a recent report in Eschmeyer’s Catalog of Fishes 2021 (as of 1 March 2021) shows that only 17 out of 24 species are validated as Tor species (

Table 1. List of reported Tor species worldwide and their validity from two different sources: Eschmeyer’s Catalog of Fishes and Pinder et al. (2019).

No.	Species	Species Status According to Eschmeyer’s Catalog of Fishes as of 1 March 2021	Species Status According to Pinder et al. (2019)
1.	Tor ater (Robert, 1999)	Valid	Valid
2.	Tor barakae (Arunkumar and Basudha, 2003)	Valid	Valid
3.	Tor chelynoides (McClelland, 1839)	Not Valid Valid as Naziritor chelynoides (McClelland, 1839)	Not Valid
4.	Tor dongnaiensis (Hoang et al., 2015)	Valid	Valid
5.	Tor douronensis (Valenciennes, 1842)	Not Valid. Valid as T. tambra (Valenciennes, 1842)	Not Valid
6.	Tor hemispinus (Chen and Chu, 1985)	Not Valid Valid as Neolissochilus hemispinus (Chen and Chu, 1985)	Not Valid
7.	Tor khudree (Sykes, 1839)	Valid	Valid
8.	Tor kulkarnii (Menon, 1992)	Valid	Valid
9.	Tor laterivittatus (Zhou and Cui, 1996)	Valid	Valid
10.	Tor macrolepis (Heckel, 1838)	Not Valid Valid as T. putitora (Hamilton, 1822)	Not Valid
11.	Tor malabaricus (Jerdon, 1849)	Valid	Valid
12.	Tor mekongensis (Hoang et al., 2015)¶	Valid	Not Valid Valid as T. tambra (Valenciennes, 1842)
13.	Tor mosal (Hamilton, 1822)	Valid	Valid
14.	Tor polylepis (Zhou and Cui, 1996)	Valid	Valid
15.	Tor putitora (Hamilton, 1822)	Valid	Valid
16.	Tor qiaojiensis (Wu, 1977)	Not Valid Valid as Neolissochilus qiaojiensis (Wu, 1977)	Not Valid
17.	Tor remadevii (Madhusoodana Kurup and Radhakrishnan, 2011)	Valid	Valid
18.	Tor sinensis (Wu, 1977)	Valid	Valid
19.	Tor soro (Valenciennes, 1842)	Not Valid Valid as Neolissochilus soro (Valenciennes, 1842)	Not Valid
20.	Tor tambra (Valenciennes, 1842)	Valid	Valid
21.	Tor tambroides (Bleeker, 1854)	Valid	Valid
22.	Tor tor (Hamilton, 1822)	Valid	Valid
23.	Tor yingjiangensis (Chen and Yang, 2004)	Valid	Valid
24.	Tor yunnanensis (Wang, Zhuang, and Gao, 1982)	Not Valid Valid as Folifer yunnanensis (Wang, Zhuang, and Gao, 1982)	Not Valid

Note: ¶ symbol is represent uncertain species.

) [32]. A recent review by Pinder et al. (2019) has concluded that there are 16 valid species of Tor, and all these species have been listed in the IUCN Red List of Threatened Species^TM (Version 2018-2) [3]. More recent versions and data from the IUCN Red List of Threatened Species^TM (Version 2020-3) have shown that there are now 18 Tor species listed as endangered species [11]. In SE Asian countries, the presence of 13 Tor species was reported previously (

Table 2. List of reported Tor species in Southeast (SE) Asian countries.

No.	Species	Common Name(s)	Reported Geographical Distribution	Status (Based on Eschmeyer’s Catalog of Fishes as of 1 March 2021 and Pinder et al. (2019))	IUCN Red List of Threatened SpeciesTM (Version 2020-3) [11]		References
					Species Status	Population Status
1.	T. ater (Robert, 1999)		Laos	Valid	Near Threatened	Unknown	[36]
2.	T. dongnaiensis (Hoang et al., 2015)	Dongnai mahseer	Vietnam	Valid	Near Threatened	Unknown	[15]
3.	T. douronensis¶ (Valenciennes, 1842)	Semah (Indonesia/Borneo Island), Pelian (Malaysia)	Malaysia, Indonesia	Not Valid/Valid as T. tambra/Unknown/Questionable/Data Deficient	Not Listed	Not Listed	[1,38]
4.	T. laterivittatus (Zhou and Cui, 1996)		Laos, China	Valid	Data Deficient	Decreasing	[16,37]
5.	T. mekongensis¶ (Hoang et al., 2015)		Vietnam	Not Valid/Valid as T. tambra/Unknown/Questionable/Data Deficient	Not Listed	Not Listed	[15]
6.	T. mosal (Hamilton, 1822)	Mosal mahseer, Copper mahseer	Myanmar, India	Valid	Data Deficient	Unknown	[3,34]
7.	T. putitora (Hamilton, 1822)	Himalayan mahseer, Golden mahseer, Putitora mahseer	Myanmar, Afghanistan, Bangladesh, Bhutan, India, Nepal, Pakistan	Valid	Endangered	Decreasing	[17]
8.	T. sinensis (Wu, 1977)	Pba daeng (Laos), Red mahseer	Vietnam, Laos, China	Valid	Vulnerable	Unknown	[15,39]
9.	T. soro (Valenciennes, 1842)	Kancra or Gemo fish	Indonesia	Not Valid/Valid as Other Species/Unknown/Questionable/Data Deficient	Not Listed	Not Listed	[33,38]
10.	T. tambra (Valenciennes, 1842)	Keureling (Indonesia), Pba tohn (Laos)	Malaysia, Thailand, Indonesia	Valid	Data Deficient	Decreasing	[3,35,38]
11.	T. tambroides (Bleeker, 1854)	Kelah, Empurau (Malaysia) Jurung (Indonesia)	Malaysia, Thailand, Indonesia	Valid	Data Deficient	Unknown	[3,35,40]
12.	T. tor (Hamilton, 1822)		Myanmar, Pakistan, Nepal, Bhutan, India, Bangladesh	Valid	Data Deficient	Unknown	[17,34]
13.	T. yingjiangensis (Chen and Yang, 2004)		Myanmar, China	Valid	Data Deficient	Unknown	[26,28]

Note: ¶ symbol is represent uncertain species.

) [1,3,15,17,26,28,33,34,35,36,37]. From these 13 reported Tor species, only ten species are considered valid by two databases, Eschmeyer’s Catalog of Fishes 2021 and a review by Pinder et al. (2019), leaving another three species invalid or their identity remaining uncertain (Table 2) [3,32]. Furthermore, all ten valid Tor species were listed in the IUCN Red List of Threatened Species^TM (Version 2020-3). Amongst the listed SE Asian Tor species, there were two species classified under “vulnerable” and “endangered” categories, two species classified under “near threatened,” and six species classified under “data deficient” (Table 2). In addition, out of these ten listed Tor species, three species show a decreasing trend in their population number, and seven species are of unknown status [11].

3. Geographical Distribution of Tor Species in the SE Asian Region

3.1. Malaysia

Peninsular Malaysia is home to two Tor species, T. tambroides and T. tambra (see Table 2), where they inhabit around 30 rivers [2]. In Peninsular Malaysia, the Tor species are called “Kelah”. Most of the Tor species in Peninsular Malaysia are found upstream of the main river basins, which are the surrounding areas of rivers that have remained conserved and untouched by development activities.

On the west coast of Malaysia, Ulu Muda River in the state of Kedah serves as home to a large population of Tor species. Other rivers in the state of Kedah inhabited by the Tor species include Telian and Gawi Rivers [2]. Meanwhile, in the state of Perak, it has been reported that the Tor species inhabit Perak, Bernam, Karawang, Kejar, Mangga, Sungkai, Sara, Sigor, and Tiang Rivers and also Temenggor Lake (Figure 2) [2]. Selangor is the most advanced and rapidly developing state in Malaysia, with a large population. Therefore, deforestation to accommodate rapid development and the growing population have resulted in the loss of rivers [41]. Furthermore, the rapid growth of industry and housing estates near existing rivers has caused pollution to the rivers [42]. Hence, Tor species have been reported only in Kancing, Langat, Semenyih, and Selangor Rivers [2]. In the state of Negeri Sembilan, the Tor species have been reported to inhabit Kampung Esok [43] and Serting Rivers [44].

On the east coast of Malaysia, the Tor species have been reported to inhabit Galas, Nenggiri, and Lebir Rivers that meet to form the Kelantan River in the state of Kelantan. However, a study in 2015 described the failure to capture the Tor species, especially T. tambroides in the Kelantan River [45,46]. From 2016 to date, there no study has been conducted to validate the disappearance of Tor species in the Kelantan River. Further, there is evidence that the Tor species inhabit Pergau Lake, Jeli Province, in the state of Kelantan [47], Kenyir Lake, and Petuang River [2] in the state of Terengganu.

A recent study has shown that the population of T. tambra in Kenyir Lake is decreasing at an alarming rate [57]. There are reports of the presence of T. tambroides and T. tambra in the Keniyam River in the National Park of Pahang and in the Pahang River, which is the longest river in Peninsular Malaysia as well as in Tembeling, Jelai, Lipis, Rompin, and Merchong Rivers in the state of Pahang [2]. Although it has been reported that rivers (Endau, Kinchin, Kemapan, Jasin, and Marong) in the state of Johor, the southern state of Peninsular Malaysia, are inhabited by the Tor species [2], recently it has been reported that the Tor population is limited only to the Endau River [58].

The Malaysian states of Sabah and Sarawak, on the island of Borneo, have large areas of untouched and conserved forest. The Tor species in Sabah are locally known as “Pelian”, while in Sarawak, the Tor species are known as “Empurau”, meaning “unforgettable taste” in Chinese characters. In the state of Sabah, T. tambra and T. tambroides are the most common species that inhabit the Moyog and Tenom Rivers [2]. On the other hand, in the state of Sarawak, the most common Tor species are T. douronensis and T. tambroides; T. douronensis has been reclassified as a synonym of T. tambra [3]. Phylogenetic analysis by Walton et al. (2017) showed that T. tambra found in Borneo is genetically different from T. tambra and T. tambroides from Indonesia and Peninsular Malaysia, which might be a new species of Tor that remains to be confirmed [13]. Generally, the Tor species in the state of Sarawak can be found in most major rivers, such as Trusan, Limbang, Rejang, Layar, Sadong, Sarawak, and Batang Lupar Rivers [51]. However, the exact number of Tor species in Sarawak remains elusive because, among the 22 river basins in Sarawak, many remain unexplored.

3.2. Thailand

In Thailand, the Tor species are less popular as a food, or protein source than other large indigenous species from the same family, such as Probarbus jullieni and Catlocarpio siamensis [59]. Thus, fewer studies have been conducted, and there are a limited number of publications related to the Tor species from Thailand. The information in the following paragraph is from previous studies done over a decade ago and there are limited recent studies on these species in Thailand. Tor species that have been reported to inhabit lakes and rivers of Thailand include T. tambroides [1], T. tambra, T. sinensis, T. douronensis, and T. putitora [50]. Amongst these species, T. tambroides is widely distributed in Thailand [35,50,60]. Nevertheless, T. tambroides has gained much interest as a high-value food fish and ornamental fish among the Thai people [59,61,62]. This has accelerated the exploitation of wild fish both as broodstock for aquaculture and a premium supply for the ornamental fish market [63]. Moreover, habitat alteration (e.g., damming or deforestation) and deterioration have been additional threats to these valuable species [64]. Therefore, it is urgent to gather information on species distribution and diversity to enable proper conservation plans before some species are lost.

There are 25 river basins in Thailand (254 sub-basins), among which the two largest rivers are the Chao Phraya and Mekong [65]. The Mekong runs from China through Myanmar and Laos, and its tributaries are home to some of the world’s most diverse species of freshwater fishes, including the Tor species [35,52,66,67]. The Chao Phraya and its tributaries occupy Thailand’s central plains. It emerges from four rivers, Ping, Wang, Yom, and Nan that merge in Nakorn Sawan and continue flowing southward until it reaches the Gulf of Thailand in Samut Prakarn, south of Bangkok. T. tambroides was reported to inhabit the Chao Phraya and its tributaries [49,68]. Besides the two major rivers, in the western part of the country, Mea Klong River is important and runs in a north to south direction from Kanchanaburi Province and reaches the Gulf of Thailand in Samut Sakorn Province, south of Bangkok. Mae Klong has been recorded as being home to Tor species (see Figure 2) [1,50].

In the northwestern part of Thailand, T. tambroides have been reported to inhabit rivers in Chiang Mai and Mae Hong Son Provinces [35]. Meanwhile, northeastern part of Thailand, from Nong Khai to Bueng Kan, Nakhon Phanom, and Kaeng Tana National Park, serves as a river basin for the 10th longest river in the world, the Mekong River, which is also home to T. tambroides [50]. In addition, T. tambroides are found to inhabit Phetchaburi Basin located in Phetchaburi Province, northwest of the Gulf of Thailand [60]. The southern region of Thailand, located on the Thailand peninsula, stretches from Kra Isthmus to the Malaysian border. There are 12 rivers in this area; among them, the Tapi River is the longest in the high mountain ranges of the southern peninsula and is home to 319 fish species, including T. tambroides, T. tambra, and T. douronensis [68]. The T. tambroides have been reported to inhabit Cheow Lan Reservoir, Khao Sok National Park in Surat Thani Province [35]. Despite its debatable taxonomic classification, T. douronensis has been reported in peninsular Thailand, including in the Chao Phraya and Mekong Basins. Furthermore, this species has also been reported in Si Sawat District, Kanchanaburi Province (central Thailand), where lies the Srinakarin Reservoir, rivers in Chiang Mai Province (northern Thailand), and also Pattani River in the south [35].

3.3. Vietnam

Only a few studies describe the presence of Tor species in Vietnam, which mainly focus on the Central Highlands in the southern region. Several Tor species have been reported to inhabit the highland riverine systems in Vietnam, including T. tambra, T. tambroides, T. sinensis, T. mekongensis, and T. dongnaiensis [15,69]. The study conducted by Hoang et al. (2015) has shown T. mekongensis and T. sinensis in Krong No River (see Figure 2) [15]. This river rises from Chu Yang Sin Mountain (>2000 m) and flows west 156 km, and meets River Krong Ana before connecting to Sre Pok River, a transboundary tributary of the Mekong River [70]. In addition, Hoang et al. (2015) have also shown the presence of T. dongnaiensis in the middle Dong Nai River, Cat Tien National Park, and in Lam Dong Province [15]. Dong Nai River is the longest inland river in Vietnam, at 586 km, starting from Lam Dong Highlands and flowing to the East Sea. A previous study by Mai et al. (1992) has reported T. tambroides in the middle and upstream of Dong Nai River. However, Hoang et al. (2015) suggested that T. tambroides described by Mai et al. (1992) is T. dongnaiensis based on morphological characteristics. Therefore, the natural distribution of T. tambroides in Vietnam is still in question [3,15].

In another study, Phan and Duong (2014) collected T. tambroides from some locations (Dak Lak, Dak Nong, and Lam Dong) of the Central Highlands where Sre Pok River flows which belongs to the Mekong River Basin [71]. However, the validity of T. tambroides from these areas remains questionable as the analysis of this species showed it belongs to T. sinensis. Given that the two species T. dongnaiensis and T. tambroides are present in separate river basins (Dong Nai and Mekong, respectively) in Vietnam and the latter is found in Laos sharing the Sesan, Sre Pok, and Sekong (3S) River Basins with Vietnam, their distribution and classification should be further investigated. Besides T. tambroides, the distribution of T. mekongensis in Vietnam is also questionable. Further genetic analysis of T. mekongensis done by Walton et al. (2017) has shown that this Tor species belongs to T. tambra [13]. Another issue about the validity of Vietnam’s Tor species is T. dongnaiensis. The genetic analysis, done by Hoang et al. (2015) to compare Vietnam’s Tor species with T. tambra (previously described as T. douronensis) from the Malaysian Borneo, should be revised because the Tor species from Malaysian Borneo did not belong to any known valid Tor species in SE Asian countries [13,15].

3.4. Myanmar

Most reports that mention the presence of Tor species in Myanmar are those from the Thanlwin River. This river originates from the Himalayan Plateau and passes through China, Thailand, and Myanmar, and flows into the Andaman Sea. Locally, the Tor species are called Nga-dauk. Several Tor species have been reported to inhabit the Thanlwin River (see Figure 2), which are T. putitora (golden mahseer), T. tambroides, and T. tor [18]. Besides these, T. yingjiangensis has been reported to inhabit the Mali Hka [26] and Irrawaddy [28] Rivers. Although Myanmar is well known as the primary source of fish globally, there is no recent study investigating the availability and population size of Tor species in Myanmar. The limited number of studies and no recent reports (in the last three years) on the Tor species raise concern over the number of Tor species which exist in Myanmar.

3.5. Lao PDR

Geographically, Lao PDR is the only land-locked SE Asian country. Therefore, the fish industry in Lao PDR is solely dependent on freshwater fish that come from rivers, reservoirs, ponds, and lakes [72]. A previous study in 1999 found the presence of T. ater, T. sinensis, and T. tambra that inhabit the Nam Theun/Kading River [36]. In 2011, a study showed that Sekong (Xe kong) River tributaries and Se Kaman/Xe Kaman River are home to several Tor species, which are T. laterivittatus, T. tambra, and T. tambroides (see Figure 2) [16]. To date, there are no other studies that have reported the population sizes of Tor species in Lao PDR.

3.6. Cambodia

Freshwater fish are captured and mostly consumed in Cambodia [73]. Nonetheless, studies on Tor species’ population sizes are unavailable. Previous studies have shown that T. tambroides, T. tambra, and T. sinensis inhabit the Mekong River in Cambodia [66,74]. Unfortunately, there is no study to show the presence of Tor species in other rivers of Cambodia or even in Tonle Sap Lake.

3.7. Indonesia

Indonesia is the largest island country and separated from mainland SE Asia by sea. Nonetheless, Indonesia somehow shares similar Tor species with some SE Asian mainland countries such as Malaysia, Thailand, Vietnam, and Cambodia. The Tor species reported in Indonesia’s rivers are T. tambroides, T. tambra, and T. douronensis [19]. The wide distribution of Tor species across SE Asia’s mainland, Borneo, Sumatra, and Java of Indonesia could be because the Mekong River system was connected with these islands’ river systems during the Pleistocene, known as the Sunda Shelf [75]. After the rising of sea levels, SE Asia separated from the Indonesian islands, as it is now.

The Tor species are locally known as Keureling fish. Starting from Sumatra, four Tor species, T. tambroides, T. douronensis, T. tambra, and T. soro, have been recorded as inhabitants of this area [76]. Currently, T. soro is classified as N. soro [77]. In Sumatra, several studies have reported the presence of Tor species. A recent study showed T. tambroides in the upstream of Wampu waters, northern region of Sumatra [19]. Another study also showed T. tambroides in the Manna and Tarusan Rivers of the western part of Sumatra and Bahorok River and Berkail River. Meanwhile, a survey conducted in the Batang Toru river system, South Tapanuli, North Sumatra, has shown several Tor species, namely T. tambra, T. douronensis, and T. tambroides (see Figure 2) [53]. Furthermore, a study in the Kreuang Sabee River, Aceh Jaya District, Indonesia showed only T. tambra [78]. Although T. douronensis has been classified as an invalid species, this species has been widely studied in Indonesia and found widely distributed in several rivers on Sumatra, namely Batang Gumanti River, Batang Antokan River, Batang Malalo River, Batang Matur River, Batang Sinuruik River, and Lubuk Mangkuih River [79].

Moving toward south of Sumatra, Java is also home to the Tor species [80]. A recent study documented T. tambroides in the rivers of West Java, particularly in Cimanuk River that flows from Pappandayan Mountain into the Java Sea [54]. Another recent study reported that T. douronensis inhabits riverine systems surrounding Cereme Mountain located in Kuningan District, West Java [81]. The river system areas inhibited by T. douronensis are Balong Dalem, Darma Loka, Cigugur, Cibulan, and Bale Kambang [81]. In Kalimantan of Borneo, T. tambroides have been reported in riverine systems of Muller Mountain in Central Kalimantan [55,56]. In addition, T. tambroides and T. tambra have been reported in the Mendalam river system, Betung Kerihun National Park, West Kalimantan. As compared to Sumatra, studies on Tor species on Java and Kalimantan are limited. However, in Bogor, South Jakarta, Java, the Research Institute for Freshwater and Fisheries Extension is actively engaged in T. douronensis and T. tambroides aquaculture and preservation research [82].

3.8. Brunei, Singapore, Timor-Leste, and the Philippines

A recent study in 2018 reported that T. tambroides and T. tambra inhabit the Brunei riverine system of the Temburong River [83]. Since then, no other Tor species study has been reported from Brunei. Singapore has no recent studies on Tor species. T. tambroides was reported in Singapore riverine systems back in 1966 [84]. However, in 1997, T. tambroides were classified as extinct in Singapore [85]. The Philippines and Timor-Leste were not part of the Sunda Shelf during the Pleistocene [86]. Therefore, the riverine systems in these two countries are not connected with any of the mainland of SE Asia; hence, Tor species have not been reported in the Philippines and Timor-Leste.

4. The Morphological Features of SE Asia’s Tor Species

Morphological features such as shape, size, fin ray, and median length have been used previously to distinguish Tor species. However, this methodology has caused confusion and debate among taxonomists [13]. The Tor species’ morphological features show high intra- and interspecies variability that is influenced by environmental factors and localities [2,13]. This has resulted in difficulty to validate the Tor species in SE Asian countries as well as in generating stable databases for Tor species. Therefore, establishing a standard morphological identification, morphometric measurement, and genetic identity analysis will prevent data variation and avoid confusion. Therefore, this review has summarized the SE Asian Tor species’ distinct morphological characters, which could help towards Tor conservation.

4.1. T. ater (Robert, 1999)

T. ater is the smallest of the SE Asian Tor species. Generally, T. ater is characterized by having a short median lobe, longitudinal dark stripe on the body, and all fins have a dark or black color (

Table 3. The morphological features of SE Asia’s Tor species. The figures in this table were drawn based on an actual sample of fish, descriptions recorded, or redrawn from photos: T. ater [36]; T. dongnaiensis [15]; T. douronensis [87]; T. laterivittatus [88]; T. mekongensis [15]; T. mosal [89]; T. putitora [89]; T. sinensis [15]; T. tambra [13]; T. tambroides, our picture and [90]; T. tor [89]; T. yingjiangensis [28]. Note: ¶ symbol is represent uncertain species.

Species	Reported Maximum Size (Standard Length for Adult, cm)	Mouth Position	Lower Median Lobe (Scale Bars Represent the Length of Lower Median Lobe)	Body Color	Lateral Scales	Distinctive Features	Reference
T. ater (Robert, 1999)	~30	Sub-terminal	Short	Dark brown with the presence of a dark longitudinal stripe	30–31	The smallest in body size among SE Asia’s Tor species. Short median lobe and longitudinal dark stripe on the body.	[52]
T. dongnaiensis (Hoang et al., 2015)	~40	Sub-terminal	Long	Silver gray and yellowish in sub-adult	23–24	Long lower median lobe without upper median projection.	[15]
T. douronensis¶ (Valenciennes, 1842)	~100	Sub-terminal	Medium	Silvery, darkish above and dark fins	24–27	Short lower median lobe and silvery and yellowish in color.	[2]
T. laterivittatus (Zhou and Cui, 1996)	~60	Sub-terminal	Long	Dark green to silver with the presence of longitudinal stripe	25–27	Long lower median lobe without upper median projection and longitudinal dark brown stripe along the middle side of the adult body.	[91]
T. mekongensis¶ (Hoang et al., 2015)	~33	Sub-terminal	Medium	Silver gray	23	Short lower median lobe, blunt rostral hood, and silvery in color.	[15]
T. mosal (Hamilton, 1822)	~270	Terminal	Long	Delicate yellowish shade below, caudal reddish orange	23–26	Terminal mouth position with long lower median lobe, yellowish color on the lower body, and fin ray counts: 13 dorsal fin rays, 17 pectoral fin rays, and 8 anal fin rays.	[3]
T. putitora (Hamilton, 1822)	~270	Sub-terminal	Varying in length	Reddish sap green, light orange fading to silvery white	23–28	Blunt rostral hood and dark longitudinal stripe in the middle of the side of the body. The color of the caudal, pelvic, and anal fins is reddish gold.	[92]
T. sinensis (Wu, 1977)	~46	Sub-terminal	Long	Darkish on the back and brownish or bronzy beneath, a dark longitudinal stripe	23–28	Long lower median lobe without upper median projection, pointed rostral hood, and a slate gray longitudinal stripe along the side of the body from head to caudal base.	[37]
T. tambra (Valenciennes, 1842)	~100	Sub-terminal	Short	Olive or dark olive, reddish	22–24	Short lower median lobe, blunt rostral hood, and reddish body color.	[13]
T. tambroides (Bleeker, 1854)	~100	Sub-terminal	Long	Silver bronze and reddish with dark fin	23–26	Long lower median lobe, present with upper median projection, pointed rostral hood, and reddish body color.	[93]
T. tor (Hamilton, 1822)	~200	Terminal	Varying in length	Grayish green	22–28	Small head and mouth, blunt rostral hood, reddish fin, silvery abdomen, and dark gray dorsal side.	[12]
T. yingjiangensis (Chen and Yang, 2004)	~20	Terminal	Short	Yellowish	24–26	Longer head, conical shape of the head, and a slightly convex body. All fins are yellowish, and there is no mid-lateral line.	[28]

) [14].

4.2. T. dongnaiensis (Hoang et al., 2015)

T. dongnaiensis has morphological characters that distinguish it from the Indonesian T. tambroides, namely, yellow to gray body color. The morphological feature of the mouth is the absence of a median projection of the upper lips, and unequal caudal fin [15]. Generally, the appearance of this species includes conical head shape, pointed snout, subterminal mouth with fleshy lips, long lower lip of median lobe, and straight and pointed rostral hood (see Table 3). However, genetic analysis done by Walton et al. (2017) shows no clear separation between T. dongnaiensis studied by Hoang et al. (2015) and T. tambra from West Java and Peninsular Malaysia [13]. Therefore, this species is postulated to be classified under T. tambra [13]. Nonetheless, a recent review by Pinder et al. (2019) suggests that T. dongnaiensis remains as a valid Tor species [3]. Therefore, a large-scale study is required to validate this species’ uncertainty.

4.3. T. douronensis (Valenciennes, 1842)

The taxonomy of T. douronensis is the most complicated amongst the Tor species. This name (T. douronensis) has been debated since 1999 and continues to date [3,13,32]. Several researchers, including Roberts (1999) and Kottelat (2013), have debated the validity of T. douronensis as there is no specific morphological feature that can distinguish between this species and T. tambra [36,52]. Hence, Kottelat (2013) has postulated that T. douronensis found in Peninsular Malaysia and Borneo (Sabah and Sarawak) is a synonym of T. tambra [52]. Despite the unstable taxonomy, researchers in Indonesia continue to use T. douronensis for species identification instead of T. tambra [94,95,96,97]. This postulation has been supported by Rahayu et al. (2015) as their study reported that T. tambroides and T. douronensis are distinct species through genetic classification, along with morphological classification [98]. Morphologically, T. douronensis has a short lower median lobe, the absence of an upper median projection, and a blunt rostral hood (see Table 3). Additionally, the color of T. douronensis is silvery with a darkish back. Haryono and Tjakrawidjaja (2006) showed that three Tor species, T. tambroides, T. douronensis and T. tambra, from the same source are distinguishable from each other through their body measurements [93]. From the study mentioned above, the body measurements that differentiate the Tor species are interorbital width, caudal peduncle length, caudal peduncle depth, head width, and body depth [93]. Interestingly, T. douronensis is different from T. tambra with its less than 10 mm interorbital width and large caudal peduncle. Additionally, genetic analysis done by Walton et al. (2017) showed that there is a clear separation in the phylogenetic tree between this species and T. tambra from West Java, Indonesia, and from Peninsular Malaysia [13]. Therefore, the authors suggest that the Tor species from Borneo might be a new Tor species. However, for further confirmation of this species’ validity, morphological and genetic comparisons between T. tambra and T. douronensis from the Mekong riverine system, Peninsular Malaysia, Borneo, Sumatra, and Java should be performed to resolve this varying taxonomy.

4.4. T. laterivittatus (Zhou & Cui, 1996)

This species’ unique characteristics are its elongated median lobe of the lower lip, the upper lip is rolled upward and backward, the presence of a longitudinal stripe along the body, and a profoundly concave dorsal (see Table 3) [37]. Amongst the Tor species., T. laterivittatus is the least studied, and therefore information on this species is mainly from fishers’ local knowledge [3].

4.5. T. mekongensis (Hoang et al., 2015)

From the word “Mekong”, it is evident that T. mekongensis inhabit the Mekong River system [15]. The general appearance of T. mekongensis is as follows: longer head, blunt snout, blunt rostral hood, subterminal mouth with fleshy lips, and a short median lobe of the lower lip (see Table 3). A study by Hoang et al. (2015) has reported that this species is a valid Tor species. Genetic analysis of T. dongnaiensis (Vietnam) and T. tambra (previously known as T. douronensis from Borneo (Malaysia)) shows a clear separation of these species’ phylogenetic tree. In contrast, genetic analysis of T. mekongensis [15] and T. tambra from West Java, Indonesia, and Peninsular Malaysia shows no difference between the two species along the phylogenetic tree [13]. Furthermore, a recent review has also suggested that T. mekongensis is a synonym of T. dongnaiensis [3]. This species’ variable taxonomy requires a new study to review the validity of all the Tor species in SE Asian countries.

4.6. T. mosal (Hamilton, 1822)

T. mosal is considered a synonym of T. putitora [99]. T. mosal is also known as the copper mahseer due to its near reddish coloration on the anal and pectoral fin, which resembles T. putitora [3]. However, the head length of this species is distinctively shorter than T. putitora (see Table 3) [100]. Furthermore, genetic analysis has shown that T. mosal is different from T. barakae, T. putitora, and T. tor [100].

4.7. T. putitora (Hamilton, 1822)

In India, T. putitora is known as the golden mahseer. The color of this species appears greenish and silvery on the side of the body, which then turns reddish yellow or golden on the anal and pectoral fin that gives it the name (see Table 3) [101]. Like T. tor, which originated from the Himalayan Plateau, this species is endemic to the Myanmar riverine system [7]. This species’ unique characteristics are an elongated and nearly straight body, a small mouth with lower jaw slightly shorter than the upper jaw, and a deeply forked caudal fin.

4.8. T. sinensis (Wu, 1977)

T. sinensis is another Tor species that predominantly inhabits the Mekong River system [67]. This species has unique characteristics that differ from other sympatric Tor species. Compared to T. ater, T. sinensis is characterized by fewer number of a lateral, predorsal and transverse row scales, a long median lobe on the lower lip, and the absence of stripes along the body (see Table 3) [15]. T. sinensis has similar features to T. tambroides; a large standard length over body depth ratio and the presence of a median projection of the upper lip (see Table 3). Compared with T. mekongensis, T. sinensis uniquely has a long rostral hood and many lateral scales (23–28). The color of T. sinensis is usually dark on the head and back with a silver gray or yellowish color along the body. Furthermore, unlike other Tor species, T. sinensis has a longitudinal stripe stretching from the head to the caudal fin.

4.9. T. tambra (Valenciennes, 1842)

Like T. tambroides, T. tambra can be found in many riverine systems of the SE Asian countries, including the Mekong River system, Thailand, Peninsular Malaysia, Sumatra, Java, and Borneo [13]. The color of this species varies depending on the locality, including reddish, olive, dark, or slightly olive [13,15]. The morphological features that differentiate T. tambra from T. tambroides are the lower median lobe’s varying size and the blunt rostral hood (see Table 3). Other features that T. tambra shares closely with other Tor species are the absence of an upper median projection and an equal caudal fin [13].

4.10. T. tambroides (Bleeker, 1854)

The body color of T. tambroides is silver/bronze/reddish. The unique morphological feature that specifically represents T. tambroides is the presence of an upper median projection (see Table 3) [93]. Other features of T. tambroides common among other Tor species include sub-terminal mouth position, long lower median lobe, pointed rostrum hood, and an equal caudal fin lobe [13,15].

4.11. T. tor (Hamilton, 1822)

The prominent characteristics that differentiate T. tor from other Tor species in Myanmar is the big head, large scales, sub-terminal mouth with an interrupted fold of the lower lip, two pairs of large barbels, and lateral line scales (see Table 3) [15].

4.12. T. yingjiangensis (Chen and Yang, 2004)

T. yingjiangensis was first identified by Chen and Yang (2004), and originated from Yunnan Province, China [28]. Briefly, T. yingjiangensis uniquely have a longer, conical head shape and a slightly convex body (see Table 3). The body color of T. yingjiangensis is yellowish and black or light brown in the middle. All fins are yellowish, and there is no mid-lateral line [28].

5. Genetic Identification of SE Asia’s Tor Species

Conventional taxonomy relies solely on morphological characters. However, it is known that phenotypic plasticity occurs in animals due to adaptation to environmental conditions [3,15,36,102]. In addition, in specific cases, variable gene expression is also observed, e.g., the saddle-back trait of Nile tilapia shows a wide range of phenotypes, from a standard dorsal fin to the lack of a dorsal fin, despite having the same genotype [103]. Phenotypic variations within the Tor species are other factors of taxonomic confusion [3,104]. Recent studies have employed molecular markers to solve taxonomic ambiguity [13,15,105] and/or assess the genetic diversity of Tor [43,106,107,108]. Most of the studies have employed mitochondrial genes, while others have used nuclear DNA markers, such as microsatellites, for species identification. This review focuses on two types of genetic markers and their application for Tor studies.

5.1. Mitochondrial DNA

Generally, animal mitochondrial DNA (mtDNA) is 16–17 kb in size and consists of two rRNA-encoding, 22 tRNA-encoding, and 13 protein-encoding genes, including NADH-ubiquinone oxidoreductase chain 1–6 (ND1–6), NADH-ubiquinone oxidoreductase chain 4L (ND4L), cytochrome b (Cytb), cytochrome c oxidase subunit I-III (COX 1–3), and lastly complex V subunits of ATPase 6 (ATP6) and 8 (ATP8) [109]. The characteristics of mtDNA are uniparental inheritance through maternal lineage, non-recombination, and higher mutation rates as compared with nuclear DNA [110]. With these characteristics, mtDNA has been used extensively for phylogenetic analyses for species identification and specification. For more than two decades, the identification and specification of Tor species has remained unstable. In this regard, different mtDNA gene sequences have been used for species identification. The most frequently used is cytochrome oxidase I (CO1 or COX1) [13,15,44,107,111] followed by cytochrome b (Cyt b) and ATPase subunits 6 and 8 (ATPase 6/8) [1,106,108]. The other mitochondrial gene used in Tor studies is the 16S rRNA [1]. Complete mitochondrion genome sequences have been reported for a few Tor species, such as T. tambroides [112] and T. tor [104].

In this review, we performed phylogenetic analyses using COX1, Cyt b, and 16S rRNA gene information databases from the National Center for Biotechnology Information (NCBI) [113]. Four phylogenetic analyses were performed using MEGA X software (Version 10.1) [114,115]. First, we performed the phylogenetic analysis for the complete sequence of the COX1 gene. In this analysis, seven complete sequences of COX1 of Tor species samples were selected, which are T. douronensis, T. putitora, T. sinensis, T. tambra, T. tambroides, and T. tor (Supplementary 1). These seven complete sequences of COX1 of Tor species were aligned together with five selected complete COX1 sequences of Neolissochilus species, N. benasi, N. hexagonolepis, N. hexastichus, N. soroides, and N. stracheyi, which acted as the out-group (see Supplementary 1) [112,116,117,118,119,120,121,122,123,124,125]. The phylogenetic trees were constructed using the maximum likelihood method and Tamura–Nei model [126]. All the samples used in the first phylogenetic analysis were used as reference samples for the subsequent phylogenetic analyses. Second, we performed the phylogenetic analysis of complete and partial/short sequences of the COX1 gene. For this phylogenetic analysis, we extended the first phylogenetic analysis by adding seventy-one short/partial sequences of COX1 of Tor species obtained from NCBI databases (Supplementary 2) [13,15,107,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142]. Then, the phylogenetic trees were constructed using the method and model used in the first phylogenetic analysis. Similar to the second phylogenetic analysis, the third and fourth phylogenetic analyses were performed using complete and partial/short sequences of Cyt b and 16S rRNA genes. Briefly, twelve complete sequences of Cyt b or 16S rRNA genes from similar Tor and Neolissochilus samples used in the first phylogenetic analysis were obtained from NCBI databases. Then, these complete sequences were aligned with a short/partial sequence of Cyt b or 16S rRNA genes of selected Tor species obtained from NCBI databases. For Cyt b phylogenetic analysis, eighty-three short/partial sequences of Cyt b of Tor species were selected (Supplementary 3) [1,108,112,116,117,118,119,120,121,122,123,124,125,127,142,143,144,145,146,147,148,149,150,151,152,153,154], while for 16S rRNA phylogenetic analysis, sixty-eight short/partial sequences of 16S rRNA of Tor species were selected (Supplementary 4) [1,31,112,116,117,118,119,120,121,122,123,124,125,127,154,155,156,157,158,159,160,161,162,163,164,165,166]. The results of these four phylogenetic analyses are described and discussed in the following sub-section.

5.1.1. Cytochrome c Oxidase Subunit I (COX1) Gene

Cytochrome c oxidase subunit I (COX1) is a gene that encodes for one of three subunits of the cytochrome c oxidase complex. The COX1 gene has been proposed as a powerful universal marker for vertebrate species identification and specification, including the Tor species [167,168]. A study by Hoang et al. (2015), covering 17 Tor species, found that most of the morphological measurements were in agreement with the results of COX1, which enabled them to validate a new species, T. dongnaiensis [15]. Furthermore, COX1 sequences confirmed the difference between T. mekongensis and T. tambra despite their morphological similarity. Without morphological data, Esa et al. reported the difference between T. tambroides and T. douronensis based on COX1 sequences [44,111]. A group of unique haplotypes was observed and subsequently proposed as a cryptic lineage of T. douronensis [111]. COX1 has also been used to study the genetic diversity of T. tambroides from Sumatra, Indonesia [107]; due to the highly conserved nature of the marker, very low genetic diversity was reported.

From our first phylogenetic analysis for complete sequences of the COX1 gene of Tor and Neolissochilus, the phylogenetic tree constructed demonstrates that T. tambroides sample JX444718.1 was clustered together with T. tambra sample KJ880044.1 and closely related to sample AP011372.1 (presumed as T. tambroides) (Figure 3), while T. douronensis, T. putitora, T. sinensis, and T. tor are not clustered together. Furthermore, all listed Neolissochilus samples were separated from Tor species. In our second phylogenetic analysis, the phylogenetic tree demonstrates that T. douronensis, T. sinensis, T. tambra, and T. tambroides are distinct species (Figure 4). Similar to Walton et al. (2017), our present phylogenetic tree analyses show that the T. dongnaiensis and T. mekongenesis samples, collected by Hoang et al. (2015), and the sample KT001033.1, collected in Malaysia (presumed as T. tambroides), are clustered together with T. tambra, collected in Indonesia and Malaysia (see Figure 4) [13,15]. In contrast with findings by Kottelat (2013) and Robert (1993; 1999), our phylogenetic tree of complete and partial/short sequences of COX1 demonstrates that T. douronensis is not closely related to T. tambra and T. tambroides [36,52,80].

5.1.2. Cytochrome b (Cyt b) and ATPase 6/8 Gene

Besides COX1, the Cyt b gene has also been used as a reliable and useful universal marker for fish identification and specification [169]. The length of the Cyt b gene is about 1143 bp with low variability. However, a longer sequence is postulated to increase or improve species identification and specification [170]. Other mitochondrial gene sequences used as universal markers are ATPase 6 and ATPase 8 genes (referred to as ATPase6/8), located next to each other and overlapping by six base pairs [108]. This gene sequence is characterized by its high mutation rate, and is suitable to distinguish species with high haplotype diversity [171]. Identification of the Tor species, based on Cyt b, ATPase6/8, and 16S rRNA, by Nguyen et al. (2008), found two distinct lineages in samples of T. douronensis from the Mekong River Basin, Sumatra, and Borneo. They suggested that the samples from the Mekong River Basin may be T. tambra [1]. Sati et al. (2013) and Sah et al. (2020) used Cyt b and ATPase6/8 to distinguish between T. putitora and T. tor [106,108].

A phylogenetic tree was constructed by aligning the partial/short sequences of the Cyt b gene (T. douronensis, T. putitora, T. sinensis, T. tambra, T. tambroides, and T. tor) with twelve complete sequences of the Cyt b gene of Tor and Neolissochilus species as a reference (Figure 5). The phylogenetic tree of Cyt b shows a similar pattern to the COX1 phylogenetic tree, where T. douronensis, T. putitora, T. sinensis, T. tor, and Neolissochilus sp. are separated from each other (see Figure 5). T. tambra and T. tambroides are clustered together but separated from other listed Tor and Neolissochilus species. These T. tambra and Tor samples presumed as T. tambroides are from Peninsular Malaysia and Sumatra, Indonesia. This finding shows that the T. tambroides in Malaysia require a re-visit and supports Walton et al. (2017) [13]. Notably, the present phylogenetic analysis indicates that the T. douronensis are grouped into two clusters. Some of the T. douronensis, clustered together with T. sinensis, are from China and Vietnam. Hence, the presumed T. douronensis could be T. sinensis. Furthermore, the T. douronensis from Indonesia, and the two T. tambroides (DQ464985.1 and EF588065.1) from Vietnam and Malaysia are closely related to Neolissochilus.

5.1.3. 16S rRNA Gene

Previous studies have shown that the 16S rRNA gene sequence is also a useful marker for species identification [167]. Despite its highly conserved nature, this sequence also shows polymorphism due to a high level of deletions and insertions [172]. Nguyen et al. (2006) used 16S rRNA as a marker for identifying T. tambroides and T. douronensis in the wild broodstock collected from Borneo (Sarawak) [31]. Their results emphasized the importance of supporting data from molecular markers for taxonomic identification as they observed two genetically distinct lineages within T. douronensis harboring the same morphological characteristics. Furthermore, interspecific hybridization has been raised as one possible reason for the presence of a T. tambroides haplotype in T. douronensis [31]. Even though the 16S rRNA gene sequence has been used and classified as a universal marker, the current finding shows that this gene is less powerful than COX1 for species identification of congener cyprinids [173].

We generated a phylogenetic tree of the Tor species using the 16S rRNA gene (Figure 6). In line with phylogenetic analyses of COX1 and Cyt b, phylogenetic analysis of 16S rRNA also was able to separate T. putitora, T. douronensis, and T. tor. Meanwhile, T. tambra and presumed T. tambroides samples were clustered together, however, they were separated from other listed Tor and Neolissochilus species. In contrast with phylogenetic analysis of Cyt b (see Figure 5), T. douronensis samples were clustered together into one cluster (see Figure 6). Nonetheless, T. douronensis from China and Vietnam were clustered into one group separated from T. douronensis samples from Indonesia and Malaysia. The T. douronensis from China and Vietnam were clustered together with T. sinensis, similarly to the phylogenetic analysis of Cyt b. In the 16S rRNA phylogenetic analysis, we discovered that the presumed T. tambroides collected in Vietnam (DQ464914.1) were clustered together with Neolissochilus sp., but in the phylogenetic analysis of Cyt b, it was clustered together with T. douronensis from China and Vietnam (see Figure 5). However, the presumed T. tambroides collected in Malaysia (EF588065.1) was clustered together with Neolissochilus sp. (see Figure 6). This was similar the phylogenetic analysis of Cyt b (see Figure 5).

The present phylogenetic analyses of COX1, Cyt b, and 16S rRNA were able to distinguish listed Tor species. Furthermore, these phylogenetic analyses also show a different or inconsistent result for similar samples. Nonetheless, from the outcome of our phylogenetic analyses and supported by the previous review, Walton et al. (2017), we would like to postulate and suggest three things: (1) T. douronensis should be considered as valid, (2) T. tambroides should also be maintained as a valid species, however, the presumed T. tambroides collected in Malaysia requires revalidation as T. tambra, (3) T. dongnaiensis should be reconsidered as a synonym of T. tambra. In addition, we present our suggested list of valid Tor species specifically in the SE Asian region in

Table 4. Suggested valid Tor species in SE Asian region.

No.	Species Name
1.	T. ater (Robert, 1999)
2.	T. douronensis (Valenciennes, 1842)
3.	T. laterivittatus (Zhou and Cui, 1996)
4.	T. mosal (Hamilton, 1822)
5.	T. putitora (Hamilton, 1822)
6.	T. sinensis (Wu, 1977)
7.	T. tambra (Valenciennes, 1842)
8.	T. tambroides (Bleeker, 1854)
9.	T. tor (Hamilton, 1822)
10.	T. yingjiangensis (Chen and Yang, 2004)

. This list will serve as a template or standard for improving or revaluating SE Asia’s Tor species taxonomy.

5.2. Microsatellite DNA Marker

Microsatellites are used for population genetics studies due to their high level of polymorphisms, co-dominant nature, and Mendelian inheritance [174]. Despite various genetic studies on Tor species, there is no study comparing all the Tor species present in SE Asian countries. A microsatellite is a repetitive sequence of DNA with specific motifs repeating up to 50 times, occurring at thousands of locations within the genome. Microsatellites are also known as simple sequence repeats or simple sequence length polymorphisms. This sequence is highly susceptible to mutation compared to other DNA regions, resulting in a high level of polymorphisms. Despite their advantages (e.g., highly polymorphic, co-dominant, and Mendelian inheritance nature), microsatellite markers require prior knowledge whereby the primers used for amplification have to be developed explicitly from flanking sequences of that species (or related species) [174]. Only a few microsatellite primers have been designed for the Tor species, e.g., 10 polymorphic loci developed from T. tambroides DNA [30]. A study by Nguyen (2008) shows that, based on nine microsatellites, they gained information useful for the conservation of T. douronensis in seven river systems in Sarawak. The study shows that the genetic variation in each population is low. Due to the high level of polymorphisms of microsatellites, it is difficult to find the “diagnostic loci” monomorphic on different alleles in different species. There are no diagnostic microsatellite loci between T. tambroides and T. douronensis; microsatellites show a longer genetic distance between species (d = 0.1412 − 0.2201) than within species (0–0.0134 for T. tambroides and 0.0128 for T. douronensis) [43]. Microsatellite analysis, when used together with the COX1 gene to compare differences between T. tambroides and T. douronensis, does not resolve the ambiguity, which suggests that microsatellite DNA is not a powerful method for species identification.

6. Conclusions

Collectively, there are 13 Tor species reported to inhabit SE Asian rivers, except in the Philippines and Timor-Leste. Nonetheless, research related to the Tor species, particularly in Myanmar, Lao PDR, Cambodia, and Brunei has remained scarce (fewer than five studies) and, therefore, requires more attention. Furthermore, out of 13 Tor species, the distribution of T. laterivittatus and T. yingjiangensis remains elusive as there are fewer than five reports and no new research has been conducted on these two species since 2011 and 2017, respectively. From this review, it is clear that limited data and the rapid decline in the Tor species population indicate an urgent need for collaborations within SE Asian countries because failure to recognize a distinct taxon may lead to its extinction.

Even though 13 Tor species have been reported, phylogenetic analysis based on genetic markers classifies T. tambroides from Malaysia and T. dongnaiensis and T. mekongensis from Vietnam as T. tambra. Furthermore, T. douronensis is not synonym of T. tambra nor T. tambroides. Therefore, based on phylogenetic analysis, we postulate that only ten species inhabit the SE Asian rivers. This would serve as a standard template for future comprehensive studies of SE Asia’s Tor taxonomy, particularly of T. tambra, T. tambroides, and T. douronensis. In terms of the speciation of SE Asia’s Tor, multiple phylogenetic analyses using sequences of different genes simultaneously and morphological analysis would enable species identification.

To date, the whole genome and transcriptome sequences of the Tor species are not available. Sequencing of the Tor species genome and transcriptome will provide a powerful tool to address questions of evolutionary biology, species identification, morphological variations, and sequences of genes related to reproduction, sex differentiation, and growth, which will be useful for conserving the Tor species.