Polymixia melanostoma, Fan & Su & Lin & Chang & Lin, 2024

Fan, You-Ci, Su, Yo, Lin, Chien-Hsiang, Chang, Chih-Wei & Lin, Hsiu-Chin, 2024, Polymixia melanostoma, a new beardfish from the western Pacific (Teleostei, Polymixiiformes, Polymixiidae), ZooKeys 1220, pp. 105-121 : 105-121

publication ID	https://doi.org/ 10.3897/zookeys.1220.125127
publication LSID	lsid:zoobank.org:pub:B55A8A7C-6424-463E-9FCE-572B5C675B1A
DOI	https://doi.org/10.5281/zenodo.14342594
persistent identifier	https://treatment.plazi.org/id/7B2FEDD0-F810-5E46-99D8-487F4F59B26E
treatment provided by	ZooKeys by Pensoft
scientific name	Polymixia melanostoma
status	sp. nov.

Treatment

Polymixia melanostoma sp. nov.

Figs 1 View Figure 1 , 2 View Figure 2 , 3 View Figure 3 , 4 A View Figure 4 , 5 B View Figure 5 , 6 View Figure 6 , 7 View Figure 7 ; Tables 1 View Table 1 , 2 View Table 2 , 3 View Table 3 , 4 New English name. Black-mouth beardfish New Chinese name. 黑口鬚銀眼鯛 View Table 4

Polymixia japonica View in CoL (non Günther): Li 2010: 12–13 (in part, specimens were mixed with P. japonica View in CoL ).

Type specimens.

Holotype: Taiwan • NMMB-P 39587 (110.2 mm SL); northern South China Sea, Pingtung, off Dong-gang fishing port ; ca 22°22'22"N, 120°27'34"E; 27 January 2023; Y. Su leg.; in bottom trawl; COI: PP 556538. GoogleMaps

Paratypes (n = 2; SL 122.5–153.0 mm): Taiwan • 1 specimen; NMMB-P 39588 (SL 122.5 mm); Taitung, off Chenggong fishing port ; ca 23°05'52.93"N, 121°22'43.05"E; 11 September 2009; purchased by C.-W. Chang; hook and line; COI: PP 556540 GoogleMaps . – New Caledonia • 1 specimen; MNHN 2014-2291 About MNHN (153 mm SL); Coral Sea , Chesterfield Islands; 21°10'2.40"S, 158°37'24.01"E; 765–778 m deep; 11 October 2005; EBISCO, at st. CP 2545 GoogleMaps .

Etymology.

The specific name melanostoma is a combination of Greek melano and stoma, meaning “black mouth”, in reference to its unique black oral cavity.

Diagnosis.

Polymixia melanostoma sp. nov. differs from its congeners in having the following combination of characters: dorsal-fin rays IV – V, 35–37; gill rakers on the outer face of first gill arch 3 + 1 + 6 = 10; S 1 6–8; S 2 12–14; pyloric caeca 40; snout rounded, with surface rough and gelatinous, its tip evidently protrude anterior margin of premaxilla; ctenii on body scales arranged in wedge shape, forming three rows; 4 th anal-fin spine long, 1.0– 1.2 in eye diameter; dorsal-fin long, 49.6–53.0 % SL; oral-branchial cavity, not including the underside of tongue, black.

Description.

Meristic and morphometric data are provided in Tables 1 View Table 1 , 2 View Table 2 . Data below are for the holotype, followed by a range of paratypes in parentheses, except where indicated.

Dorsal-fin rays V, 35 (IV – V, 36–37); pectoral-fin rays 17 (15–16); pelvic-fin rays i + 6 (i + 6); anal-fin rays IV, 14 ( IV, 14–16); principal caudal-fin rays 9 + 9 = 18 (9 + 9 = 18), uppermost and lowermost rays unbranched; procurrent caudal-fin rays 5 (6) dorsally and 5 (5) ventrally; lateral-line scales 35 (35–39); S 1 6 (7–8); S 2 12 (14); S 3 15 (16, n = 1); gill rakers on the outer face of first gill arch 3 + 1 + 6 = 10 (3 + 1 + 6 = 10); pseudobranchial filaments 27 (33); pyloric caeca 40; branchiostegal rays 7 and only posterior 4 visible. Vertebrae 12 + 17 = 29 (12 + 17 = 29); supraneural and pterygiophore insertion formula: 0 / 0 // 0 / 1 + 1 / 1 + 1 / 1 + 1 / 1 + 1 / 1 + 1 / 1 + 1 / 1 + 1 / 1 + 1 / 1 + 1 / 1 + 1 / 1 + 1 + 1 / 1 + 1 / 1 + 1 / 1 + 1 / 1 + 1 / 1 + 1 / 1 + 1 + 1 / 1 + 1 + 1 / 2 (0 / 0 // 0 / 1 + 1 / 1 + 1 / 1 + 1 / 1 + 1 / 1 / 1 + 1 / 1 + 1 / 1 + 1 + 1 / 1 + 1 + / 1 + 1 + 1 / 1 + 1 / 1 + 1 / 1 + 1 + 1 / 1 + 1 / 1 + 1 + 1 / 1 + 1 / 1 + 1 + 1 / 1 + 2, n = 1).

Body rather slender, longer than deep, depth at dorsal-fin origin 2.8 (2.5–3.0) in SL. Head large, its length 2.9 (2.8–2.9) in SL; from snout to forehead rising gently; upper profile in front of dorsal fin slightly concave; forehead narrow, its width 6.1 (7.5–7.6) in HL; eyes large, 2.8 (2.6–3.0) in HL; snout rounded, its surface rough and gelatinous with several small bumps, tip protruding anteriorly from premaxilla, its length 6.1 (5.8–6.1) in HL; space between eyes convex and rather narrow, interorbital width 3.0 (3.1–3.4) in HL.

Mouth large, posterior end of maxilla extending distinctly beyond vertical through posterior margin of eye. Nostrils close together but separated by narrow membranes, both immediately in front of anterior margin of eye and below horizontal through of eye; posterior nostril much larger than anterior one. No knob at symphysis of dentaries. Two supramaxillae, with anterior one triangular and posterior one rectangular, rounded posteriorly, not covering posterior portion of maxilla; postventral corner of maxilla exposed. Posterior-ventral margin of preopercle serrated with tip slightly pointed. Lower-jaw barbels, length 1.2 (1.0–1.1) in HL, its end exceeds pelvic-fin origin.

Most portions of lateral and medial surfaces of premaxilla and dentary covered with villiform teeth. Palatine and ectopterygoid with narrow band of villiform teeth; vomer with an oblong patch of villiform teeth. Endopterygoid with a large patch of villiform teeth. Gill rakers rod-shaped, somewhat laterally compressed, with villiform teeth on inner surfaces; those in outer row of the first arch longest; rakers on inner row of the first arch and both inner and outer rows of the second to third arches short, forming bumps; those on the fourth arch forming bumps; small tooth patches forming bumps, present on midline of all four outer arches. Villiform, teardrop-like tooth patches present on the fifth ceratobranchial. Small villiform teeth patch on the second pharygobranchial forming oval patches. Large, teardrop-like tooth patches on the third pharyngobranchial.

Body covered with firmly attached ctenoid scales; ctenii on body scales arranged in a wedge shape, forming three rows (Fig. 3 A View Figure 3 ). Isthmus with ctenoid scales; gular region naked, without scales.

Dorsal fin long, 2.0 (1.9–2.0) in SL. Dorsal-fin spines progressively longer posteriorly, length of the longest spine 0.9 (0.8–0.9) in eye diameter; outer margin of soft rays slightly concave; the first soft ray longest, and gradually shorter posteriorly. Anal-fin spines progressively longer posteriorly, length of the longest spine rather long, 1.2 (1.0–1.2) in eye diameter; outer margin of soft rays slightly concave; first soft ray longest. Pectoral fin short, 1.6 (1.5–1.8) in HL, its tip not reaching lateral line when adpressed to body. Pelvic fin without spine, its end slightly exceeds through pectoral-fin end vertical. Caudal-fin concave and pointed. Pyloric caeca pale, unbranched.

Size.

Moderately small species. The biggest specimen examined was 153.0 mm SL.

Coloration.

When fresh, body silvery, with dorsum slightly dusky (Fig. 1 A View Figure 1 ). Tip of anteriormost nine dorsal-fin soft rays black, forming distinct spot (paratype). Anal-fin rays pale, second to fourth spines and anteriormost five soft rays with few black pigmentations. Caudal fin grayish. Pelvic fin pale, with black pigmentations on first, fifth, and seventh rays.

Body color, when preserved, similar to when fresh, slightly yellowish (Figs 1 B View Figure 1 , 2 View Figure 2 ). Snout semitransparent. Oral cavity, including dorsal surface of tongue black (Fig. 4 A View Figure 4 ). Underside of tongue pale. Inner side of opercle and peritoneum black (Fig. 5 A View Figure 5 ). Tip of lower jaw without black spots.

Otolith.

The sagittal otolith was taken from NMMB-P 39588 (122.5 mm SL) and measured 6.8 mm in length (Fig. 6 View Figure 6 ). Otolith rhomboidal (length / height ratio 1.36), moderately thick (Fig. 6 A View Figure 6 ). Dorsal margin gently raised, lobed, highest at central; ventral margin slightly crenulated, regularly curved, deepest slightly anterior to central; posterior margin blunt, crenulated, not extending posteriorly; anterior margin triangular with incised notch; rostrum short and broad; antirostrum pointed; inner face convex; outer face slightly concave; sulcus centrally positioned, well divided into ostium and cauda; ostium wider than cauda, short, oblong, nearly filled with colliculum; cauda elongated, strongly curved at posterior with tip directing ventrally; cristae well developed; dorsal depression shallow, just above crista superior.

Distribution.

This species is known from specimens collected from southwestern and eastern Taiwan and the Chesterfield Islands of New Caledonia, suggesting a broad distribution in the western Pacific Ocean. Inhabits at depth down to 778 m (based on MNHN 2014-2291).

Genetic analysis.

The phylogenetic tree of Polymixia was reconstructed by the maximum-likelihood method with HKY + G + I ( Hasegawa et al. 1985) nucleotide substitution models. The monophyly of P. melanostoma sp. nov. specimens is strongly supported by a bootstrap value of 99 % (Fig. 7 View Figure 7 ).

The pairwise genetic distance analysis with the K 2 P model reveals an average interspecific distance of 4.4–14.5 % between P. melanostoma sp. nov. and the other six congeneric species (Table 4 View Table 4 ). The shortest genetic distance is with P. japonica , while the farthest is with P. berndti .

Comparison.

Polymixia melanostoma sp. nov. can be distinguished from 10 of the 12 congeneric species in having fewer gill rakers (3 + 1 + 6 = 10 vs 3–13 + 1 + 6 – 13 = 11 – 24 in P. lowei , P. japonica , P. berndti , P. fusca , P. yuri , P. longispina , P. salagomeziensis , P. busakhini , P. sazonovi , and P. carmenae ; Table 2 View Table 2 ; Kotlyar 1996; Caixeta et al. 2024). Among these congers, only P. nobilis has a count that overlaps with P. melanostoma sp. nov. Borden et al. (2019) suggested that P. nobilis might not only occur in the Atlantic Ocean but also the Pacific. However, in comparison to the description and data provided by Kotlyar (1996), P. melanostoma sp. nov. differs from P. nobilis in having fewer pyloric caeca (40 vs 108 in P. nobilis ), fewer S 2 (12–14 vs 15–16), different vertebral formula (12 + 17 = 29 vs 14 + 15 = 29), a longer head (34.2–35.4 % SL vs 30.8–32.3 % SL), a longer dorsal fin (49.6–53.0 % SL vs 41.3–44.3 % SL), and a more protruding snout (vs slightly protruding).

Although the number of gill rakers for Polymixia hollisterae was not documented by Grande and Wilson (2021), P. melanostoma sp. nov. differs from P. hollisterae in having more dorsal-fin rays (IV, 35–37 vs V, 31–32), more pyloric caeca (40 vs 30), and higher numbers of S 1 (6–8 vs 5), S 2 (12–14 vs 10), and S 3 (15–16 vs 12–14).

Compared with the species co-occurring in Taiwan ( Polymixia japonica , P. longispina , and P. berndti ), P. melanostoma sp. nov. exhibits meristic counts overlapping with P. japonica and shares distinctive characteristics, including a black branchial cavity (Fig. 5 View Figure 5 ) and ctenii on scales distributed in a wedge shape (Fig. 3 View Figure 3 ). It differs from P. japonica in having fewer number of gill rakers (3 + 1 + 6 = 10 vs 3–4 + 1 + 7 – 9 = 11 – 14; Table 2 View Table 2 ), more dorsal-fin rays (IV – IV, 35–37 vs IV – VI, 31–34; Table 2 View Table 2 ), a longer dorsal fin (49.6–53.0 % SL vs 42.5–48.3 % SL; Table 3 View Table 3 ), a fully black oral cavity (vs partly black; Fig. 4 View Figure 4 , Table 2 View Table 2 ), and a more protruding snout (vs not protruding; Table 2 View Table 2 ).

Additionally, P. melanostoma sp. nov. is also similar to P. longispina in having a long fourth anal-fin spine (12.0–15.2 % SL, mean 14.0 vs 12.6–22.9 % SL, mean 15.1; Table 3 View Table 3 ) but differs from it in having less gill rakers (3 + 1 + 6 = 10 vs 4 + 1 + 6–9 = 11 – 14; Table 2 View Table 2 ), more dorsal-fin rays (IV, 35–37 vs V, 28–32; Table 2 View Table 2 ), more pyloric caeca (40 vs 26–30; Table 2 View Table 2 ), more S 2 (12–14 vs 8–11; Table 2 View Table 2 ), a shorter predorsal length (46.3–47.1 % SL vs 49.1–58.5 % SL; Table 3 View Table 3 ), a longer dorsal-fin (49.6–53.0 % SL vs 40.7–46.4 % SL; Table 3 View Table 3 ), a black oral cavity (vs white; Fig. 4 View Figure 4 ; Table 2 View Table 2 ), a black branchial cavity (vs white; Fig. 5 View Figure 5 ; Table 2 View Table 2 ), a more protruding snout (vs slightly protruding; Table 2 View Table 2 ), and the wedge-shaped distribution of ctenii on scales (vs vertical; Fig. 3 View Figure 3 ; Table 2 View Table 2 ).

Polymixia melanostoma sp. nov. differs from P. berndti in having less gill rakers (3 + 1 + 6 = 10 vs 3–4 + 1 + 7 – 8 = 11 – 13; Table 2 View Table 2 ), more dorsal-fin rays (IV, 35–37 vs IV – VI, 28–31; Table 2 View Table 2 ), more S 2 (12–14 vs 8–11; Table 2 View Table 2 ), a shorter predorsal length (46.3–47.1 % SL vs 48.5–54.7 % SL; Table 3 View Table 3 ), a longer dorsal-fin (49.6–53.0 % SL vs 40.7–47.1 % SL; Table 3 View Table 3 ), a black oral cavity (vs white; Fig. 4 View Figure 4 ; Table 2 View Table 2 ), a black branchial cavity (vs white; Fig. 5 View Figure 5 , Table 2 View Table 2 ), and the wedge-shaped distribution of ctenii on scales (vs vertical; Fig. 3 View Figure 3 , Table 2 View Table 2 ).