Malacosarcus macrostoma ( Günther, 1878 ), A ASIZP

Su, Yo, Lee, Mao-Ying & Ho, Hsuan-Ching, 2024, Redescription of Malacosarcus macrostoma (Günther, 1878) from the abyssal zone off Taiwan, northwestern Pacific Ocean (Beryciformes, Stephanoberycoidei, Stephanoberycidae), ZooKeys 1220, pp. 93-104 : 93-104

Malacosarcus macrostoma ( Günther, 1878)

Figs 1 , 2 , 3 , Tables 1 , 2 English name: Large-mouth pricklefish Chinese name: 大口軟冠鯛

Scopelus macrostoma Günther, 1878: 186 (type locality: Mid Pacific, 0°33'S, 154°34'W, depth 2,425 fathoms (4,435 m). Holotype: BMNH 1887.12. 7.11).

Malacosarcus macrostoma : Günther 1887: 30 (new genus and new combination). Goode and Bean 1896: 182 (in part). Grey 1956: 191 (listed). Ebeling and Weed 1973: 416 (in part). Kotlyar 1996: 261 (in part). Kotlyar 2004: 2 (listed). Merrett and Moore 2005: 1708 (in part, compared to the new genus and species described). Mundy 2005: 301 (listed, Hawaiian Islands). Nonaka et al. 2021: 153, fig. 9 B (in part, larval record from the Hawaiian Islands).

Stephanoberycidae sp.: Boehlert and Mundy 1992: 83, fig. 4 (larval record from Hancock Seamount, Hawaiian Islands).

Specimens examined.

Taiwan • ASIZP 73637 , 65.5 mm SL, off southeastern Taiwan , Philippine Sea, 22°3'38.16"N, 121°10'6.95"E, R/V Ocean Researcher I, sta. CP 413, beam trawl, depth 4,412–4,458 m, 12 Jun. 2008 ; ASIZP 73644 , 55.7 mm SL, off southeastern Taiwan , Philippine Sea, 22°7'32.15"N, 122°5'35.87"E, R/V Ocean Researcher I, sta. CP 415, beam trawl, depth 4,806–4,813 m, 14 Jun. 2008 ; ASIZP 73646 , 61.2 mm SL, off southeastern Taiwan , Philippine Sea, 22°12'28.07"N, 122°6'56.16"E, R/V Ocean Researcher I, sta. CP 415, beam trawl, depth 4,801–4,813 m, 15 Jun. 2008 . All collected by M.-Y. Lee.

Diagnosis.

A species of Stephanoberycidae characterized by the absence of dorsal-and anal fin-spines (vs fin spines present in other genera, sometimes absent in Acanthochaenus ); 11–12 anal-fin rays (vs 9–10 in Acanthochaenus ); GRI 7–8 + 1 + 15–19 = 23–27 (vs 5–7 + 1 + 13–14 = 19–21 in Abyssoberyx and 12–15 + 1 + 24–27 = 37–40 in Stephanoberyx ); lateral line forms flaps in approximately 42–46 vertical rows (vs lateral line without distinct vertical flaps in Acanthochaenus and Stephanoberyx ); vertebrae 10 + 20–21 = 30–31 (vs 10–11 + 21–23 = 32–34 in Abyssoberyx ); body scales cycloid and deciduous (vs body scales ctenoid and adherent in Acanthochaenus and Stephanoberyx ); and origin of pelvic fin situated nearer to pectoral fin than to anal fin (vs pelvic fin situated nearer to anal fin than to pectoral fin in Acanthochaenus ).

Description of Taiwanese specimens.

Meristic and morphometric data are provided in Tables 1 and 2 .

Dorsal-fin rays 13–14. Pectoral-fin rays 12–13 / 12 – 13. Pelvic-fin rays 4–6 / 4 – 5. Anal-fin rays 11–12. Principal caudal-fin rays 10 + 9, uppermost and lowermost rays unbranched; procurrent caudal-fin rays 9 and 10–11 on upper and lower lobes, respectively. GRI 7–8 + 1 + 15–18 = 23–27 (total); GRII 6–7 + 1 + 15–17 = 22–25; GRIII 3 + 1 + 14 = 18; GRIV 1–2 + 0–1 + 10–12 = 12–14. Pseudobranchial filaments 5–6 (n = 2). Lateral-line flaps in approximately 42–46 vertical rows. Vertebrae 10 + 20–21 = 30–31; branchiostegal rays 8.

Body slender, greatest depth 4.1–4.3 in SL; body laterally compressed. Head oval, length 2.7–2.8 in SL; its height 1.3–1.6 in HL; upper profile of head slightly rounded, with nearly straight profile to dorsal-fin origin; forehead slightly convex, its height 6.1–7.3 in HL; eye diameter 4.6–4.9 in HL; tip of snout rounded, extending slightly before premaxilla, its length 3.9–4.6 in HL; interorbital width 2.9–3.1 in HL.

Mouth oblique, upper-jaw length 1.4–1.6 in HL; posterior end of maxilla rounded, exceeding beyond vertical through posterior margin of eye; lower jaw slightly larger than upper jaw, length 1.4 in HL; its anterior tip protruding before upper jaw when closed. Two nostrils situated in front of eye; both at same horizontal through center of eye; both nostrils rounded and nearly same in size. No trace of nasal organs in all specimens (possibly shrunk during preservation). Symphysis of premaxillae notched and naked. Symphysis of dentaries forming single and blunt knob. Supramaxilla single, with needle-like process anteriorly and rectangular, slightly oval process posteriorly; its posterior tip in advance of tip of maxilla.

Head skeletons and their ridges delicate. Opercle with one central ridge but not forming spine. Small spines on posteroventral margins of both inner and outer ridges of preopercle. Posttemporal ridge rounded, with small spines on its outer margin. Both premaxilla and dentary with villiform teeth on outer and medial surfaces. Palatine and vomer edentate.

Gill rakers on outer face of all four arches rod-shaped and laterally compressed, their inner surfaces covered with small teeth; rakers on outer row of first arch longer than remainder, longest gill raker in 4.0– 4.6 in HL; rakers on inner surfaces of outer three arches absent or forming minute bumps; no tooth patches present between rakers on all four arches. Fourth gill arch largely attached to the wall leaving slit between arch and gill chamber. No tooth on fifth ceratobranchial. Oval, conical tooth patch on second epibranchial arch. Large, moon-shaped conical tooth patch on fourth pharyngobranchial. Gill filaments present on all four rakers. Gill filaments on first arch very short, 8.4–14.9 in length of longest opposite rakers. Pseudobranch present, short and poorly developed.

Body scales cycloid and deciduous, embedded under skin; those covered by pectoral fin smaller than rest. Lateral-line scales and abdominal scutes absent. No scales on gular region and isthmus. Cycloid scales present on cheek.

Dorsal fin situated posteriorly, its origin about same vertical of anus. Pectoral-fin length 2.3–2.5 in HL; its origin at horizontal through ventral margin of eye; its tip reaching vertical through anus. Pelvic-fin length 5.0 in HL; its origin behind pectoral-fin base and tip reaching anus when adpressed. Anal-fin origin at vertical through fifth dorsal-fin ray; its posterior end at same vertical through that of dorsal fin. Caudal fin moderately small, forked. All fin rays delicate, with smooth surfaces.

Skins on surface of lateral line forming many vertical rows of flaps. Its main branch forming canal, originating behind and below posttemporal bone; its anterior portion gently curved down, becoming nearly straight on posterior portion. Anus situated immediately anterior to anal-fin origin. Caudal peduncle slender, with postdorsal and postanal length 1.4–1.5 and 1.3 in HL, respectively; its height 5.1–5.7 in HL. Light organs absent.

Coloration.

When fresh (Fig. 1 ), body pale and somewhat translucent; head and abdominal region uniformly black; all fins pale; lateral-line flaps forming vertical dark bands on lateral side of body. When preserved (Fig. 2 ), body and fins uniformly pale; oral cavity, including underside of tongue, inner face of operculum, and gill arches brown; gill rakers pale.

Osteology

(Fig. 3 ). Three supraneurals. First dorsal-fin pterygiophore inserts between eighth and ninth or seventh and eighth vertebra. Pleural ribs present on sixth to tenth vertebra; epineurals present. Anal-fin origin below twelfth vertebra.

Size.

A rather small species, reported up to 8.9 cm ( Günther 1878).

Distribution.

Known from specimens collected from the tropical central Pacific Ocean at depths 2,777 – 4,434.8 m ( Günther 1878; Ebeling and Weed 1973; Mundy 2005). Our specimens were collected from the northwestern Pacific at depths 4,412 –4,813 m (Fig. 4 ).

Figures

Figure 1. Fresh specimens of Malacosarcus macrostoma (Günther, 1878) A ASIZP 73637, 65.5 mm SL, left-right reversed B ASIZP 73644, 55.7 mm SL, left-right reversed C ASIZP 73646, 61.2 mm SL.

Figure 2. Preserved specimens of Malacosarcus macrostoma (Günther, 1878) A ASIZP 73637, 65.5 mm SL B ASIZP 73644, 55.7 mm SL C ASIZP 73646, 61.2 mm SL. Photos by Y. - C. Hsu.

Figure 3. X-radiographs of Malacosarcus macrostoma (Günther, 1878) A ASIZP 73637, 65.5 mm SL B ASIZP 73644, 55.7 mm SL C ASIZP 73646, 61.2 mm SL.

Figure 4. Distribution map of Malacosarcus macrostoma (Günther, 1878). Data source: triangle = this study; star = type locality; circle = other records (Günther 1887; Ebeling and Weed 1973; Boehlert and Mundy 1992; Nonaka et al. 2021); square = possible records (Jamieson et al. 2021).