Neostethus geminus, Parenti, 2014
Parenti, Lynne R., 2014, A new species of Neostethus (Teleostei; Atherinomorpha; Phallostethidae) from Brunei Darussalam, with comments on northwestern Borneo as an area of endemism, Raffles Bulletin of Zoology 62, pp. 175-187 : 177-181
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Material examined. All specimens collected in Brunei Darussalam. Holotype: USNM 409956 View Materials (sinistral male, 23.3 mm SL), Tutong Dist., Sg. Birau, trib. of Sg. Tutong, just above where it enters Sg. Tutong, E of Bandar Tutong, 4°48'0" N, 114°40'38" E, coll. L GoogleMaps . R. Parenti, H. K. Larson, Hj. Ramlee, Hj. Bahrin, Abu Bakar & Bujong, 25 August 1997 . Paratypes: Brunei Museum / USNM 409980 View Materials (40, 10 sinistral males, 10 dextral males, 10 females, 10 imm., 16.8–23.0 mm SL) , USNM 409963 View Materials , (95, 14 sinistral males, 12 dextral males, 27 females, 42 imm., 11.3–25.7 mm SL, of which one sinistral male, one adult female and one imm. male were cleared and stained solely with alcian blue and one sinistral male was cleared and stained solely with alizarin) , BMNH 2013.5.10.2-4, sinistral male, 23.8 mm SL, dextral male, 23.5 mm SL, female, 18.8 mm SL; CAS 235597 View Materials , sinistral male, 23.7 mm SL, dextral male, 23.0 mm SL, female, 19.5 mm SL ; FMNH 121800 View Materials , sinistral male, 23.4 mm SL, dextral male, 23.8 mm SL, female, 19.8 mm SL ; MCZ 170559 View Materials , sinistral male, 22.4 mm SL, dextral male, 24.5 mm SL, female, 18.3 mm SL ; ZRC 54117, sinistral male, 23.8 mm SL, dextral male, 24.5 mm SL, female, 18.8 mm SL, all collected with the holotype . USNM 409957 View Materials (133, 17 sinistral males, 15 dextral males, 25 females, 76 imm., 12.5–24.8 mm SL, of which one dextral male, one sinistral male, and two adult females were cleared and counterstained), Belait District, Sg. Dalit, just above where it enters Sg. Belait, 4°34'0" N, 114°12 ′ 0 ″ E, coll. L GoogleMaps . R. Parenti, H. K. Larson, Hj. Ramlee, Hj. Bakana, Hj. Bahrin, M. Wong & Yussof, 27 August 1997 ; USNM 356847 View Materials (10, 13– 24 mm SL), Tutong Dist., Sg. Tutong, upstream from Bandar Tutong near Kampung Panchor Dulif , 4°46'04" N, 114°36'12" E, coll. L GoogleMaps . R. Parenti, H. K. Larson, Hj. Ramlee, Zainal & Hj Ariffin, 23 August 1997 ; USNM 409958 View Materials (10, 9– 24.8 mm SL), Belait Dist., Sg. Teraban, trib. of Sg. Belait, just above where it enters main river channel, and in main channel, 4°34'41" N, 114°10'59" E, coll. L GoogleMaps . R. Parenti, H. K. Larson, Hj. Ramlee, Hj. Bahrin & Yussof, 26–27 August 1997 ; USNM 409960 View Materials (adult female, 19 mm SL), Tutong Dist., Sg, Gandang , trib. of Sg. Penyatang, coll. L . R. Parenti, H. K. Larson, Hj. Ramlee, Hj. Bakana, Hj. Bahrin, Abu Bakar & Hj. Ariffin, 24 August 1997 ; USNM 365135 View Materials (47, 10– 20.5 mm SL), Belait Dist., Sg. Limatak, just above where it enters Sg. Belait, 4°34'20" N, 114°11'33" E, coll. L GoogleMaps . R. Parenti, H. K. Larson, Hj. Ramlee, Hj. Bakana, Hj. Bahrin, M. Wong & Yussof, 27 August 1997 .
Nontype specimens (all poorly preserved or distorted): USNM 409959 View Materials (90, 7– 24 mm SL), from the type locality ; USNM 389677 View Materials (4, alcohol fixed) , USNM 389674 View Materials (2, alcohol fixed), Belait District, Sg. Dalit, just above where it enters
Genus Neostethus Regan, 1916
Neostethus geminus , new species *
Genus Gulaphallus Herre, 1925
Gulaphallus panayensis ( Herre, 1942) Sg. Belait, 4°34'0" N, 114°12'0" E, coll. L. R. Parenti, H. K. Larson, Hj. Ramlee, Hj. Bakana, Hj. Bahrin, M. Wong & Yussof, 27 August 1997.
Differential diagnosis. Neostethus geminus and N. bicornis are distinguished from all other phallostethid fishes by mature males with two elongate ctenactinia (vs. one elongate and one short ctenactinium in other Neostethus ), immature males with a brown blotch on the pelvic fin rays of the proctal side, which fades with growth and maturity, and females with a fleshy, hoodlike fold or flap that includes the anus, genital pore and urinary pore. These characters were considered diagnostic of N. bicornis by Parenti (1989). Neostethus geminus differs from N. bicornis in being smaller (reaching no more than 25.7 mm SL vs. 31 mm SL) and having a relatively compact priapium with a foreshortened, broad aproctal axial bone that meets but does not overlap the pulvinular appendage (vs. a relatively elongate priapium with a long and narrow aproctal axial bone that overlaps the pulvinular appendage medially; Figs. 3 View Figure 3 , 4 View Figure 4 ), thin, nearly translucent, broad papillary bone expanded distally into a tab (vs. a short papillary bone; Figs. 3,4), and females with a thickened fleshy, hoodlike fold that includes the anus, genital pore and urinary pore (vs. a thin fold of skin; Fig. 6 View Figure 6 ), and a thickened ridge just posterior to the fold and separated from it by a gap (vs. lacking a ridge and a gap; Fig. 6 View Figure 6 ).
Description. Meristic data for the new species are summarised in Table 2. A small, laterally compressed species, maximum size recorded 25.7 mm SL. No vestigial pelvic-fin rays or bones in adult females; males with parts of pelvic and pectoral fins modified into priapium that is either sinistral (holotype, 46 male paratypes) or dextral (42 male paratypes). Two prominent externalised subcephalic bones: an elongate first ctenactinium arises on either left or right side of body and articulates with posterior extent of left or right (proctal) axial bone, curves gently along the left or right side of the head and the somewhat expanded tip lies just ventral to the lower jaw ( Figs. 2A View Figure 2 , 3A View Figure 3 ). A curved second ctenactinium about one-half the length of the first arises on the same side of the body and articulates with the posterior extent of left or right (proctal) axial bone ( Figs. 3A View Figure 3 , 4C View Figure 4 ) reaches the fibrous, oval pulvinulus which is lateral to, and covers articulation point of, inner pulvinular and proctal axial bones. Papillary bone with numerous thin, bony segments, confluent posteriorly to form a broad, elongate, translucent tab with faint striations. Aproctal axial bone short and broad, meets posterior extent of pulvinular appendage ( Figs. 3A View Figure 3 , 4C View Figure 4 ). Immature males with a small pelvic fin or fins on the proctal side of the body. Mature males with two small bundles of rudimentary pelvicfin rays in the wall of the membranous sac at the posterior extent of the priapium. Pleural ribs of fourth vertebra in males expanded anteroposteriorly, their distal tips meet on right side of proctal axial bone in sinistral males and left side in dextral males; parapophysis on right side greatly expanded and oriented anteriorly in sinistral males, on left side in dextral males. First pleural rib on third vertebra in females.
Females with a thickened fleshy, hoodlike fold that includes the anus, genital pore and urinary pore and a thickened ridge just posterior to the fold ( Fig. 6A View Figure 6 ). Ventral dermal keel extends from body just posterior to thickened ridge in females or constricted body just posterior to the priapium in males, to just before the anal-fin origin ( Fig. 5 View Figure 5 ). Swim bladder spans four to five body segments anterior to anal-fin base.
Neurocranium and jaws like those illustrated for Neostethus bicornis by Roberts (1971b), with little modification. Posttemporal bone forked, lower limb short and connected via a ligament to the basicranium. Jaw teeth conical, in a single uneven row, small medially and progressively larger distally. Paradentary bone edentulous. Caudal fin forked. Caudal skeleton with two thin epurals, autogenous parhypural, and a dorsal and a ventral hypural plate. Principal caudal-fin rays i,7–8/7–8,i [i,7/7,i]. Pectoral fin narrow and elongate, with 10–11  rays. Two dorsal fins, the first with one short, thickened ray followed by a thinner second ray in males, both supported by a single, elongate pterygiophore; the second fin with 6–7  rays, the second through fourth or fifth branched, the first and last articulated, but not branched. Anal-fin rays 15–16 , the first ray short and unsegmented. Vertebrae 35–37  (precaudal 16–18 + caudal 19–20, including half centrum). Branchiostegal rays 5. Scales on body of moderate size, deciduous, 34–36  in a transverse series.
Colour in life. Body largely translucent with melanophore pigmentation pattern as in preservative, below. Specimens from the type locality at capture had a bright orange blotch on the caudal peduncle and a bright orange band at midbody, just anterior to the anal fin (see Remarks, below).
Colour in preservative. Preserved, formalin-fixed, specimens ( Figs. 2 View Figure 2 , 5 View Figure 5 ) with ground colour pale yellow. Dark brown melanophores scattered on dorsal surface of head, just ventral to the midline, on operculum and priapium, along basal portion of anal fin, pectoral fin, dorsal and ventral midline, and abdomen where they are concentrated into a blotch in some specimens. An arc of small brown melanophores on the posteroventral rim of the orbit. All fin rays with thin, black, interrupted to complete line of melanophores on margin. A discrete, thin black line along midlateral, intermuscular septum from pectoral fin to caudal fin base. Dorsal and ventral extent of hypural plates with indistinct black blotch. Body scales with a posterior margin of small brown melanophores. Immature males with a brown blotch on the pelvic-fin rays of the proctal side and a line of brown melanophores on the ventral surface of the body from about the position of the first through third pleural rib. Medial portions of membranous sac at the posterior extent of the priapium in mature males dark brown.
Distribution and habitat. Neostethus geminus was collected from six localities in the Belait and Tutong districts of Brunei, clustered near Kuala Belait and Kuala Tutong ( Fig. 7 View Figure 7 ). At the type locality, N. geminus was collected using dip nets and a scoop net along with other fish species representative of a Southeast Asian nipah forest biota including Ambassis sp., Scatophagus argus , Tetraodon leiurus , Gobiopterus brachypterus , and Brachygobius doriae ; a mangrove colubrid snake, Cerberus sp., was also seen. Neostethus borneensis was the only other phallostethid species collected with N. geminus . The brackish water was clear and brown with a temperature of 27°C.
Etymology. geminus, Latin for twin or double referring to the close morphological similarity of the new species to its inferred sister species, N. bicornis , and to the paired or double ctenactinia in both species.
Remarks. Collections examined here of both N. bicornis and N. geminus contain roughly even numbers of sinistral and dextral males that, together, number somewhat more than the total number of females (see Material Examined).
A bright orange blotch on the caudal peduncle in freshly caught specimens of Phallostethus cuulong was illustrated by Shibukawa et al. (2012: fig. 1). Bright orange-yellow markings on the caudal peduncle and, rarely, at the base of the anal fin were reported by Roberts (1971a) for Phenacostethus posthon and P. smithi ; he noted that such markings were unknown in Neostethus . There are few photographs of live or freshly caught Neostethus ; a freshly-caught dextral male N. bicornis from Singapore has orange-yellow blotches at the base of the caudal-fin rays (see below). Other atherinomorph fishes may have orange markings on the caudal fin; the North American Desert Pupfish, Cyprinodon macularius has a lemon yellow to orange caudal peduncle ( Page & Burr, 2011). These markings fade rapidly upon formalin-fixation and preservation in alcohol. The distribution of the caudal peduncle and mid-body orange markings in atherinomorphs is unknown and, therefore, I do not include the markings as diagnostic of the new species.
Departamento de Geologia, Universidad de Chile
Zoological Reference Collection, National University of Singapore
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