ATHERINIFORMES / ATHERINIDAE & PHALLOSTETHIDAEATHERINIFORMES – silversides and allies
Many of fishes of the order Atheriniformes have well-separated
dorsal fins and abdominal pelvic fins (see photograph of a speciesof Atherinomorus, right), and are somewhat similar superficiallyto the mugiliform fishes (pp. 312–317). This order comprises
6 families (Nelson, 2006; Wiley & Johnson, 2010), including ahighly specialized family Phallostethidae that has a reduced or no
first dorsal fin and a peculiar bilaterally-asymmetric copulatoryorgan under the throat in the males ("priapium", see below
column).
Within the Atheriniformes, 3 and 5 species of Atherinidae
and Phallostethidae were hitherto recorded from the Mekong(Vidthayanon, 2008; Shibukawa et al., 2012; our unpublisheddata). During our field surveys in 2007–2013, we found 5 speciesof phallostethid fishes from the middle and lower Mekong,although we failed to collect the atherinid fishes. In the Mekong,all of these phallostethids except for Phenacostethus smithi (p.
320) are known only from brackish-water areas (e.g., mangrovecreeks) around estuaries of Vietnam; only P. smithi is also foundAtherinomorus sp. (cf. aetholepis) from Phú Quốc Island, Vietnam (CTU-P 4258)

in the middle reaches of the Mekong in Cambodia and Thailand.
Three species of atherinid fishes were recorded from theMekong Delta by Vidthayanon (2008): Atherinomorus duodecimalis, Atherinomorus lacunosus, and Hypoatherina valenciennei(as valenciennesi [sic.]). The photographed specimen shown
above, provisionally identified as a species of Atherinomorus
sp. (cf. aetholepis), was collected from Phú Quốc Island, off thesouth-western coast of Mekong Delta of Vietnam.

A Phallostethus cuulong (left side)anus

toxactinium

Male of Neostethus bicornis, Sóc Trăng, Vietnam(priapium is encircled by yellow dashed line) (photo: KS)

Priapium fishes
Fishes of the atheriniform family Phallostethidae (except fora single marine species Dentatherina merceri, usually placedin the distinct subfamily Dentatherinae from the other phallostethids) have a unique bilaterally-asymmetrical copulatoryorgan, termed "priapium", under the throat in male, and, thus,
are commonly known as priapium fishes. Using the priapium,males of these phallostethids grasps/holds onto the females,and fertilizes the eggs internally. The phallostethid fishes are,however, not live-bearers (as in the poeciliids, p. 323); following the copulation, the female lays eggs (Smith, 1927, 1945;
Villadolid & Manacop, 1934; Parenti, 1989, 2005; Grier &Parenti, 1994). The priapium is composed of fleshy (including
seminal papilla and pulvinulus) and bony elements (presumed
to be derived from the pelvic-fin structure, e.g., ctenactinium
and toxactinium, as well as modified cleithra and pleural ribsfor suspension) (Regan, 1913; Parenti, 1989), and the detailed
strucure of the priapium brings a useful information for identifying the genera/species. The terminology of its details follows
Parenti (1989), and is roughly summarized in the right figure.

secondctenactinium

seminal papillaurogenital opening

B Phallostethus cuulong (right side)

toxactinium

seminal papillapulvinulus
second ctenactinium

C Neostethus bicornis (left side)first ctenactinium
second ctenactinium

318

Phallostethus cuulong Shibukawa, Tran & Tran,2012

Family: Phallostethidae (FC: 250)
Size: 2.5 cm SL (Shibukawa et al., 2012: 46).

A

Distribution: Mekong Basin in Vietnam.

Bmidlatetral orange spot at end of caudal peduncle
first dorsal fin absent

C

toxactinium distinct,
curved and rod-likelong-based anal fin,with 24–27 rays

D
7 serrae on second
ctenactinium of adult maleE

A creek at Sóc Trăng Province, Vietnam, where Phallostethus cuulong was collected (photo: KS)

Notes: A medium-sized species of priapiumfishes, hitherto
known only from small magnrove creeks and canals aroundbrackish estuaries of the Vietnamese Mekong. In this region,
Phallostethus cuulong is frequently collected with the otherpriapiumfishes (e.g., Phenacostethus smithi and Neostethus
lankesteri), but is less abundant than the others. The photographbelow shows a habitat in Sóc Trăng Province of Vietnam, where
Phallostethus cuulong was collected.
Members of the Phallostethidae are small-sized, transparent
surface-swimming fishes, known from Southeast Asian countries.
They have a priapium, which is a unique bilaterally-asymmetricalcopulatory organ under the throat in males, and are commonly
known as priapium fishes (see column on left page). The family
comprises 4 genera, and, of these, 3 genera (Neostethus, Phallostethus, and Phenacostethus) are found in the Mekong (Shibukawaet al., 2012).
Phallostethus differs from the other phallostethid genera inhaving long-based anal fin with 24–27 rays (vs. 22 or less inthe other genera) and serrated second ctenactinium (vs. second
ctenactinium absent or, if present, not serrated) (Parenti, 1989;
Shibukawa et al., 2012). Phallostethus comprises 3 allopatricspecies, viz., P. dunckeri from Johor of Malay Peninsula, P. lehi
from northwestern Borneo, and P. cuulong from the Vietnamese
Mekong, and can be distinguished from one another by, e.g.,
the number of serrae on the second ctenactinium in adult males
and vertebrae (see Parenti, 1996; Shibukawa et al., 2012).

A) CTU-P 2494 (photo: LXT); B) CTU-P 2492 (photo: LXT); C) CTU-P 2495 (poto: LXT); D) CTU-P 2473 (preserved specimen, photo: LXT);
E) CTU-P 2327 (photo: LXT)

PHALLOSTETHIDAE

319

PHALLOSTETHIDAE
Phenacostethus smithi Myers, 1928

Distribution: Mekong Basin in Thailand, Cambodia, and Vietnam; coastal basins in western Cambodia, Thailand to peninsular
Malaysia.
Notes: A small-sized species of priapiumfishes, found in middle
and lower reaches of rivers, canals and brackish estuaries. Phenacostethus smithi is locally abundant particularly in slow-flowing
rivers and turbid canals in the Vietnamese Mekong, but is easily
overlooked, due to its diminutive size.
Phenacostethus is similar to Phallostethus (rather than Neostethus) in having a toxactinium, a long rod-like external bone at
the anterior part of the priapium. At least in the Mekong species,
fishes of Phenacostethus and Phallostethus have an orange spot
at/around the caudal-fin base (vs. absent in Neostethus). Phenacostethus is, however, distinguished from Phallostethus by having short-based anal fin with 14–15 rays (vs. 24–28 rays in
Phallostethus), minute first dorsal fin (vs. absent), and a reduced,non-serrated second ctenactinium (vs. serrated) (Parenti, 1989,1999; Shibukawa et al., 2012). Three species are known in Phenacostethus, and, of these, 2 species, P. smithi and P. posthon(below), are found in the Mekong. Phenacostethus smithi differsfrom the sympatric P. posthon in the origin of the first dorsal
fin, which is above or slightly behind midlength of the anal-finbase (vs. above or slightly behind th eend of anal-fin base in P.
posthon), and ruffed seminal papilla (vs. smooth). The remaining
one, P. trewavasi, is hitherto known only from Sarawak, Malaysia(Parenti, 1986, 1989). The recent record of P. trewavasae fromthe Cambodian Mekong by Rainboth et al. (2012: 76, pl. 49, fig.
1025) needs confirmation; their photographed specimen, takenfrom a floodplain near Phnom Penh, has a moderately large eyelens (vs. small to minute in P. trewavasae, see Parenti, 1986,1989), and, based on the photograph, we cannot find any discrepancies from P. smithi, that is common in southern Cambodia.

Phenacostethus posthon Roberts, 1971
Family: Phallostethidae (FC: 250)

AfemaleBC malemidlatetral orange spot at end of caudal peduncle

toxactinium distinct,
curved and rod-likeorigin of first dorsal fin above
or slightly behind midlengthof anal-fin baseDmale

male seminal
papilla ruffed

E femalemaleAmale

Size: 1.8 cm SL (Roberts, 1971: 13).
Distribution: Mekong Basin in Vietnam; Malay Peninsula.
Notes: A small-sized species of priapiufishes, found in lower
reaches of rivers, canals, and brackish estuaries; it is usually
collected together with a similar congener Phenacostethus smithi(above) in the Mekong. Tran et al. (2013: 81) misidentified afemale specimen of this species as P. smithi.
Phenacostethus posthon closely resembles the sympatric congener P. smithi (see "Notes" of P. smithi, above). All of our photographed specimens of females of P. posthon have an additional
orange marking around the origin of the anal fin (vs. absent in P.
smithi); the position of the orange marking at the end of caudal
peduncle is also somewhat different from that of P. smithi.

short-based anal fin,with 14–15 rays

BC female
D femaleorigin of first dorsal fin above or slightly behind end of anal-fin baseorange spot at ventral part of end of caudal peduncle

E maletoxactinium distinct,
curved and rod-like320

F femalemale seminal papilla smoothshort-based anal fin,with 14–15 rays

A and C) CTU-P 1255 (photo: LXT); B and E) CTU-P 2678 (photo: LXT); D) IFREDI-P 2268 (photo: PT)

Size: 1.6 cm SL (Roberts, 1971: 10).

A and E) CTU-P 3626 (photo: LXT); B) CTU-P 3678 (photo: LXT); C and F) CTU-P 2156 (photo: LXT); D) CTU-P 2827 (photo:LXT)

Family: Phallostethidae (FC: 250)male

A maleFamily: Phallostethidae (FC: 250)
Size: 3.0 cm TL (Kottelat et al., 1993: 95).
Distribution: Mekong Basin in Vietnam; Western Pacific andAndaman Sea.
Notes: A medium-sized species of priapiufishes, found in brackish estuaries; it is commonly collected from small pools andcreeks in mangrove forests of the Mekong Delta, but appears tobe less abundant than Neostethus lankesteri (below).
Neostethus is readily distinguished from the other 2 phallostethid genera known from the Mekong, viz., Phallostethus (p.
319) and Phenacostethus (left page), by lacking externalized
toxactinium, a long rod-like bone articulating with the anterior
end of the proctal axial bone (see column in p. 318). Whenalive, fish of Neostethus have a nearly transparent body, with noorange spot at the caudal-fin base (vs. orange spot present at leastin the Mekong species of Phallostethus and Phenacostethus).
Eleven species are known in Neostethus (Parenti, 1989; Parenti& Louie, 1998); of these, at least 2 species, Neostethus bicornis
and N. lankesteri, were recorded from the Mekong (Shibukawaet al., 2012). These 2 Mekong species can be distinguished by
the morphology of priapium and genital papilla in male andfemale, respectively (see photos shown here); the examinationof these characteristics needs magnification.

D maleno toxactinium
2 ctenactiniums

B maleC femaleno orange spot aroundcaudal-fin base when alive

short-based anal fin,with 15–16 rays

E femaleurogenital opening covered byhood-like (not hook-like) fleshy flapin female (see also N. lankesteri, below)

Neostethus lankesteri Regan, 1916
Family: Phallostethidae (FC: 250)
Size: 3.0 cm SL (Kottelat et al., 1993: 95).
Distribution: Mekong Basin in Vietnam; Western Pacific andAndaman Sea.
Notes: A medium-sized species of priapiufish, found in brackish estuaries. Neostethus lankesteri is one of the most common
species of priapiumfishes in the mangrove creeks and adjacentwaters of the Mekong Delta. See also the similar-looking congener, N. bicornis, above.

C maleno toxactinium
single long ctenactinium, witha ventral triangular projection

A maleno orange spot around caudal-fin base when alive

B femaleshort-based anal fin,with 16–17 rays

D femaleurogenital opening covered by
hook-like (not hood-like) fleshy flapin female (see also N. bicornis, above)

321A) CTU-P 1191 (photo: KS); B) CTU-P 2271 (photo: LXT); C) one of CTU-P collection (photo: LXT); D)
one of CTU-P collection (photo: LXT)

Neostethus bicornis Regan, 1916A) one of CTU-P collection from Bến Tre, Vietnam (aquarium photo: KS); B) CTU-P 4574 (photo: HVM); C) CTU-P 1133 (photo: KS); D) one of CTU-P collection (photo:LXT); E) one of CTU-P collection (photo: LXT)

PHALLOSTETHIDAE