Mekong River Commission

Fishes of the Mekong – how many species are there?

By Kent G. Hortle*

Recent records suggest that about 850 ‘freshwater' fish species occur in the basin, but many more marine and coastal fishes may at times enter the waters of the Mekong delta

Introduction
Loss of biodiversity is a major concern for biologists, who warn that we are now well into 'the century of extinctions' during which conservation biology is likely to become an increasingly important discipline (Dubois 2003). While there is increasing understanding of the need to manage habitat to maintain biodiversity, there is much less awareness that rational management of ecosystems faces a major '‘taxonomic impediment', the incompleteness and inaccuracy of inventories of biodiversity, which are the starting point for any conservation program.

Tropical rivers are generally among the most diverse ecosystems and their biodiversity is particularly threatened by habitat changes and introduction of exotic species (Coates et al. 2003, Dudgeon et al. 2006). The Mekong system is thought to be an international 'hotspot' of unusually high biodiversity, so there is an urgent need for an up-to-date inventory of the system's biodiversity, both in an absolute sense and relative to other larger tropical rivers, as well as accurate information on individual species’ distribution and migration patterns.

One element of biodiversity is species richness, the number of species present in a defined area, an easily understood notion and one that is useful in comparisons between systems. The fish species richness of the Mekong system is a topic of particular interest, given the importance of fisheries to Mekong peoples and the claim by many authors that the system has unusually high biodiversity.

It should also be noted that apart from species richness (the subject of this article), other features of biodiversity ensure that the Mekong deserves the title of a biodiversity hotspot and should be given a high priority in conservation. These include the apparently large number of fish families (Kottelat, 2001), a significant proportion of endemic species (about 24%) which include several large or giant fishes, the persistence of the natural flood pulse, and an extraordinary dependence on fisheries by expanding rural populations (Hortle 2009).

What is a species?
The biological species concept (BSC) has been generally accepted by zoologists as the most useful and practical definition of a species as 'actually or potentially reproductively isolated' population(s) of organisms (de Queiroz 2005)¹. The BSC can be seen as a special case of the broader 'evolutionary species concept', which covers asexually reproducing organisms and fossils, and which defines each species as a separately evolving lineage (Wiley 1978). Although there is a lively and ongoing debate about the nature and definition of 'species' (de Queiroz 2005; Hey 2006), in practical terms testing for reproductive isolation is usually not feasible, and genetic studies to reveal phylogeny (evolutionary relatedness) are too expensive to be applied to the many wild populations which require classification. To deduce relationships and to diagnose 'species' taxonomists continue to rely on morphology-variation in features, such as number of gill rakers, number of scales, counts of fin rays or even shape and coloration². Taxonomists generally adjust classifications when evidence is found of intermediates between what were thought to be reproductively isolated populations, or if genetic studies indicate the presence of sympatric but reproductively isolated populations of morphologically similar fishes. The very limited resources available to study in the Earth’s biodiversity in detail, and differences in opinion between taxonomists, ensure that revisions to classification of species that have been described on morphological grounds are likely to continue for the foreseeable future.

The need to adjust for taxonomic effort
The number of species recorded from any particular system tends to increase as species are collected and recorded for the first time (new records) and as new species are found and described, with the rate of addition of species tending to decrease over time to an asymptote. Figure 1 suggests that there are still likely to be many more fish species described from the Mekong catchment, as is also evident from listings of un-described species in recent literature (see below). However, this is not an unusual situation for tropical river systems, indeed it seems likely that the Mekong has by now received comparable or more attention than many other large tropical river systems which are less accessible or subject to security risks. It is also highly likely that in many other tropical river systems flow and habitat alterations and the impact of species introductions may have caused the extinction of many fish species before they could be described. This inference can be reasonably drawn based on the situation in well-studied regions: for example in North America 61 taxa of inland fish are thought to have become extinct and about one third of all inland fish species are considered imperilled (Jelks et al. 2008).

Some other factors that affect species richness
As well as possible artefacts introduced by differences in taxonomic effort, comparisons between systems should take account of other factors which are well-known to be correlated with species richness; for example, the ecoregion in which the system is situated (Abell 2008), the fact that tropical systems are generally richer in species than temperate systems, and the 'species-area effect': more species are found in larger areas, other factors being equal. Welcomme (1985 Fig 5.1) used data from 47 major rivers to show that fish species richness is well correlated with the size of the river catchment, one indicator of available habitat area. Lévêque et al. (2008) provide updated graphs of the relationship between the size of some major river systems and their species richness, and de Silva et al. (2007 Fig 1.) show that land area alone accounts for over 70% of the variance in published 'finfish' species estimates between countries in east and southeast Asia. It is to be expected therefore that the Mekong will support more species than other smaller tropical river systems in the Oriental region because of the large extent of available aquatic habitat within its catchment of about 800,000 km².

The limits of the Mekong
Defining a river system's boundaries by its catchment is straightforward, but defining its seaward extent is more problematic. MRC publications generally define the boundary geographically as a line across the mouth, but a more useful ecological boundary would be based on salinity. In the dry season, saline water extends at least 60 km inland, whereas in the wet season a surface layer of fresh water extends several kilometres offshore in the river's plume. Although not well-documented for the Mekong, it can be assumed that the downstream limit of distribution of strictly freshwater species, as well as the penetration of marine species into the estuary changes with season. Estuary fishes tend to prefer water of a particular salinity so they are also likely to move seasonally. Any reference to species counts of the system should therefore refer to whether the figure includes fishes normally found in brackish or marine waters that may only be present in the 'Mekong' seasonally or occasionally.

Published species estimates
Published estimates of the number of species in the Mekong system appear to have increased rapidly in the last few years, as can be seen from the following brief chronology:

• Taki (1978) reviewed information up to the early 1970s and estimated that there were about 300 species known from the lower Mekong basin.
• Welcomme (1985 Fig 5.1) estimated that there were about 600 species in the Mekong, and ranked the system third in terms of its species richness after the Amazon and the Zaire, both of which are larger rivers
• Rainboth (1996) recorded 475 species from Cambodia and estimated that the Mekong "has about 1200 species of fish" (p. 14) and reiterated this estimate in an interview (Jensen 1996). This estimate however was clearly his expectation of the total that might be eventually found after more collection and taxonomic work.
• Kottelat (2001 p19) stated that there were documented records of about 700 species from the Mekong basin, and because there were no data to support claims of higher figures he suggested that they should not be quoted.

Several authors provided similarly high but unsubstantiated figures, perhaps based on Rainboth’s estimate, but misused his figure as if it were a count of species actually recorded or a minimum estimate. In some cases, authors implied that the species count was for the river, when many species are found only in upland tributaries or in the estuary or in specialised habitats such as lakes or caves. Some of these recent species estimates include the following.

• 'There are 1200 recorded fish species' (Sverdrup-Jensen 2002).
• 'It (the Mekong) is a hot spot for biodiversity with between 1200 and 1600 described species of fish' (Campbell 2002).
• 'There are at least 1200 and possibly as many as 1700 (species of fish) living in the Mekong Basin' (Coates et al. 2003).
• 'There are more than 1200 species in the river' (sic) (MRC 2003).

The MRC Fisheries Programme published the first compendium of species of the Mekong, the Mekong Fish Database (MFD), in 2003 and widely distributed it as a CD to fisheries scientists in the region. The MFD contained information on 898 indigenous fishes as well as 24 introduced species that were recorded from or thought likely to be present in the lower Mekong basin and in Yunnan. The fishes can be grouped according to their known occurrence in water of different salinities, using information which has been obtained from FishBase (Froese and Pauly 2009).

Freshwater only:	539 species
Freshwater-brackish:	79 species
Fresh- brackish marine:	113 species
Brackish only:	4 species
Brackish and marine:	115 species
Marine:	48 species

The marine species and many of the brackish/marine species have only been recorded from the sea off the mouth of the Mekong or from nearby marine waters, and records for several of the freshwater species are questionable, so the MFD supports an estimate of about 750 species likely to be found at times within freshwaters of the system, which can be loosely defined as ‘freshwater’ species³. This figure is quite consistent with Kottelat’s estimate, nevertheless higher unsubstantiated estimates continued to be published, for example:

• 'The number of fish species is at least 1200' (Mattson 2004)
• 'The number of fish species that has been found in the (Mekong basin) exceeds 2000' (van Zalinge et al. 2004).

The citation of species numbers seems to represent a case of 'escalation bias', where a figure is quoted that supports a commonly-held belief (the Mekong has 'high' biodiversity) while ignoring the actual counts of species from the system published by Kottelat in 2001 and in the MFD in 2003. The variability between recent published estimates of species richness certainly creates a credibility issue for those seeking to promote conservation of the system's biodiversity, as well as highlighting the practical problems of classification and nomenclature facing those engaged in field studies or management of habitat for fisheries.

 

Figure 1: The Sunda shelf

Showing the maximum extent of sea level regression over the last 250,000 years, the -120 m contour.
Based on (Voris 2000)

 

Figure 2: Species described from the Mekong River system

Cumulative count does not include species recorded that were first described from elsewhere

 

Table 1: Species 'split' since material was compiled for publication of the MFD in 2003

The newly described Mekong species is shown in bold after the species which was formerly considered to occupy a large range on both sides of the Sunda shelf, including the Mekong system

Sister species	Revised distribution summary
Amblyrhynchichthys truncatus (Bleeker, 1851)	Malay Peninsula and Borneo
Amblyrhynchichthys micracanthus Ng & Kottelat, 2004	Lower and Middle Mekong to Vientiane, Chao Phraya, Mae Khlong, Tapi River
Bagrichthys macracanthus (Bleeker, 1854)	Sumatra, Peninsular Malaysia, northern Borneo (Sarawak)
Bagrichthys majusculus Ng, 2002	Lower and middle Mekong River system
Balantiocheilos melanopterus (Bleeker, 1851)	Kalimantan, Malay Peninsula, Sumatra
Balantiocheilos ambusticauda Ng & Kottelat, 2007	Lower and middle Mekong upstream to Nam Ngum, Chao Phraya
Hemiarius stormii (Bleeker, 1858)	Eastern Malay Peninsula and west Borneo
Hemiarius verrucosus (Ng, 2003)	Lower Mekong to Khone Falls and possibly Bang Pakong R., Southeast Thailand
Kryptopterus schilbeides (Bleeker, 1858)	Borneo, Java, Sumatra and Malay Peninsula
Kryptopterus paraschilbeides Ng, 2003	Lower and middle Mekong, Mae Khlong, Bang Pakong R., Chao Phraya
Kryptopterus schilbeides (Bleeker, 1858)	Eastern Sumatra and western and southern Borneo
Kryptopterus paraschilbeides Ng, 2003	Lower and middle Mekong upstream to Vientiane
Ompok hypophthalmus (Fang & Chaux, 1949)	Java and southern Borneo (Barito River drainage)
Ompok urbaini Ng, 2003	Mekong, Chao Phraya and Pasak River drainages
Ompok rhadinurus Ng, 2003	Sumatra, the Malay Peninsula and Borneo
Ompok eugeneiatus (Vaillant, 1893)	Peninsular Malaysia, central Sumatra and western Borneo
Ompok pinnatus Ng, 2003	Lower Mekong River and Chao Phraya drainages
Pangasius kunyit Pouyaud Teugels & Legendre, 1999	Sumatra and Kalimantan (southern Borneo)
Pangasius mekongensis Gustiano, Teugels & Pouyaud, 2003 Lower	Mekong River system
Pangasius sabahensis Gustiano, Teugels & Pouyaud, 2003	Sabah, northern Borneo
Pangasius polyuranodon Bleeker, 1852	Sumatra and Borneo
Pangasius elongatus Pouyaud, Gustiano & Teugels, 2002	Chao Phraya, Mekong and Bang Pakong basins
Pangasius mahakamensis Pouyaud, Gustiano & Teugels, 2002	Mahakam River, East Kalimantan, Borneo
Polynemus dubius Bleeker, 1853	Malay Peninsula, Sumatra, Kalimantan
Polynemus aquilonaris* Motomura, 2003	Chao Phraya and Mekong River basins
Wallago leerii Bleeker, 1851	Southern Thailand, Malay Peninsula and western Indonesia
Wallago micropogon Ng, 2004	Lower and middle Mekong to Luang Phabang, and middle Chao Phraya

* also includes material formerly identified as Polynemus longipectoralis

An updated species count
Since the MFD was published there has been considerable taxonomic work which has resulted in many changes to classification of Mekong system fishes, which include the following.

• 'Splitting' of species: in at least 12 cases a species that was considered widespread in rivers that formerly ran across the Sunda shelf has been 'split' into 'sister' species, a mainland Indochinese (including Mekong) species and one or more 'Sundaic' species, found in river systems to the south (Figure 2, Table 1). Such splitting does not change the species count for the Mekong, only the name applied to the Mekong species. But it can cause confusion and double-counting when distribution records for the Sundaic species are not updated, which is a problem for example in the current version of FishBase. Examples of split species are shown in Table 1 with updated distribution information.
• Description of new species: new species are still being described, either from new collections or when taxonomists re-examine existing collections which may have been stored in museums for many years. At least 16 new inland species have been described from the Mekong River system since the MFD was published in 2003 (Table2), adding to the species count for the system.
• 'Lumping': two or more species formerly thought to be distinct are synonymised when additional examination shows that there is overlap in the range of variation of diagnostic characters between populations that have been named as separate species, or when it is found that a species has been named more than once. Since 2003 'lumping' of species has reduced the count for the Mekong system by about 20 species, more than compensating for the effect of description of new species.
• Revisions of groups: several groups (genera or families) have been revised since the MFD was published, with many new generic names created to better define relationships between species. For example, the widespread genus Botia, which includes several common Mekong species, was split into seven genera by Kottelat (2004), with Mekong species now falling within three genera. Thus the red-tailed botia, formerly Botia modesta is now known as Yasuhikotakia modesta. A revision of the catfish family Ariidae has also led to many new names for genera (Marceniuk and Menezes 2007). Such name changes are relatively easy to follow by updating species lists using FishBase and do not affect the species count for the system.
• Additional distribution records: species that are already described from elsewhere are recorded for the first time in collections from the Mekong system. Such information has not been updated in any systematic way since the MFD was published. Summary distribution records are often woefully inadequate for assessing whether it is likely that a species is present in the area of interest, in some cases simply reading "Asia", while original locality data on museum specimens are often incomplete or erroneous. The lists of fishes published in ecological surveys typically include incompletely identified species (usually noted as 'sp1', 'sp2' etc.) or may contain errors (for example non-Mekong species), hardly surprising where there are many similar species of fishes that are difficult for non-specialists to identify correctly. Ideally, distribution data for inland species in FishBase would accurately identify the major river basins in which each species has been definitely recorded, but it is a major task to check and carefully summarize and update such information. A review and crosschecking of Mekong system species listed in Baird et al. (1999), Kang et al. (2009), Kottelat (2001), Rainboth (1996) and Vidthayanon (2008) suggests that there are about 100 additional species that have been recorded from freshwaters of the Mekong basin, although most of these are not formally described or are of uncertain taxonomic status. Therefore it is reasonable to estimate that there are about 850 'freshwater' species known from the system.

Table 2: New species described from the Mekong River system Since material was compiled for publication of the Mekong Fish Database in 2003

Species	Distribution summary
Scizothorax nudiventris Yang Chen & Yang, 2009	Upper Mekong, Yunnan
Bangana brevirostris Liu & Zhou, 2009	Upper Mekong, Yunnan
Mekongina lancangensis Yang, Chen & Yang, 2008	Upper Mekong, Yunnan
Minyclupeoides dentibranchialus Roberts, 2008	Lower Mekong, Cambodia
Oreoglanis jingdongensis Kong, Chen & Yang, 2007	Upper Mekong, Yunnan
Pareuchiloglanis abbreviatus Li, Zhou, Thomson, Zhang & Yang, 2007	Upper Mekong, Yunnan
Pareuchiloglanis prolixdorsalis Li, Zhou, Thomson, Zhang & Yang, 2007	Upper Mekong, Yunnan
Tonlesapia tsukawakii Motomura & Mukai, 2006	Great Lake, Tonlé Sap drainage in Cambodia.
Polynemus bidentatus Motomura & Tsuwaki, 2006	Mekong delta, Viet Nam
Pseudobagarius filifer (Ng & Rainboth, 2005)	Tonlé Sap drainage near Phnom Penh, Cambodia
Pseudobagarius nitidus (Ng & Rainboth, 2005)	Mekong River downstream of Khone Falls near Cambodia-Lao border
Akysis fuliginatus Ng & Rainboth, 2005	Lower Mekong River in northern Cambodia
Betta stiktos Tan & Ng, 2005	Mekong Drainage, Cambodia
Schistura bannaensis Chen, Yang & Qi, 2005	Upper Mekong, Yunnan
Oreoglanis macronemus Ng, 2004	Xieng Khouang, Lao PDR
Hemimyzon ecdyonuroides Freyhof & Herder, 2002	Headwaters of Se San River, Viet Nam

 

The count of all fish species from 'the river system' is likely to be heavily influenced by the coastal species which enter fresh water for short periods, or marine species which may cross the geographical boundary (a line across the mouth) during dry periods. Systematic collection and accurate identification of fishes from the delta region would do much to remove the uncertainty in species counts and resolve the discrepancies in published estimates. If marine and coastal 'visitors' are included, there are about 1100 species recorded from or thought likely to occur in the system at times based on their occurrence in the Mekong's plume. More species will continue to be found, but at present there are no grounds for quoting higher estimates.

Conclusion
The available data indicate that there are about 850 freshwater fish species recorded from the Mekong (a total that includes some undescribed species of uncertain status), with a total estimate of about 1100 if the possible coastal or marine visitors are included. As mentioned above, comparison with other river systems should take into account the biogeographic province or ecoregion, the area of each system, the relative taxonomic effort that has been applied, and the extent to which biodiversity may have been lost before it could be documented. If it is evident that the Mekong does indeed have more species than expected for a river of its size and location, it would be of interest to explore the possible causes, which might include its recent geological history and the geography of the system.

Unless great care is taken, comparisons with other river systems may lead to erroneous conclusions. For example, according to FishBase there are 405 species recorded from the Zaire (Congo) and 1212 species recorded from the Amazon, but these are clearly under-estimates, both because existing records have not been systematically compiled and because many species remain to be described. Roberts (1975) recorded 650 fishes from the Zaire basin and Tedesco et al. (2005) quoted a figure of 686 species, but Lévêque et al. (2008) cautioned that existing information for this basin needs to be synthesised and that many new discoveries are anticipated. For the Amazon, Tedesco et al. (2005) used an estimate of 1761 species and Lévêque et al. (2008) recorded 2416 species, but Kottelat (2001) cited an opinion that the number of species in South America could more than double with new collections and taxonomic effort. The Amazon's catchment is about seven times and the Zaire five times the size of the Mekong's, so it is likely that they support more fishes, but at present it is impossible to know whether any of these large river system's species richness is above or below the average that would be predicted for large tropical rivers of their size.

Acknowledgments
The Water Studies Centre of Monash University kindly provided office space and library facilities to assist in the preparation of this paper. I thank Eamonn Hortle for assisting with compilation of species information from FishBase and Chris Burridge and Chris Barlow for helpful comments on a draft manuscript.

*Mr Hortle was a technical officer at the MRC Fisheries Programme from 2001 to 2005 and currently works as a consultant on fisheries and environment

References
Abell (2008) Freshwater ecoregions of the world: a new map of biogeographic units for freshwater biodiversity conservation. BioScience 58, 403-414.

Baird IG, Inthaphaisy V, Kisouvannalath P, Phylavanh B, Mounsouphom B (1999) The Fishes of southern Lao. Ministry of Agriculture and Forestry, Vientiane, Laos. 161 pages. (In Lao).

Berra TM (2001) Freshwater Fish Distribution. Academic Press: San Diego, USA. 604 pages.

Campbell IC (2002) The Mekong River - water, people and fisheries in a politically complex river basin. Paper presented in 'Enviro Flows 2002 4th Ecohydraulics Conference, 4th February 2002, Capetown, South Africa.' 15 pages.

Coates D, Ouch P, Suntornratana U, Nguyen TT, Viravong S (2003) Biodiversity and fisheries in the Mekong River Basin. Mekong Development Series 2: 1-30.

de Queiroz K (2005) Ernst Mayr and the modern concept of species. Proceedings of the National Academy of Sciences 102: 6600-6607.

de Silva SS, Abery NW, Nguyen TTT (2007) Endemic freshwater finfish of Asia: distribution and conservation status. Diversity and Distributions 13: 172-184.

de Queiroz K (2005) Ernst Mayr and the modern concept of species. Proceedings of the National Academy of Sciences 102, 6600-6607.

Dubois A (2003) The relationships between taxonomy and conservation biology in the century of extinctions. Comptes Rendus Biologies 326, Supplement 1: 9-21.

Dudgeon D, Arthington AH, Gessner MO, Kawabata Z-I, Knowler DJ, Lévêque C, Naiman RJ, Anne-Hélène, Prieur-Richard, Soto D, Stiassny MLJ, Sullivan CA (2006) Freshwater biodiversity: importance, threats, status and conservation challenges. Biological Reviews 81: 163-182.

Froese R, Pauly D (2009) (Eds) FishBase. World Wide Web Electronic Publication. www.fishbase.org, version (03/2009).

Hey J (2006) On the failure of modern species concepts. Trends in Ecology and Evolution 21, 447-450.

Hortle KG (in press) Fisheries of the Mekong River basin. Ch 9 in: The Mekong: Biophysical Environment of an International River Basin. (Ed. IC Campbell). Elsevier Publishers Amsterdam, the Netherlands.

Jelks HL, Walsh SJ, Burkhead NM, Contreras-Balderas S, Diaz- Pardo E, Hendrickson DA, Lyons J, Mandrak NE, McCormick F, Nelson JS, Platania SP, Porter BA, Renaud CB, Schmitter-Soto JJ, Taylor EB, Warren ML (2008) Conservation status of imperiled North American freshwater and diadromous fishes. Fisheries 33: 372-386.

Jensen JG (1996) 1,200 different fish species in the Mekong basin. Catch and Culture 2: 4-5.

Kang B, He D, Perrett L, Wang H, Hu W, Deng W, Wu Y (2009) Fish and fisheries in the Upper Mekong: current assessment of the fish community, threats and conservation. Reviews in Fish Biology and Fisheries.

Kottelat M (2001) Fishes of Laos WHT Publications Pty Ltd: Colombo, Sri Lanka. 198 pages. Kottelat M (2004) Botia kubotai, a new species of loach (Teleostei: Cobitidae) from the Ataran River basin (Myanmar), with comments on botiine nomenclature and diagnosis of a new genus. Zootaxa 401: 1-18.

Lévêque C, Oberdorff T, Paugy D, Stiassny MLJ, Tedesco PA (2008) Global diversity of fish (Pisces) in freshwater. Hydrobiologia 595: 545–567.

Marceniuk AP, Menezes NA (2007) Systematics of the family Ariidae (Ostariophysi, Siluriformes), with a redefinition of the genera. Zootaxa 1416: 1-126.

Mattson NS (2004) Conservation and enhancement of fisheries - the case of the lower Mekong basin. Paper presented in 'INTECOL, 7th International Wetlands Conference, 23-30 July 2004'. Utrecht, the Netherlands 16 pages.

MRC (2003) State of the Basin Report 2003. Mekong River Commission: Phnom Penh, Cambodia. 300 pages.

Rainboth WJ (1996) Fishes of the Cambodian Mekong. FAO: Rome, Italy. 265 pages + plates pages.

Rieppel O (2006) The PhyloCode: a critical discussion of its theoretical foundation. Cladistics 22: 186-197.

Roberts TR (1975) Geographical distribution of African freshwater fishes. Zoological Journal of the Linnean Society 57, 249-319.

Sverdrup-Jensen S (2002) Fisheries in the lower Mekong Basin: status and perspectives. MRC Technical Paper 6: 1-95.

Taki Y (1978) An analytical study of the fish fauna of the Mekong Basin as a biological production system in nature. Research Institute of Evolutionary Biology Special Publications 1: 1-77.

Tedesco PA, Oberdorff T, Lasso CA, Zapata M, Hugueny B (2005) Evidence of history in explaining diversity patterns in tropical riverine fish. Journal of Biogeography 32 1899-1907.

van Zalinge NP, Degen P, Pongsri C, Nuov S, Jensen JG, Nguyen VH, Choulamany X (2004) The Mekong River system. RAP Publication 2004: 335-357.

Voris HK (2000) Maps of Pleistocene sea levels in Southeast Asia: shorelines, river systems and time durations. Journal of Biogeography 27: 1153–1167.

Vidthayanon C (2008) Field Guide to Fishes of the Mekong Delta. (Mekong River Commission: Vientiane, Lao PDR).

Welcomme RL (1985) River fisheries. FAO Fisheries Technical Paper 262: 1-358.

Wiley EO (1978) The evolutionary species concept reconsidered. Systematic Zoology 27, 17-26.

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¹ An alternative 'phylogenetic' system based only upon degree of genetic relationship has been developed recently and is being increasingly used in cladistics (Rieppel 2006).

² The basis for the current system of scientific classification (taxonomy) of animals was set out over 200 years ago by Linnaeus in his Systema Naturae, with rules now formalised in the Code set by the International Commission on Zoological Nomenclature (www.iczn.org).

³ It should be noted that 'freshwater' species have been defined by some authors as only those within primary freshwater families, i.e. families which have a long history of evolution in freshwaters, and if this approach were used the total for the Mekong system would fall to about 530 species because many Mekong fishes are within peripheral division families, those with recent marine ancestors. See: Berra TM (2001) Freshwater Fish Distribution. Academic Press: San Diego, USA.