Malakichthys formosus Ng & Liu & Joung 2023, sp. nov.

Malakichthys formosus sp. nov.

Figs. 2–5, Table 1

Holotype. NMMB-P37683 (85.4 mm SL), ca. 22°36’ N, 120°16’ E, off Donggang, southwestern Taiwan, northwestern Pacific Ocean, depth 300–400 m, 23 July 2022.

Paratypes. Five specimens, 64.9–74.1 mm SL. NMMB-P36784 (72.1) (dissected); NMMB-P37685 (71.9), NMMB-P37686 (64.9), NMMB-P37687 (71.2), NMMB-P37688 (74.1), collected with the holotype.

Diagnosis. No paired spines on chin; proximal-middle radial of FAP curved, with cone-shaped hollow divided by lamellar septum; hollow shallow, its lower part of the hollow terminates at the midway between the upper part and the base of the first anal-spine; lateral wing well developed, extending from the upper part of hollow to the base of the anal-spine; anterior surface roundly convex; vane long, extending from the level of lateral wing to the base of anal-spine; membranous edge of opercle with a prominent triangular tip; pectoral-fin tip not reaching the vertical level of anus; lateral-line scales 53–56.

Description. Dorsal-fin rays IX-I, 10 (9–10); anal-fin rays III, 8 (7–8); pectoral-fin rays 13 (13–14); pelvic-fin rays I, 5 (5); scales on lateral line 54 (53–56); scales above lateral line 5 (5–6); scales below lateral line 12 (11–12); branchiostegal rays 5 (5); gill rakers 9 (8–9)+1+20 (20–22); vertebral count 15 (15) Other morphometrics are shown in Table 1.

Data of the other species are obtained from Yamanoue & Matsuura (2004).

Body slightly elongate, laterally compressed. Dorsal contour of head rather straight. Head, cheek and body with caducous ctenoid scales; bases of soft portions of fins scaled; gular region with very tiny scales; opercle naked. Snout snort, blunt, its length 52.3 (46.7–59.9) % orbit length. Mouth oblique, posterior margin of upper jaw reaching to or slightly beyond a vertical line through orbit. Supramaxillary absent. Eyes large, its length 38.5 (32.3–38.4) % head length. Interorbital space slightly concave. Villiform teeth in band on jaws, vomer, and palatine. No canine teeth. No paired spines on chin. Preopercle with double edges, posterior edge heavily serrated; opercle with two weak spines, interopercle posteroventral margin slightly serrated; subopercle anteroventral margin weakly serrated, its posterior edge membranous, extended to form a long, triangular tip. Gill rakers very slender, with a pointed tip. Nostrils elliptical, close to each other, large, length about half of the orbit diameter, situated just anterior to orbit. Anus close to the anal-fin origin, situated at about the same level of the 1 st dorsal-fin soft ray. Lower end of gill opening just reaching a vertical line through posterior end of anterior nostril. Lateral almost straight, except for the elevated anterior part.

Single dorsal fin, spinous and soft portions of dorsal fin separated; first dorsal-spine very short, its length not reaching half of the length of the second dorsal-spine; the first dorsal-fin soft ray longest. Anal-fin spine slender, the first anal-fin soft ray longest. All dorsal- and anal-fin rays branched. The 2 nd pelvic-fin soft ray longest. Pectoral-fin tip not reaching the vertical level of anus. Caudal fin forked.

Anterodorsal tip of proximal-middle radial of FAP elongated (Fig. 4, 5). Proximal-middle radial of FAP with a short and shallow cone-shaped hollow, partitioned by a lamellar septum into upper and lower parts; the upper part cone-shaped part encloses the posterior part of swim bladder; the lower part is an empty space, closed by lamellar septum. The end of the lower part of the hollow terminates at the midway between the upper part and the base of the first anal-spine. Lateral wing well developed on shaft portion of proximal-middle radial, extend from the upper part of hollow to the base of the anal-spine. Anterior surface roundly convex. Vane long, extend from the level of lateral wing to the base of anal-spine.

Coloration. (When fresh) Dorsal half of body light brown, demarcated from the silvery white lower half of body. A small black spot present on the opercle. All fins translucent; membrane of spinous portion of dorsal fin often with black margin. Scale pockets with distinct black margins. Peritonium pale in the holotype (In preservative) very similar to coloration when fresh.

Size. To at least 85.4 mm SL. The dissected specimen measuring 72.1 mm TL does not process developed gonads.

Distribution. Known only from off Donggang, Pingtung County, southwestern Taiwan, northern South China Sea, northwestern Pacific, at a depth range of 300– 400 m.

Etymology. The species name, formosus, is derived from the historical name of Taiwan, signifying its connection to the type locality where the new species was first described.

Comparisons. Malakichthys formosus sp. nov. can be readily distinguished from its congeners, except M. mochizuki, by the following combination of the characters: from M. barbatus, M. elegans, M. griseus, M. similis and M. wakiyae by the absence of paired spines on the chin (vs. present), the presence of a lamellar septum at the hollow of the proximal-middle radial of FAP (vs. pterygiophore shape different; Yamanoue & Matsuura 2004: fig. 1), having more lateral-line scales (53–56 vs. 45–50), shallower body depth (23.6–27.8 vs. 28–42 % SL), a shorter distance between snout and the pelvic-fin origin (33.5–36.8 vs. 37–45% SL), and a shorter third anal-fin spine (10.1–11.8% vs. 12–20% SL); and from M. levis by having a lamellar septum at the hollow of the proximal-middle radial of FAP (vs. absence of lamellar septum), more lateral-line scales (53–56 vs. 49–53), fewer scales below lateral line (11–12 vs. 13–16), a shorter distance between snout and the dorsal-fin origin (34.8–37.3 vs. 37–41% SL), a shorter distance between snout and the pelvic-fin origin (33.5–36.8 vs. 36–44% SL), and a shallower head (head depth 21.2–24.0 vs. 24–30% SL).

According to Yamanoue & Matsuura (2001), M. formosus sp. nov. is most similar to M. mochizuki. Both species share the characters of the absence of paired spines on the chin, and the presence of a lamellar septum at the hollow of the proximal-middle radial of FAP. However, some discernible differences in the morphologies of the pterygiophores are present between the two species: in M. formosus, the hollow is shallow that extends to the midway between the upper part of hollow and the base of anal-fin spine only (vs. extending well beyond the midway between the upper part of hollow and the base of anal-fin spine in M. mochizuki); lateral wing well developed, extending from the level of upper part of hollow towards the base of the anal-spine (vs. poorly developed); anterior surface with a roundly convex margin (vs. margin straight); vane long, extending from the level of the upper part of hollow towards the base of anal-fin spine (vs. vane short, extending from the level of lower part of hollow towards the base of anal-fin spine). It further differs from M. mochizuki in having more lateral-line scales (53–56 vs. 50–52), fewer scales below lateral line (11–12 vs. 13–16), a shorter distance between snout and the dorsal-fin origin (34.8–37.3 vs. 37–41% SL), a shallower head (head depth 21.2–24.0 vs. 24–28% SL), and having fewer gill rakers on lower limb (20–22 vs. 23–25).