Phenacostethus sikat, Parenti & Lumbantobing & Haryono, 2023

Parenti, Lynne R., Lumbantobing, Daniel N. & Haryono, 2023, Description of a new species of Phenacostethus (Atheriniformes: Phallostethidae) endemic to Kalimantan Selatan, Indonesian Borneo, reveals deep mtCOI divergence among miniature species, Raffles Bulletin of Zoology 71, pp. 553-571 : 557-559

Phenacostethus sikat , new species

( Figs. 2–5 , Tables 1–5)

Holotype. MZB 25501 , sinistral male, 13.4 mm SL, Indonesia, Provinsi Kalimantan Selatan, Kabupaten Tanah Laut, Kecamatan Jorong , Jorong River , along Jalan Ahmad Yani , road from Pelaihari to Simpangempat , (Field no. TGK 01 ), 3°58.794 S, 114°56.375 E, elevation 20.7 m asl, collected by D. Lumbantobing, D. Rudaya, and A. Daely, 7 August 2007.

Paratypes. MZB 25502 , 10 : 4 dextral males, 12.5–15.2 mm SL; 2 sinistral males, 11.8–12 mm SL; 4 females, 11.5–13.6 mm SL . ZRC 64468 , 3 : dextral male 13.5 mm SL; 2 females, 11–11.4 mm SL ; USNM 443824 , 12 : 3 dextral males, 12.2–14.5 mm SL; 2 sinistral males, 11.2–12 mm SL; 4 females, 14.3–15.3 mm SL, from which 4: 1 sinistral male, 12 mm SL, 1 dextral male, 14.5 mm SL, and 2 females, 14.7–14.8 mm SL, have been cleared-and-counterstained for bone and cartilage, collected with the holotype . USNM 393660 , 7 : 3 dextral males, 11.5–12.6 mm SL, 4 females, 12–15.1 mm SL, Jorong River , along Jalan Ahmad Yani , the road from Pelaihari to Simpangempat, (Field no. TGK 32), 3°58.48 S, 114°56.23 E, elevation 20.7 m asl, collected by D. Lumbantobing, D. Rudaya, and A. Daely, 25 August 2007 ; USNM 443826 , 1 sinistral male fixed in 95% ethanol, of which the posterior portion of the body was used for DNA extraction . USNM 393627 , 1 possibly dextral, immature male, 11 mm SL, Asem-asem River , along Jalan Ahmad Yani , the road from Pelaihari to Simpangempat, (Field no. TGK 02), 3°54.419 S, 115°04.828 E, elevation 18.1 m asl, collected by D. Lumbantobing, D. Rudaya, and A. Daely, 8 August 2007 .

Differential diagnosis. Phenacostethus sikat is distinguished from all other species of phallostethids by a unique, brush-shaped seminal papilla in males with some 15 folds of tissue in the holotype ( Figs. 2 , 3 , 5 ). This differs from the skin at the distal end of the genital or seminal papilla of male Phe. smithi , its inferred sister species based on morphology, which forms prominent ruffles or crenulations, but these are fewer in number—6 to 9 and more distantly spaced than the folds of Phe. sikat . Males of Phe. sikat and Phe. smithi are either sinistral or dextral: the toxactinium, the prominent, curved holding or clasping component of the priapium ( Bailey, 1936), arises on the right or left side of the head and curves strongly under head towards the aproctal side of the body. The seminal papilla is on the aproctal side opposite the articulation point of the toxactinium. In congeners Phe. posthon and Phe. trewavasae , the distal portion of the seminal papilla is smooth, not ruffled, and males are exclusively dextral ( Phe. posthon ) or sinistral ( Phe. trewavasae ). Females of Phe. sikat lack rudimentary pelvic fins, as does Phe. trewavasae ; these are present in female Phe. posthon and Phe. smithi (see below) as well as Phallostethus ( Shibukawa et al., 2012: fig. 3). Phenacostethus sikat has a second ctenactinium (one of the elongate priapial bones) that is curved and pointed and extends beyond the ventral body profile ( Figs. 2 , 3 , 5 ). Males of Phe. smithi may have either a pointed or blunt second ctenactinium ( Figs. 3 , 5 ). Males of Phe. sikat differ further from those of Phe. smithi in having an ossified, as opposed to cartilaginous or absent, inner pulvinular bone ( Parenti, 1989: fig. 2). The new species has a unique pigmentation pattern with few or no melanophores along the horizontal septum anterior to the anal-fin origin ( Figs. 2 , 4 ) as opposed to a more complete line of melanophores in congeners, including a putative undescribed species.

Description. Meristic data for the new species are summarised in Table 2. A small, slender laterally compressed species, maximum size recorded 15.3 mm SL. No vestigial pelvic-fin rays or bones in adult females; males with parts of pelvic and pectoral fins modified into a bilaterally asymmetric priapium that is either sinistral (4 male specimens, including holotype) or dextral (11 male specimens): prominent externalised subcephalic bone a toxactinium arising on right or left side of body, articulates with proctal axial bone via a broad base, and curves under the head towards left or right side of body. Priapium fully developed only in largest males. The seminal papilla large and prominent with 15 folds of tissue in the holotype. Cartilaginous pulvinular pad lateral to and covering articulation point of toxactinium and proctal axial bone; small inner pulvinular bone just anterior to point of articulation ( Fig. 5 ). Cartilaginous pulvinular pad suboval. Curved and pointed second ctenactinium articulates with posterior base of aproctal axial and posterior infrasulcar bones ( Fig. 5 ). Papillary bone large with a broad base that articulates with basipenial and prepapillary bones. Papillary and slender penial bone support seminal papilla. Paired priapial ribs (ribs that do not articulate with a vertebra in adults) each with a posterior flange, elongate dorsoventrally, meet just dorsal to the proctal or aproctal axial bone and adjacent to the antepleural cartilage. First articulated rib on fourth vertebra. Ventral dermal keel extends from base of priapium in males or urinary opening in females, to just before anal-fin origin ( Fig. 2 ). Posttemporal bone simple, no ventral limb. Preamaxillary ascending process thin and elongate; rostral cartilage rectangular, longer in anterior-posterior axis. Jaw teeth unicuspid, pointed, in a single uneven row: small medial, teeth separated by a gap from larger, distal teeth. Paradentary bone edentulous, largely cartilaginous. Ventral gill arches with three ossified basibranchials. Hyoid bar with four to five [four] branchiostegal rays. Caudal fin forked, dorsal and ventral rays form incipient lobes. Caudal skeleton with two thin epurals, autogenous parhypural, and a dorsal and a ventral hypural plate.

Principal caudal fin rays i,6–7/7,i [i,7/7,i]. Pectoral fin narrow and elongate, with 9 rays. Two dorsal fins, the first with one short, thickened unsegmented ray supported by a single, elongate pterygiophore; the second dorsal fin with 6 rays. Anal-fin rays 14–15 [14], the first ray short and unsegmented. Vertebrae 32–34 [34] (precaudal 13–15 + caudal 19–21, including ural centrum). Scales on body small, deciduous, not counted as many are lost.

Colour in life. Phenacostethus sikat is translucent in life with an orange spot at base of caudal fin. Melanophore pigmentation pattern as in preservative (below).

Colour in preservative. Ground colouration pale yellow. Colour pattern in alcohol like that of congeners (as in Roberts, 1971a): discrete melanophores scattered on dorsal surface of head and anterior portion of body, along horizontal septum, around orbit, on operculum and priapium, and along basal portion of anal fin, dorsal midline, and ventral midline. Orange spot at base of caudal fin not preserved. Few to no melanophores along the horizontal septum just anterior to a point above the anal-fin origin; from that point, a discrete row of dash-shaped, evenly spaced melanophores continues posteriorly to the caudal-fin base. A translucent, membranous dome on top of the head, flattened in most alcohol specimens.

Distribution and habitat. Known from three collections in southeastern coastal drainages of Kalimantan Selatan, as detailed above, with field numbers TGK 01, TGK 02, and TGK 32 ( Figs. 6 , 7a ). These three coastal stream habitats were at altitudes of 18.1 to 20.7 m asl. Other species collected at these sites and catalogued in the USNM include members of other exclusively freshwater or estuarine taxa in the families Anabantidae , Aplocheilidae , Bagridae , Balitoridae , Osphronemidae , Channidae , Cyprinidae , and Tetraodontidae . Another specimen, Phe. sp. 1 (USNM 443825; Fig. 4 ), that was highly divergent from the new species in mtCOI sequence data (see below), was collected from a coastal stream habitat at 7 m asl (field no. TGK 18; Fig. 7b ). Other species collected with it and catalogued in the USNM included those in the family Cyprinidae but are otherwise largely estuarine or coastal taxa in the families Clupeidae , Eleotridae , Gobiidae , and Zenarchopteridae . See discussion under Biogeography, below, for more details on habitat.

Etymology. The specific epithet sikat, Bahasa Indonesia for brush, to refer to the distinct, brush-shaped external morphology of the seminal papilla of adult males of the new species.

Remarks. Specimens of the new species are in fair to good condition. The alizarin red bone stain in the four cleared-and-counterstained specimens faded soon after preparation and the skeleton could not be restained. We suspect that the specimens are partially decalcified. Likewise, radiographs of the specimens did not clearly reveal characters of the skeleton. MicroCT scans are used to illustrate the priapial skeleton and some details of external anatomy. Meristic data are recorded for the holotype and cleared-and-counterstained paratypes.

Figures

Fig. 2. Phenacostethus sikat. A, Holotype, MZB 25501, male, 13.4 mm SL. The caudal fin is damaged and incomplete. B, Paratype, USNM 443824, female, 13.8 mm SL. Bars = 1 mm.

Fig. 3. Phenacostethus sikat, holotype, MZB 25501, male, 13.4 mm SL. A, lateral view of head and priapium; B, ventral view of head and priapium; C, lateral view of priapium focused on brush—like seminal papilla. Phenacostethus smithi, USNM 329582, male, 16.1 mm SL. D, lateral view of head and priapium; E, ventral view of head and priapium; F, lateral view of priapium focused on ruffled seminal papilla. Bars = 1 mm.

Fig. 4. Anterior portion of the body of specimens from which the caudal portion was removed in the field. DNA was extracted from the caudal portion for mtCOI (barcode) analysis. Above, Phenacostethus sikat, USNM 443826; below, Phenacostethus sp. 1, USNM 443825. Bars = 1 mm.

Fig. 5. MicroCT scans to illustrate skeletal and some soft tissue anatomy of the priapium in A, Phenacostethus sikat, MZB 25501, holotype, left lateral view of a sinistral male; and B, Phenacostethus smithi, USNM 329581, left lateral view of a dextral male. Names of priapial components follow Parenti (1989). The distal tip of the toxactinium, the prominent, anterior, hooked bone, is damaged in both specimens. Bars = 1 mm.

Fig. 6. Collection localities of freshwater and coastal fishes from Kalimantan that included specimens of the genus Phenacostethus. TGK01/ TGK32 and TGK02, localities of Phenacostethus sikat. The holotype was collected at TGK01. TGK18 is the locality of Phenacostethus sp. 1, represented by one specimen (USNM 443825) distantly separated from Phe. sikat in mt COI data (see Table 4 and Discussion). Blue dots represent other localities sampled in August 2007 which included no phallostethid specimens. Yellow lines are roads. Base map from Microsoft Encarta Interactive World Atlas 2001.

Fig. 7. Two stream habitats in Kalimantan Selatan sampled in 2007: A, Type locality of Phenacostethus sikat, the Jorong River at 20.7 m above sea level (3°58.794 S, 114°56.375 E). B, Sampanahan River at Sampanahan Hulu at 7 m above sea level (2°37.745 S, 116°11.170 E).