Biogeography 9.
7–11. Aug. 20, 2007

Second Japanese Record of a Threadfin, Eleutheronema rhadinum(Perciformes, Polynemidae), with Distributional Implications
Hiroyuki Motomura1*, Masahide Ito2, Mayumi Takayama1, Yuriko Haraguchi1 andMizuki Matsunuma3
1The Kagoshima University Museum, 1-21-30 Korimoto, Kagoshima, 890-0065 Japan
2Kasasa Fishery Cooperative Society, 2-6772 Kataura, Kasasa, Minami-satsuma, Kagoshima,897-1301 Japan
3Faculty of Fisheries, Kagoshima University, 4-50-20 Shimoarata, Kagoshima, 890-0056 Japan

Abstract. A single specimen (420.4 mm in standard length) of an East Asian endemic threadfin (Perciformes:
Polynemidae), Eleutheronema rhadinum (Jordan and Evermann), was collected in a small set net at a depth of
27 m off Kasasa (East China Sea), Kagoshima, southern Japan, on 28 October 2006. The specimen represents
the second confirmed record for the species from Japanese waters (the first record being of a single specimen
from Aomori, northern Japan). Distributional implications for Japanese E. rhadinum are noted.
Key words: Polynemidae; Eleutheronema rhadinum; distribution; Japan; Kuroshio Current.

Introduction
The threadfin, Eleutheronema rhadinum (Perciformes: Polynemidae), originally described as Polydactylus rhadinus by Jordan and Evermann (1902)
from Taiwan Province, China (Motomura, 2004a),
has usually been treated as a junior synonym of E.
tetradactylum (Shaw, 1804) (e. g. Weber and deBeaufort, 1922; Kagwade, 1970; Motomura et al.,2001). Recently, however, Motomura et al. (2002)
recognized P. rhadinus as a valid species of
Eleutheronema Bleeker, 1862 and redescribed it indetail along with designation of a neotype, and a discussion of intraspecific variations and morphologicalchanges with growth. Intraspecific variations in lateral line squamation on the caudal-fin membrane in
E. rhadinum were discussed by Motomura (2003).
Motomura et al. (2001) reported a single large
specimen (739.0 mm standard length) of E.
*Corresponding author: motomura@kaum.kagoshimau.ac.jp

tetradactylum from off the mouth of Azuma River,
Fukaura, Aomori, northern Japan as the first recordof the species from Japanese waters. Subsequently,
Motomura et al. (2002) re-identified the Aomori
specimen as E. rhadinum, showing that E.
tetradactylum does not occur in East Asia where it isreplaced by the endemic E. rhadinum. Eleutheronema rhadinum has currently been recorded on thebasis of collected specimens from northern Vietnam(Motomura, 2003; one specimen), China, including
Taiwan Province (Motomura et al., 2002; 19 specimens), and Japan (Motomura et al., 2001; as E.
tetradactylum, one specimen).
During an ichthyological survey of inshore watersof Kagoshima, southern Kyushu Island, Japan, withthe support of the Kasasa Fishery Cooperative Society, Kagoshima, we collected a single specimen of
Eleutheronema (420.4 mm standard length) in asmall set net at a depth of 27 m off Kasasa, Minamisatsuma, Kagoshima. The specimen was identified as
E. rhadinum on the basis of meristics, morphometrics, lower-jaw morphology and coloration, and rep-−7−

Eleutheronema rhadinum from Kagoshima, Japan

resents the second reliable record for the species
from Japan.
Counts and measurements followed Motomura etal. (2002). Pectoral-fin ray counts included only
those interconnected by a membrane, the lower freerays being considered separately; counts of pectoral
filaments begin with the anterior element. Standard,
fork and total lengths are expressed as SL, FL and
TL, respectively. The specimen is deposited at theKagoshima University Museum, Japan (KAUM).
Comparative material examined are listed in Motomura et al. (2002) and Motomura (2003) — E. rhadinum: 20 specimens (including neotype of Polydactylus rhadinus Jordan and Evermann), 82–739
mm SL, northern Japan, China (including Taiwan
Province), and northern Vietnam; E. tetradactylum:
113 specimens (including neotype of Polynemus
tetradactylus Shaw), 48–375 mm SL, Persian Gulf toPapua New Guinea and northern Australia; E. tridactylum: 34 specimens (including holotype of
Polynemus tridactylus Bleeker), 60–255 mm SL,
Thailand to Indonesia.
Eleutheronema rhadinum (Jordan and Evermann)[Japanese name: Minami-konoshiro](Figs 1–3)
Polydactylus rhadinus Jordan and Evermann, 1902:
351, fig. 20 (type locality: Linkou, Taipei, Taiwan
Province, China, based on a neotype designatedby Motomura et al., 2002).
Eleutheronema tetradactylum (not of Shaw); Shen
S-C, 1984: 98, pl. 98, figs 361-1a, b (Taiwan
Province of China); Senou, 2000: 968 (East
China Sea); Motomura et al., 2001: 41, fig. 1(Aomori Prefecture, northern Japan).
Eleutheronema rhadinum; Motomura et al., 2002:
50, figs 2, 3, 7, 9, 11 (Japan and China including
Taiwan Province); Senou, 2002: 968 (Japan);Motomura, 2003: 33, fig. 1 (Long Chau Bay,northern Vietnam).
Material examined. KAUM–I. 956, 420.4 mm SL(455.0 mm FL, 546.0 mm TL), east of Sakinoyama,

Kasasa, Minami-satsuma, Kagoshima, southern partof Kyushu, southern Japan, 31˚25.44´N, 130˚
11.49´E, 27 m depth, small set net, 28 Oct. 2006, collected by Y. Nakao and M. Ito.
Description. First dorsal fin with 8 spines; second
dorsal fin with 1 spine and 15 soft rays; anal fin with
3 spines and 15 soft rays; pectoral fin with 18 rays
(exclusive of filaments); pectoral filaments 4 on eachside of body; pelvic fin with 1 spine and 5 soft rays;
pored lateral-line scales 95; scale rows above lateralline 13, below 18; gill rakers on upper limb 5, lower
limb 6. Head length 26.7% of SL; body depth at firstdorsal-fin origin 24.4; body depth at second dorsalfin origin 27.6; body width at pectoral-fin base 14.3;
snout length 4.0; eye diameter 4.3; orbit diameter
5.1; interorbital width 6.0; postorbital length 18.7;
upper-jaw length 14.5; depth at posterior margin of
maxilla 2.7; length of tooth plate 7.6; pre first dorsalfin length 31.9; pre-second dorsal-fin length 58.3;
pre anal-∞n length 59.0; pelvic-fin origin to anal-fin
origin 38.3; pelvic-fin origin to anal-fin origin 23.8;
second dorsal-fin base length 15.4; anal-fin baselength 18.7; longest pectoral-fin ray (2nd) length
19.7; longest pectoral-filament (4th) length 18.0;
pectoral-fin base including pectoral-filaments base
6.8; longest pelvic-fin soft ray (1st) length 11.3;
longest first dorsal-fin spine (3rd) length 15.2; second dorsal-fin spine length 7.0; longest second dorsal-fin soft ray (2nd) length 18.2; longest anal-fin
spine (3rd) length 6.3; longest anal-fin soft ray (1st)length 14.9; caudal-peduncle length 24.8; caudalpeduncle depth 11.0; upper caudal-fin lobe length
33.8; lower caudal-fin lobe length 30.7.
Body oblong, compressed; body depth 4.1 in SL.
Maxilla covered with scales; posterior margin of
maxilla extending well beyond a vertical throughposterior margin of adipose eyelid; width of toothband on upper and lower jaws greater than space (onsymphysis) separating tooth bands on opposing premaxillae; anterior parts of lower jaw with small teeth
extending onto lateral surface, adjacent portion of lip
absent; upper-jaw lip absent, small teeth extending
onto lateral surface; teeth villiform in broad bands onvomer, palatines and ectopterygoids; vomer with−8−

Hiroyuki Motomura, Masahide Ito, Mayumi Takayama, Yuriko Haraguchi and Mizuki Matsunuma

12Fig. 1. Fresh specimen of Eleutheronema rhadinum. KAUM–I. 956, 420.4 mm standard length,east of Sakinoyama, Kasasa, Minami-satsuma, Kagoshima, Kyushu, southern Japan.
Fig. 2. Head of Eleutheronema rhadinum, showing anterior two-thirds of lower jaw with small
teeth extending onto lateral surface, adjacent portion of lip absent. KAUM–I. 956, 420.4 mmstandard length.

deciduous tooth plates on both sides. Preopercularmargin serrated. Base of second spine in first dorsal
fin slightly more robust than other spines. Posterior
tip of pectoral fin not reaching a vertical throughposterior tip of depressed pelvic fin; pectoral-fin
insertion well below midline of body; all pectoral fin
rays unbranched. Fourth pectoral filament longest,
extending beyond a vertical through pelvic-fin insertion, but not reaching to pectoral-fin tip; third
filament extending beyond a vertical through pelvicfin origin; first filament shortest, first and second
filaments not reaching a vertical through pelvic-fin
origin. Upper and lower caudal fin lobes notfilamentous. Lateral line extending from upper gill
opening to caudal-fin; lateral line bifurcating on middle of caudal fin; the upper branch simple, not reaching to posterior margin of caudal fin; the lower
branch secondarily bifurcating, both secondarybranches reaching to posterior margin of lower caudal-fin lobe. Swimbladder absent.
Fresh coloration (Fig. 1). Body grayish-silver
dorsally, pale yellowish-silver ventrally; lateral surface of abdomen shining greenish-yellow; ventral
surface of head and trunk white. Adipose eyelidtransparent; tooth band on upper and lower jaws
pinkish white. First dorsal fin dense black distally;
second dorsal fin grayish. Pectoral fins dense black;pectoral filaments white. Pelvic fins whitish, with
scattered melanophores anteriorly. Anal fin pale
gray; anterior parts of base and distal margin of the

fin whitish. Caudal fin grayish with black narrowmargin.
Remarks. The present specimen was identified as
belonging to the genus Eleutheronema, owing tohaving the following characters given as diagnosis of
the genus by Motomura et al. (2002): pectoral-fin
insertion well below midline of body; eye diameter
greater than snout length; anterior parts of lower jaw
with small teeth extending onto lateral surface, adjacent portion of lip absent (Fig. 2); width of toothband on upper and lower jaws greater than space (onsymphysis) separating tooth bands on opposing premaxillae; 3 or 4 pectoral filaments, longest filament
shorter than distance between snout tip and first dorsal-fin origin; pectoral-fin base length (includingbase of pectoral filaments) less than upper-jaw
length; upper and lower caudal-fin lobes not filamentous; and swimbladder absent.
The Indo-West Pacific genus Eleutheronema wasrevised by Motomura et al. (2002), who recognized
three valid species: E. rhadinum, E. tetradactylum
and E. tridactylum (Bleeker, 1845). Comparisonsamong the three species of Eleutheronema were
given by Motomura et al. (2002) in detail. The present specimen, from Kagoshima, had the following
characters: 18 pectoral-fin rays; 4 pectoral filaments;
15 second dorsal-fin soft rays; 95 pored lateral-line
scales; 13 scale rows above lateral line, 18 below; 5
upper series gill rakers, 6 lower, 11 total; vomer withdeciduous tooth plates on both sides; and pectoral-fin−9−Eleutheronema rhadinum from Kagoshima, Japan

Fig. 3. Distributional records of Eleutheronema rhadinum in Japan. Star and circle represent localitiesof the specimens examined in this study andreported by Motomura et al. (2001), respectively.

membranes black. These characters agreed closelywith those of the neotype and non-type specimens ofE. rhadinum redescribed by Motomura et al. (2002).
Morphometrics for the present specimen (seeDescription) were also very close to the range of values of E. rhadinum given in Motomura et al. (2001:
table 1, Japanese specimen), Motomura et al. (2002:
table 1, Chinese and Japanese specimens, including
the neotype) and Motomura (2003: table 1, Vietnamese specimen).
In Japanese waters, E. rhadinum (as E.
tetradactylum) has to date been reported only from
Aomori, northern part of Honshu Island, northern
Japan (Motomura et al., 2001, 2002; Fig. 3). Accordingly, the present specimen (KAUM–I. 956, 420.4mm SL) represents the second confirmed record forthe species from Japan (based on collected specimen).
Eleutheronema rhadinum is an important commercial fish in China, including Taiwan, its annual
catch ranging from 1,140 to 16,041 t between 1970
and 1999 (Motomura, 2004b: table 3). Although it isabundant around Taiwan (H. Motomura, pers. obs.,

at Taipei Fish Market, May 2005), the paucity of
specimens from Japan (no specimens having beenreported from the Ryukyu Islands, adjacent to Taiwanese waters) suggests that the Japanese specimensof E. rhadinum were most likely accidentally transported from around Taiwan or China by the
Kuroshio Current, normally flowing well west of the
Ryukyu Islands. It is highly unlikely that the species
reproduces around Japan.
Nine specimens (KAUM–I. 967–974, 1003,
150.0–175.0 mm SL) of a carangid fish, Atule mate
(Cuvier in Cuvier and Valenciennes, 1833), which ismainly distributed in Indo-Pacific tropical waters(Smith-Vaniz, 1999) and formerly known from only
three specimens from Japanese waters (Suzuki,
1962; Yoshino and Yoshigou, 2003; Miyahara et al.,2006), were also collected at the same place and time
(October 2006) as the present specimen of E. rhadinum. These unusual records of A. mate and E. rhadinum from inshore Japanese waters indicate that theKuroshio Current, conveying tropical continental
shelf fishes, probably flowed more strongly and closer to the shore in October 2006 than usual.

Acknowledgments
We greatly appreciate Yusaku Nakao (Kasasa
Fishery Cooperative Society, Kagoshima, Japan) for
opportunities to examine the present specimen of
Eleutheronema rhadinum, and Nami Kita (KAUM)
and Junko Omori (Kagoshima Aquarium) for theircuratorial assistance. The first author thanks Yuko
Motomura (Kagoshima, Japan) for her assistanceand Graham S. Hardy (Ngunguru, New Zealand),who read the initial manuscript and offered helpfulcomments. This study was supported in part by
Grant-in-Aids for Scientific Research (A) by theJapan Society for the Promotion of Science, Tokyo,
Japan (19208019) and for Young Scientists (B) by
the Ministry of Education, Science, Sports and Culture, Japan (19770067).

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Hiroyuki Motomura, Masahide Ito, Mayumi Takayama, Yuriko Haraguchi and Mizuki Matsunuma

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− 11 −(Accepted November 10, 2006)