ELOPIFORMES / ELOPIDAE
ELOPIFORMES – tenpounders
Fishes of the order Elopiformes are silvery active swimmers,
found in coastal marine waters, brackish estuaries and theadjacent freshwater areas. Despite their herring- or cyprinidlike appearances, the elopiform fishes are actually related to theanguilliform eels (pp. 46–51). Namely, fishes of these 2 orders
are placed in the single group Elopomorpha, together withAlbuliformes and Saccopharyngiformes (both of which are notyet recorded from the Mekong). Likewise the Anguilliformes
and the other 2 elopomorph orders, fishes of the Elopiformes
have a particular larval stage, called "leptocephalus" (or, inplural, "leptocephali") that has a transparent, laterally compressed, leaf- or ribbon-like body. The leptocephali of elopiform
fishes differ from the anguilliform leptocephali by having
forked caudal fin (vs. rounded or ponted in the latter).
Two families are recognized in the Elopiformes, both ofwhich are found in the Mekong Basin: Elopidae (tenpounders
or ladyfishes) and Megalopidae (tarpons). These are very

compact families, which comprises only a single living genus
in each. The elopid genus Elops looks very different than themegalopid genus Megalops, but they share various primitive
characteristics such as a gular plate and a bony median plate on
the underside of the throat (see photograph below).

Elops hawaiensis Regan, 1909

A

gular plate

lower jaw
ventral view of head of Elops hawaiensis

CTU-P 1387 (photo: KS)premaxillary toothplate (of upper jaw)

Family: Elopidae (FC: 069)
Size: slightly less than 100 cm SL (Smith, 1999: 1620).
Notes: A silvery coastal marine fish, commonly enters to brackish estuaries and shrimp-culture ponds.
According to Whitehead (1962), Elops comprises 6 living
species, 2 of which, E. hawaiensis and E. machnata, are known
from the Indo-Pacific region. During our field surveys on 2007–2013, we collected some specimens of E. hawaiensis fromestuarine areas of the Vietnamese Mekong. Another species, E.
machnata, was reported from the Mekong Delta by Vidthayanon(2008). Elops hawaiensis and E. machnata are similar-looking
species, but they can be differentiated by the shape of theirlower jaw; namely, E. hawaiensis has a non-projecting lower
jaw and the complete premaxillary tooth band is exposed
when the mouth closed, whereas in E. machnata the lower jaw
projects and covers the anterior part of the premaxillary toothband when the mouth is closed. Vertebral counts also differ(68–70 in E. hawaiensis vs. 63–64 in E. machnata) (Whitehead,1962). See also the photographs of E. machnata (taken at PhúQuốc Island, southern Vietnam), below.

Bjaws equal, lower jaw
not projecting anteriorly
beyond upper jaw (seealso text)

Cjaws extending posteriorly
well beyond a verticalthrough rear edge of eye

Dscales on body minute
Similar-looking congener, Elops machnata (nonMekong specimen, taken from Phú Quốc Island,Vietnam; photo: KS); arrow indicates the projecting
lower jaw (right photograph)

52A) Marketed fish (Bến Tre, Vietnam, not preserved, photo: KS); B–D) CTU-P 1387 (photo: LXT)

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

MEGALOPIDAE
A

Megalops cyprinoides (Broussonet, 1782)

B

Family: Megalopidae (FC: 070)
Size: 55 cm TL (Smith, 1999: 1622).

Notes: Only a single Indo-Pacific species of the tarpon family
Megalopidae, found in marine coastal areas and estuaries.
As for Elops hawaiensis (left page), freshly-collected fish arecommonly sold at small markets adjacent to shrimp-culture
ponds in the Mekong Delta of Vietnam. Rainboth (1996b)
recorded this species from the Cambodian Mekong. During oursurveys in 2007–2013, we could not confirm this fish from theCambodian Mekong, although, in Cambodia, it is commonly
seen at the markets in western coastal regions (west of theCardamom range).
Megalops cyprinoides is readily distinguished from theother fishes in the Mekong by its large eyes, jaws and scales,
and a long, filamentous last ray of dorsal fin.

DA and B) CTU-P 4907 (photo: HVM); C) CTU-P 5157 [photo: HVM (C) and KS (D)]Distribution: Mekong Basin in Vietnam (?Cambodia); IndoPacific.

last ray of dorsal fin
greatly elongate
eye very large

Cscales on body large

a large gular plate underside ofhead (like Elops, see left page)53