ANGUILLIFORMES / ANGUILLIDAEANGUILLIFORMES – eelsgill opening (yellow line;not across isthmus)

Synbranchid eels
→ pp. 344–346

gill opening (yellowline; continuousacross isthmus)

AFamily: Anguillidae (FC: 074)
Size: 90 cm SL (Kottelat, 2001a: 26).
Distribution: Mekong Basin in Laos, Thailand, Canbodia, andVietnam; Indo-Pacific.
Notes: A catadromous species, usually found in freshwater
rivers and streams, and brackish estuaries; it is also found insmall upland streams above large water falls.
Anguilla, the only genus in the family Anguillidae, differsfrom the other eels of the Mekong by having projecting lower
jaw and minute sclaes on the body. Two species, viz., A. marmorata and A. bicolor, are known from this region. Anguilla marmorata differs from A. bicolor (below) by having mottled bodyand much longer dorsal fin, which extends anteriorly wellbeyond a vertical line through the anus.

Bdorsal-fin origin
anal-fin origin

body mottled

minute scales on bodylower jaw projecting
beyond upper jawAnguilla bicolor McClelland, 1844

dorsal-fin origin

Family: Anguillidae (FC: 074)
Size: 100 cm TL (Kottelat et al., 1993: 7).
Distribution: ?Mekong Basin in Cambodia, Vietnam, andChina (Yunnan); Indo-West Pacific.
Notes: A catadromous species, usually found in brackish estuaries and adjacent freshwater areas. Some researchers have recorded Anguilla bicolor from the Mekong (e.g., Kottelat, 1989a;
Rainboth, 1996b; Vidthayanon, 2008, as A. bicolor pacificus;
Rainboth et al., 2012), although Roberts in Roberts & Warren(1994: 108) noted, "I have not yet seen any specimens orphotos of Mekong Anguilla that could be identified as Anguillabicolor." During our field surveys in 2007–2013, we could notconfirm any specimens of A. bicolor from the Mekong. A recent
record of a specimen of A. bicolor from the Mekong in Yunnan(Chen et al., 2010a) needs re-confirmation regarding its origin.

46A) UNMF uuncatalogued (caudal part heavily damaged, photo: CG); B) aquarium fish, collected from the Mekong in Ubon Ratchathani (photo: KS)

Anguilla marmorata Quoy & Gaimard, 1824

Anguilliform eels

lower jaw projecting
beyond upper jaw

minute scaleson bodyanal-fin origin
body not mottled

Non-Mekong specimen, taken at an aquarium shop in Japan (photo: KS)

Several groups of eels and/or eel-like fishes are found in theMekong, such as anguilliform eels (pp. 46–51), swamp eels
(synbranchids, pp. 344–346), earthworm eels (chaudhuriids, p.
347), spiny eels (mastacembelids, pp. 348–351), and eel gobies(a part of gobiids, pp. 482–485). These fishes have a flexible,
markedly elongated snake- or worm-like body; scales on thebody are absent or, if present, usually minute, thin and easily
overlooked. Of these eels/eel-like fishes found in the Mekong,
the anguilliform eels are strikingly similar to the synbranchideels in general appearance, and they merely differs externallyby having a pair of gill openings usually not continuous at
the ventral surface (vs. gill openings merging into a single
slit across the isthmus; see right figures). Despite their great
similarities, the anguilliform eels are not closely related to thesynbranchid eels (see also introduction of the elopiform fishes,
pp. 52–53).
The anguilliform eels are marine-oriented fishes; althoughseveral species are known to enter brackish and/or freshwater
areas, they spawn eggs in the marine waters. This is also true ofall Mekong species shown here. Many of the Mekong species
are known from the lower reaches or brackish estuaries, except
for one, Anguilla marmorata, which goes up to Laos.

MURAENIDAE
Gymnothorax sp.
Family: Muraenidae (FC: 079)
Size: 38.8 cm TL (Rainboth et al., 2012, pl. 8, fig. 154, as
Gymnothorax sp. 2).
Distribution:Mekong Basin in Vietnam; Gulf of Thailand.

gill opening a small
crescent-like slit

no pectoral fins

133 vertebrae

dorsal and anal fins with a narrow white margin

CTU-P 4851 (photo: LXT)

head and body pale gray or beige
dorsally, whitish ventrally, with nopaticular markings

Various kinds of estuarine eels, including at least 18 individuals of Gymnothorax sp. (yellow arrows), being dried (Bến Tre,Vietnam, photo: KS)

Notes: A relatively small-sized species of moray, found in coastal marine waters and in brackish estuaries. It is presumed to be
an undescribed species (K. Hatooka, pers. comm.). Both of 2 unidentified species of Gymnothorax of Rainboth et al. (2012: 58,
pl. 8, figs 153 and 154) appear to be conspecific with this fish.
The photographed specimen shown here was collected from afish market at Sóc Trăng, Vietnam; the photograph below shows
various small eels (being dried), including Gymnothorax sp.
(present species), some ophichthids (e.g., Pisodonophis boro), andOphisternon bengalense (a synbranchid), at Bến Tre, Vietnam.
Eels of the Muraenidae, commonly known as morays, havea small rounded gill opening and no pectoral fins. Many of themuraenids are confined to marine waters, but some can enter
brackish estuaries and the adjacent freshwater areas. At least 2species, Gymnothorax pseudothyrsoides and Gymnothorax sp.
(shown here), are known from the Mekong Delta (Rainboth etal., 2012; Tran et al., 2013), as well as some other species withdoubtful identification. Note that the photographed specimensof a muraenid Strophidon sathete of Vidthayanon (2008: 29, fig.
21) and that of Rainboth et al. (2012, pl. 8, fig. 157) are actually
Ophichthus lithinus (an ophichthid) and the other unnamed
species of Gymnothorax, known from the Gulf of Thailand (T.
Hibino, pers. comm.), respectively; the latter is currently being
investigated by T. Hibino and his collaborators.

47

MORINGUIDAE & OPHICHTHYIDAE
Moringuidae, indet. gen. & sp.
Family: Moringuidae (FC: 076)
Size: 39.8 cm TL (CTU-P 1498).
Distribution: Mekong estuary in Vietnam.

Neenchelys parvipectoralis Chu, Wu & Jin, 1981
Family: Ophichthyidae (FC: 081)

headdorsal-fin origin(behind anus)

caudal partthroat swollen, with numerous
branchiostegal rays (true of allophichthyids)

Size: 32.8 cm TL (Ho et al., 2013: 12).
Distribution: Mekong estuary in Vietnam; China, Taiwan, andcentral Vietnam (off Nha Trang).
Notes: A relatively small-sized worm eel, found in coastal
marine waters (to 300 m depth). The photographed specimen
shown here was collected from brackish estuary of Cung HầuRiver (a distributary of the Mekong) in Trà Vinh Province ofVietnam, together with an additional specimen.
Family Ophichthidae comprises 2 subfamilies, viz., Myrophinae and Ophichthinae (Smith & McCosker, 1999; Nelson,2006). Of these, the ophichthine eels are easily distinguishedfrom the other eels by their hard, pointed and finless tip of
the tail (as shown in Ophichthus and Pisodonophis in thenext page), and they are commonly known as "snake eels".
At least 7 species of the ophichthine eels, including 5 speciesof Ophichthus (O. lithinus, O. lumbricoides, O. macclellandi,
O. rutidodermatoides, and O. rutidoderma) and 2 speciesof Pisodonophis (P. boro and P. cancrivorus), were hitherto
recorded from the Mekong (Rainboth et al., 2012); 3 suchspecies are shown in the next page. Note that the photographed
specimen of a muraenid Strophidon sathete of Vidthayanon(2008: 28, fig. 21) is actually Ophichthus lithinus (Rainbothet al., 2012, pl. 9, figs. 178; T. Hibino, pers. comm.). The
ophichthids of the other subfamily Myrophinae, the worm
eels, have a distinct caudal fin, as in Neenchelys shown here.
Like the ophichthines, the myrophines can be distinguishedfrom the non-ophichthid eels by their numerous, overlapping
branchiostegal rays (can be seen externally in the ventral view).
Neenchelys can be distinguished from the other myrophine
genera by having the characteristic of the following combination: long, conical and acutely pointed head and posterior nost-48

CTU-P 1498 (photo: KS)

position of anus
(at posterior halfof body)

pectoral fin
rudimentalhead

tip of tail soft,
with shortcaudal fin

caudal partril above the upper lip (McCosker, 1982; Smith & McCoster,1999). Neenchelys comprises 9 species (Hibino et al., 2012; Hoet al., 2013). Of these, N. parvipectoralis resembles N. mccoskeri(known from Japan and Taiwan) and N. microtretus (Red Sea) in
having a very small pectoral fin (shorter than snout), but differsin having 138–148 total vertebrae (vs. 172–184 and 151 in N.
mccoskeri and N. microtretus, respectively) (Ho et al., 2013).
Initial identification of our Mekong specimens was made byH.-S. Ho (Academia Sinica, Taipei) (see Ho et al., 2013: 12).
This species, N. parvipectoralis, is the only species of the genus
from the Mekong Basin. Rainboth et al. (2012: 58) listed thecongener N. buitendijki in their book of fishes of the "GreaterMekong Ecosystem" (a term meaning the entire region directly
affected by the Mekong), but it was based on the record by
Fourmanoir & Nhung (1965) from Nha Trang, central Vietnam;
identification of Fourmanoir & Nhung's specimen(s) needs
confirmation, since Ho et al. (2013) recorded N. buitendijki fromIndia, Indonesia and Malaysia in their review of Neenchelys.

CTU-P 1163 (photo: KS)

Notes: It is known only by a single specimen, collected fromthe tidal canal in Bến Tre, Vietnam.
Eels of the family Moringuidae resemble some ophichthids
in general appearance, but they have a caudal fin (vs. mostophichthids which lack a caudal fin) and lack numerous overlapping branchiostegal rays (Smith, 1999); see also "Notes" of
Neenchelys parvipectoralis, below. Two genera, viz., Moringua
and Neoconger, are known in the family (Smith, 1989, 1999).
The specimen from the Mekong resembles Neoconger by
its short body (for moringuids) and slightly projecting snout,
however the other characteristics (positions of anus and dorsal/
anal-fin origin and counts of vertebrae and fin rays) match withMoringua (rather than Neoconger). This specimen also has 130
vertebrae, 110 dorsal-fin rays, 119 anal-fin rays, and no lateralline pores; the body depth at anus is 3.5% of TL. A poorlyknown species Moringua floresiana, described by Weber & deBeaufort (1916) from the Flores Islands, is possibly identicalto this species, although this nominal species was questionably
assigned as a junior synonym of M. raitaborua by Kottelat(2013c). This species can be easily distinguished from all otheranguilliform eels of the Mekong by the positions of anus andorigin of the dorsal fin (see right figure).

OPHICHTHYIDAE
Ophichthus rutidoderma (Bleeker, 1852)
Family: Ophichthyidae (FC: 081)
Size: 95 cm TL (Rainboth, 1996b: 58).
Distribution: Mekong Basin in Cambodia and Vietnam; IndoWest Pacific.

Pisodonophis boro (Hamilton, 1822)
Family: Ophichthyidae (FC: 081)

body greatly elongate (comparing withPisodonophis boro, below)

throat swollen, with numerous branchiostegal rays (true of all ophichthyids)head

Size: 100 cm TL (Rainboth, 1996b: 58).

caudal tip pointedand hard, withoutcaudal fin

Distribution: Mekong Basin in Vietnam; Indo-West Pacific.
Notes: A large-sized snake eel with hard and pointed caudal tip,commonly found in brackish estuaries and adjacent freshwater
areas.
Pisodonophis resembles Ophichthus (above) in generalappearance and in particular the hard and pointed caudal tip, but
it is distinguishable by having molariform or granular vomerine
teeth (vs. non-molariform/granular in Ophichthus). At least 2
species of Pisodonophis were recorded from the Mekong, and,in this region, P. boro is more common than P. cancrivorus(below). Pisodonophis boro is readily distinguished from P.
cancrivorus by having more elongate body (see photos) andthe dorsal-fin origin behind the tip of the pectoral fin (vs. origin
above the pectoral fin in P. cancrivorus).
Rainboth (1996) suggested possible occurrences of Pisodonophis boro and P. cancrivorus from the Cambodian Mekong.
During our surveys in the Indochinese Mekong in 2007–2013,
we collected these 2 species from the estuarine areas of only
Vietnam.

Pisodonophis cancrivorus (Richardson, 1848)

caudal partvomerine teeth
not molariform/
granularIFREDI-P 4840 (photo: PT)

headvomerine teeth
molariform or
granularcaudal part

dorsal-fin origin behind tip ofpectoral fin (vs. above pectoral
fin in P. cancrivorus, below)

CTU-P 1421 (photo: KS)

Notes: A large-sized snake eel with a hard and pointed caudal
tip, found in brackish estuaries and the adjacent freshwater
areas. The photographed specimen shown here was collectedby "Dai", a large-scale fisheries using bag nets, in the Tonle
Sap River, Cambodia.
This eel resembles the estuarine eels of the genus Pisodonophis (below), but can be separated by having a more elongatedbody and non-molariform/granular vomerine teeth (vs. molariform or granular in Pisodonophis). In addition to this species,
Rainboth (1996b: 58) noted "at least three additional speciesof this genus are recorded from Vietnam and may also occur in
the Cambodian Mekong." However, he did not give any namesof the species of Ophichthus other than O. rutidoderma.

dorsal-fin origin above pectoral fin (vs. behind
tip of pectoral fin in P. boro, above)

Family: Ophichthyidae (FC: 081)
Size: 75 cm TL (Rainboth, 1996b: 58).
Notes: A large-sized snake eel with a hard and pointed caudal
tip, commonly found around the estuarine areas of the Vietnamese
Mekong.
Of the 3 similar-looking species of the ophichthid eels recorded from the Mekong (viz. Ophichthus rutidoderma, Pisodonophis
boro and P. cancrivorus, all of them are shown in this page), P.
cancrivorus differs from the other 2 by having a non-elongated,deeper body (see photographs, right). The photographed specimenof P. cancrivorus in Vidthayanon (2008: 31, fig. 23) from theMekong Delta has a markedly elongate body, and it appears to be
not of this species.

caudal tip pointed andhard, without caudal fin

CTU-P 4915 (photo: HVM)

Distribution: Mekong Basin in Vietnam; Indo-West Pacific.

vomerine teeth
molariform or
granular49

50

Marketed fish, including Muraenesox bagio (Muraenesocidae), Scatophagus argus (Scatophagidae), Epinephelus coioides (Serranidae), Lates calcarifer (Latidae), and Megalops cyprinoides (Megalopidae), beside a road at Bến Tre, Vietnam (photo: KS)

MURAENESOCIDAE
Muraenesox bagio (Hamilton, 1822)
Family: Muraenesocidae (FC: 084)
Size: 180 cm TL (Smith, 1999: 1676).
Distribution: Mekong estuary in Vietnam; Indo-West Pacific.
Notes: A large-sized species of pike congers, found in thecoastal marine areas with muddy bottoms; it is commonly enters
adjacent brackish estuaries.
Eels of the family Muraenesocidae have a long and acutesnout, large teeth on jaws, a developed pectoral fin, and a large
gill opening (the interspace between the sides of gill openings ismuch smaller than the gill opening length). Two muraenesocidgenera, Muraenesox and Congresox, have been recorded from
the Mekong Delta. Superficially Muraenesox resembles Congresox, but it differs in having more slender vomerine teeth with arounded cross-section (vs. triangular and compressed in Muraenesox).
Two species of Muraenesox, M. bagio and M. cinereus, arefound in the Mekong; both are shown here. Muraenesox bagio
is readily distingished from the congener M. cinereus by having
much more slender and acute snout, and only 33–39 lateral-line
pores before th eanus (vs. 40–47 in M. cinereus).
Although we have not collected any voucher specimensfrom the Mekong, occurrences of 2 species of Congresox areexpected from the estuarine areas. Actually the one of them, C.
talabon, was recorded from the Mekong Delta by Vidthayanon(2008: 28, fig. 19); he noted that the species is an "uncommon
visitor of Delta." Congresex talabon resembles C. talabonoides, but it differs in having longer pectoral fin (pectoral-fin
length of 3.2 in the head in C. talabon vs. 4.1 in the head in C.
talabonoides) and a slightly higher count of lateral-line pores
before the anus (41–42 vs. 35–40) (Smith, 1999).

snout elongate and pointed

gill opening large (interspace
between both sides of gill
openings much smaller thanlength of gill opening)
33–39 lateral-line
pores before anus

vomerine teeth prominent, nearly
triangular and compressed

CTU-P 1484 (photo: KS)

snout narrow andacutedorsal view of headtip of lower jaw with enlarged teeth

Muraenesox cinereus (Forsskål, 1775)
Family: Muraenesocidae (FC: 084)
Size: 80 cm TL (Smith, 1999: 1677).
Distribution: Mekong estuary in Vietnam.
Notes: Another species of Muraenesox found in the Mekong
estuary. See "Notes" of the congener M. bagio, above.

vomerine teeth
prominent,
nearly triangularand compressed

40–47 lateral-line
pores before anus

CTU-P 1818 (photo: LXT)

snout broader

tip of lower jaw withenlarged teeth
dorsal view of head51