Polymixia melanostoma Fan, Su, Lin, Chang & Lin, 2024, sp. nov.
Creators: name: Fan, You-Ci
name: Su, Yo
name: Lin, Chien-Hsiang
name: Chang, Chih-Wei
name: Lin, Hsiu-Chin
Publisher: name: Zenodo
Year: 2024
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Polymixiiformes
Polymixiidae
Polymixia
Polymixia melanostoma

description: Polymixia melanostoma sp. nov.
Figs 1, 2, 3, 4 A, 5 B, 6, 7; Tables 1, 2, 3, 4 New English name. Black-mouth beardfish New Chinese name. 黑口鬚銀眼鯛
Polymixia japonica (non Günther): Li 2010: 12–13 (in part, specimens were mixed with P. japonica).
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.
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. – New Caledonia • 1 specimen; MNHN 2014-2291 (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.
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, 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). 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). 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, 2). Snout semitransparent. Oral cavity, including dorsal surface of tongue black (Fig. 4 A). Underside of tongue pale. Inner side of opercle and peritoneum black (Fig. 5 A). 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). Otolith rhomboidal (length / height ratio 1.36), moderately thick (Fig. 6 A). 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).
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). 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; 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) and ctenii on scales distributed in a wedge shape (Fig. 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), more dorsal-fin rays (IV – IV, 35–37 vs IV – VI, 31–34; Table 2), a longer dorsal fin (49.6–53.0 % SL vs 42.5–48.3 % SL; Table 3), a fully black oral cavity (vs partly black; Fig. 4, Table 2), and a more protruding snout (vs not protruding; 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) but differs from it in having less gill rakers (3 + 1 + 6 = 10 vs 4 + 1 + 6–9 = 11 – 14; Table 2), more dorsal-fin rays (IV, 35–37 vs V, 28–32; Table 2), more pyloric caeca (40 vs 26–30; Table 2), more S 2 (12–14 vs 8–11; Table 2), a shorter predorsal length (46.3–47.1 % SL vs 49.1–58.5 % SL; Table 3), a longer dorsal-fin (49.6–53.0 % SL vs 40.7–46.4 % SL; Table 3), a black oral cavity (vs white; Fig. 4; Table 2), a black branchial cavity (vs white; Fig. 5; Table 2), a more protruding snout (vs slightly protruding; Table 2), and the wedge-shaped distribution of ctenii on scales (vs vertical; Fig. 3; 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), more dorsal-fin rays (IV, 35–37 vs IV – VI, 28–31; Table 2), more S 2 (12–14 vs 8–11; Table 2), a shorter predorsal length (46.3–47.1 % SL vs 48.5–54.7 % SL; Table 3), a longer dorsal-fin (49.6–53.0 % SL vs 40.7–47.1 % SL; Table 3), a black oral cavity (vs white; Fig. 4; Table 2), a black branchial cavity (vs white; Fig. 5, Table 2), and the wedge-shaped distribution of ctenii on scales (vs vertical; Fig. 3, Table 2).
description: Published as part of 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), pp. 105-121 in ZooKeys 1220 on pages 105-121, DOI: 10.3897/zookeys.1220.125127