Neostethus bicornis Regan, 1916

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 : 181-184

Neostethus bicornis Regan, 1916

( Figs. 1 , 2B , 3B , 4A,B , 5B , 6B , 8 )

Neostethus bicornis Regan, 1916:14–15 , text figure 11A, B, original description.

Ceratostethus bicornis ( Regan, 1916) . – Myers, 1937, description of new monotypic genus.

Ceratostethus bicornis ( Regan, 1916) . – Herre, 1942, characters, distribution.

Ceratostethus bicornis ( Regan, 1916) . – Roberts, 1971b, osteological description.

Neostethus bicornis Regan, 1916 . – Parenti, 1989, meristic data, comparative anatomy and phylogenetic analysis.

Differential diagnosis. Neostethus bicornis and N. geminus are distinguished from all other phallostethid fishes by mature males with two elongate ctenactinia (vs. one elongate and one short ctenactinium as 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 that includes the anus, genital pore and urinary pore. I ( Parenti, 1989) considered these characters diagnostic of N. bicornis which differs from the new species, N. geminus , in being larger (reaching 31 mm SL vs. no more than 25.7 mm SL), having a priapium with a relatively long and narrow aproctal axial bone that overlaps the pulvinular appendage medially (vs. relatively compact priapium with a foreshortened, broad aproctal axial bone that meets but does not overlap the pulvinular appendage; Figs. 3 , 4 ), a short, translucent papillary bone (vs. a thin, nearly translucent, broad papillary bone expanded distally into a tab; Figs. 3 , 4 ), and females with a thin fleshy, hoodlike fold that includes the anus, genital pore and urinary pore (vs. a thick fold; Fig. 6 ), and no thickened ridge just posterior to the fold (vs. having a ridge; Fig. 6 ).

Description. A small, laterally compressed species, maximum size recorded 31 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 or dextral. 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. 2B , 3B ). 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. 3B , 4A, B ) 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, not forming an elongate tab. Aproctal axial bone long and narrow, overlaps the pulvinular appendage medially. Immature males with a small pelvic fin or fins on the proctal side of the body. Mature males with two small bundles of rudimentary pelvic-fin 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 thin fleshy, hoodlike fold that includes the anus, genital pore and urinary pore and no thickened ridge just posterior to the fold ( Fig. 6B ). 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 ).

Neurocranium and jaws as illustrated by Roberts (1971b). Post-temporal bone forked, lower limb short and connected via a ligament to the basicranium. Jaw teeth conical, in a single uneven row, smaller 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. 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 most adult males and females, both supported by a single, elongate pterygiophore; the second fin with 5–6 rays, the second through fourth or fifth branched, the first and last articulated, but not branched. Anal-fin rays 13–16, the first ray short and unsegmented. Vertebrae 36–37 (precaudal 17–18 + caudal 18–20, including half centrum). Branchiostegal rays 5–6. Scales on body of moderate size, deciduous, 31–37 in a transverse series.

Colour in life. Body and fins nearly transparent in life. Other pigmentation as in alcohol specimens described below. A fresh specimen of a dextral male captured during the October 2012 Straits of Johore Biodiversity Survey, Singapore, photographed by Arthur Anker (http://www.flickr.com/ photos/artour_a/8147112311; Accessed 12 December 2013) has yellow blotches at the base of the caudal-fin rays. Dark brown melanophores that line the dorsal portion of the body cavity and anterior portion of the testis and gut are also visible. The reddish colour of the gills in this specimen is an exaggeration, the result of its being placed on ice.

Colour in preservative. Preserved, formalin-fixed specimens with ground colour pale yellow, brownish in specimens from Sabah. 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 medium to dark brown.

Distribution and habitat. Widespread in coastal, brackish water habitats from Thailand, Peninsula Malaysia, Singapore, East Malaysia ( Sarawak and Sabah), Kalimantan, Indonesia (see Parenti & Louie, 1998), Brunei, and Palawan Is., Philippines ( Fig. 1 ).

Material examined.Peninsula Malaysia: Lectotype: BMNH 1937.2.9 :4 (imm. dextral male, 20.9 mm SL), designated by Parenti (1989); Paralectotypes: BMNH 1937.12.9 : 5–6 (female, 16.7 mm SL; imm. male, 19.1 mm SL), Kuala Langat, Selangor, coll. G. Duncker. Thailand: Central Thailand : brackish fish farming ponds in area of Samut Sakhon, ca. 25 km S of the center of Bangkok, USNM 358614 (5, 20– 23.5 mm SL), coll. A. Whitehouse, 1 December 1997 . Chanthaburi: ca. 1/ 2 km up from mouth of Chantiburi R. (near Tah Chalap), MCZ 47306 (adult sinistral male, CT micrograph), coll. T. R. Roberts & P. Wongrat, 5 May 1970 . Chumporn Prov., Klong Panangtak in vicinity of Banthup Tanhot ca. 5 km from Chumporn town , CAS 63196 (2, sinistral male, sinistral female, 22 mm SL), coll. H. A. Fehlmann, 22 May 1960 . Singapore: CAS-SU 31133 (10, 3 sinistral males, 7 dextral males, 26–28 mm SL), coll. A. W. C. T. Herre, 1934. USNM 102142 (8, sinistral male, 4 dextral males, 3 females; 21.0–28.0 mm SL), coll. A. W. C. T. Herre, 18 March 1934 . Kranji R., CAS-SU 35783 (52, 13 sinistral males, 9 dextral males, 27 females, 3 imm., 15–28 mm SL, of which 2 sinistral male, 2 dextral male and 6 females have been cleared and stained solely with alizarin), coll. A. W. C. T. Herre, March 1937 . Seletar R., CAS-SU 67161 (10, 2 sinistral males, 6 dextral males, 1 female, 1 imm, 18.1–27.8 mm SL), coll. E. Alfred, 11 February 1966 . Sg. Buloh just outside of nature reserve, small stream entering Straits of Johore , USNM 348392 (50+, sinistral male, dextral male, female cleared and stained solely with alizarin), USNM 373890 (4 pres. in ethanol), coll. L. R. Parenti, H. K. Larson, K. Lim & N. Sivasothi, 5 September 1997 . East Malaysia, Sarawak: Kuching, Sarawak R., N. bank at Kampung Tupung , USNM 325053 (30, imm. 5–16.5 mm SL). Brunei: Tutong Dist.: Kuala Tutong at Tg. Tanah Palang , USNM 409961 (45, 9 sinistral males, 10 dextral males, 14 adult females, 9 imm., from which one dextral male and one female were cleared and stained solely with alcian blue and one dextral male was cleared and stained solely with alizarin; 12–29 mm SL), coll. L. R. Parenti, H. K. Larson, Hj. Ramlee, Hj. Bahrain, Abu Bakar & Zainal, 23 August 1997 . Kuala Tutong, small stream that enters main river (4°47'09" N, 114° 36'56" E), USNM 409964 (317, of which one sinistral male, one dextral male, and 2 adult females were cleared and counterstained), coll. L. R. Parenti, H. K. Larson, Hj. Ramlee, Hj. Bahrin, Abu Bakar & Bujang, 25 August 1997 . Lubok api-api, Kuala Tutong (4°45'12" N, 114° 35'18" E), USNM 356849 (50 +, 10–25 mm SL), coll. L. R. Parenti, H. K. Larson, Hj. Ramlee, Hj. Bahrin, Abu Bakar, & Hj. Ariffin, 24 August 1997 . Sg. Penabai, trib. of Sg. Tutong where it enters Sg. Tutong (4°46'29" N, 114° 36'16" E), USNM 356846 (50+, 12–29.5 mm SL), coll. L. R. Parenti, H. K. Larson, Hj. Ramlee, Hj. Bahrain, Abu Bakar & Zainal, 23 August 1997 . Temburong Dist.: Sg. Mataiang on Hutan Simpan Selirong at Forest Dept. jetty, (4°53'26" N, 115°07'06" E), USNM 364887 (30, 7– 28.5 mm SL), coll. L. R. Parenti, H. K. Larson, Hj. Ramlee, Hj. Ahmad & Pinde, 30 August 1997 . Sg. Raya, Trib. of Sg. Temburong (4°48'11" N, 115°03'47" E), USNM 409934 (8, 8.5–28.7 mm SL), coll. L. R. Parenti, H. K. Larson, Hj. Ramlee, Hj. Bakaria, Hassan & Pinde, 21 August 1997 . Pulau Pitu at sg. that enters Sg. Temburong just downstream from confluence with Sg. Labu (4°45'57" N, 115°04'44" E), USNM 365125 (2, 13.5–14.5 mm SL), coll. L. R. Parenti, H. K. Larson, Hj. Ramlee, Hj. Bakaria, Hassan & Pinde, 21 August 1997 . Brunei- Muara Dist.: Sg. Kalmasi, trib. of Sg. Brunei , (4°50'44" N, 114°57'18" E), USNM 378045 (21, 11.2–28.6 mm SL), coll. L. R. Parenti, H. K. Larson, Hj. Ahmad & Hj. Junggal, 16 August 1997 . Sg. Pulau Berambang, Tg. Bakaka, shore and stream that enters Brunei Bay , (4°54'04" N, 115°01'25" E), USNM 356128 (27, 10– 28.2 mm SL), coll. L. R. Parenti, H. K. Larson, Hj. Ramlee, Hj. Ahmad & Pinde. 30 August 1997 . Sg. Mambangan, trib. of Sg. Brunei, from above to where it enters Sg. Brunei, opposite Kampung Dato Gandi , USNM 356848 (61, 8– 28. 6 mm SL), USNM 389678 (5 pres. in ethanol), coll. L. R. Parenti, H. K. Larson, Hj. Ahmad & Hj. Junggal, 15 August 1997 . Sg. Dolhakim, trib. stream of Sg. Brunei , USNM 364888 (17, 9.5–27.5 mm SL), coll. L. R. Parenti, H. K. Larson, Hj. Ahmad & Hj. Junggal, 17 August 1997 . Main wharf [Bandar Seri Begawan], CAS- SU 61764 (dextral male, 24 mm SL), coll. J. A. Tubb, 30 July 1948 . East Malaysia, Sabah: East Coast Residency, Kinabatangan Dist. , Little Kretam R., winding embayment into nipah swamp, mouth of Ayer Terjun, FMNH 51726 (87, 8 dextral males, 7 sinistral males, 53 adult females, 29 imm., 8.5–26 mm SL), coll. R. F. Inger, 10 May 1950 ; near mouth of Kretam Kechil R., FMNH 51727 (6, dextral male, 2 sinistral males, 3 adult females, 20–24.5 mm SL), coll. R. F, Inger, 25 May 1950 ; fork of East and West Gaja, in West Fork , 100 yds.; trib. of Kretam Kechil R., FMNH 51728 (41, 10 dextral males, 15 sinistral males, 14 adult females, 2 imm., 15.5–25 mm SL), coll. R.F. Inger, 22 May 1950 ; mouth of Pinang R. (trib. of Little Kretam R.), FMNH 51729 (26, 8 dextral males, 10 sinistral males, 6 adult females, 2 imm., 10–27 mm SL), coll. R. F. Inger, 10 June 1950 . Philippines: Palawan, Nakoda Bay , USNM 98838 (3, sinistral male, 2 adult females, 26–29 mm SL), USNM 98839 (3, sinistral male, 2 adult females, 27–31 mm SL), unnamed river SE of Maricaban Is., USNM 150632 (2, dextral male, sinistral male, 30.0 mm SL), coll. Albatross, 31 December 1908 ; Malampaya Sound, Malampaya R., USNM 98840 (1 sinistral male, 30.0 mm SL), coll. Albatross, 26 December 1908 .

Remarks. Although Regan (1916) did not report first dorsalfin rays in his description of N. bicornis , a single first dorsalfin ray was found subsequently in his type material ( Myers, 1937: 141). All specimens examined here have at least one thickened first dorsal-fin ray. The small, second dorsal-fin ray is absent in female N. geminus and some females and males of N. bicornis examined here. Number of fin rays could be verified only in cleared and stained specimens.

One 28 mm SL adult male specimen of N. bicornis from Brunei has two priapia ( Fig. 8 ), a rare condition that has been reported previously as a developmental anomaly ( Roberts, 1971b: 417). Either one or both or neither of the priapia may have been functional in life; the first ctenactinium on the left side of the specimen illustrated here is incomplete either due to developmental arrest or damage. No parts of the specimen, other than those of reproductive morphology, are obviously duplicated. Such individuals may represent monozygotic twins—one sinistral and one dextral male—a hypothesis supported by the observation that sinistral and dextral males have been collected in about equal numbers (see FMNH material, above).

A 215 bp fragment of the mitochondrial cytochrome c oxidase subunit 1 (COI), the DNA barcode ( Ward et al., 2005), for N. bicornis from Thailand was sequenced as part of a study by Sparks & Smith (2004).

Figures

Figure 1. Localities of some phallostethines discussed herein. The genus Phallostethus (circles) comprises three species: P. dunckeri (Muar), P. lehi (Sarawak and Brunei), and P. cuulong (Vietnam). Neostethus lankesteri (squares): Muar, Singapore, Bangkok, Kuching, and Brunei.Neostethus bicornis (stars):type locality,Muar, and collections examined here from Singapore, Thailand, Kuching, Brunei, Sabah, and Palawan. Neostethus borneensis (hexagon): Brunei, Sabah and Coron Is. Neostethus geminus, Brunei (cross). Each symbol may represent more than one locality. Details on the collection localities of N. geminus are in the text and Figure 7.

Figure 2. A, Neostethus geminus, new species. Holotype, USNM 409956, sinistral male, 23.3 mm SL; B, Neostethus bicornis, USNM 409961, sinistral male, 27.0 mm SL.

Figure 3. Comparison of external form of priapium in adult males. A, Neostethus geminus, Brunei Museum/USNM 409963, sinistral male, 25.2 mm SL; B, Neostethus bicornis, USNM 409964, sinistral male, 27.0 mm SL. First ctenactinium, gray; second ctenactinium, black. Abbreviations: aab, aproctal axial bone; pv, pulvinulus, pva, pulvinular appendage, p, seminal papilla, p*, seminal papilla and enlarged papillary tab.

Figure 4. Comparison of external form and extent of development of the priapium in immature males. A, Neostethus bicornis, modified from Regan (1916: fig. 11 B) of a dextral male, 25 mm TL.Abbreviations: pv., pulvinulus, pva., pulvinular appendage, ct., ctenactinia, p., seminal papilla, ef., efferent groove; B, Neostethus bicornis, USNM 409964, immature sinistral male, 24 mm SL (photo is reversed); C, Neostethus geminus, Brunei Museum/ USNM 409963, immature dextral male, 23 mm SL. Pectoral-fin rays were removed from specimen in bottom photo. Arrows point to where anterior extent of aproctal axial bone and posterior extent of pulvinular appendage overlap (B) or meet (C). An asterisk (*) marks the broad papillary tab.

Figure 5. A, Neostethus geminus, new species. Paratype, Brunei Museum/USNM 409963, adult female, 22.7 mm SL; B, Neostethus bicornis, USNM 409961, adult female, 25.0 mm SL.

Figure 6. Comparison of position of the anus and urogenital openings in mature females. A, Neostethus geminus, new species, paratype, Brunei Museum/USNM 409963, adult female, 20.9 mm SL. B, Neostethus bicornis, USNM 409964, adult female, 26.3 mm SL. Abbreviations: an, anus; ov, oviduct aperture or genital pore, u, urinary pore.Pectoral-fin rays have been removed in both specimens.

Figure 8. Neostethus bicornis. USNM 409961, adult male, 28.5 mm SL, with two priapia, one on the left side of the body, the other on the right side. Priapium on the right side appears complete; that on the left side has an incompletely formed first ctenactinium.