Stolephorus astrum, Hata & Motomura & Hiroyuki, 2024

Hata, Harutaka, Motomura, & Hiroyuki, 2024, A new species of anchovy, Stolephorus astrum (Teleostei: Clupeiformes: Engraulidae), from Palau, Micronesia, Raffles Bulletin of Zoology 72, pp. 150-161 : 153-157

Stolephorus astrum , new species

(English name: Astral Anchovy) ( Figs. 1 , 2A ; Tables 1–5)

Holotype. CAS 230413 , 48.1 mm SL, ca. 300 yards (approx. 274 m) from mouth of channel draining waters from Ngeremeskang and Ngkebeduul Rivers , Karamado Bay, Babeldaob Island, Palau (7°29′47″N, 134°31′31″E), 10 feet (approx. 3.05 m) depth (water brown, turbid), 7 September 1955, 1630–1730 hours.

Paratypes. 16 specimens: 32.5–54.8 mm SL. CAS 230408 , 11 specimens, 35.8–54.8 mm SL, KAUM –I. 188269, 43.1 mm SL, KAUM –I. 188270, 32.5 mm SL, USNM 445732 , 35.3 mm SL, USNM 445733 , 34.9 mm SL, 0–50 feet (approx. 15.2 m) off muddy shore of mangrove estuary in Kamiyangaru Bay , Babeldaob Island, Palau (7°26′29″N, 134°29′4″E), 2–10 feet (approx. 0.61–3.05 m) depth (water very muddy; current ca. 0.25–1 knot); CAS 248277 , 54.2 mm SL, collected with holotype .

Diagnosis. A species of Stolephorus with the following combination of characters: maxilla short, 15.9–17.8% of SL, its posterior tip just reaching to or slightly beyond anterior margin of preopercle (just short of anterior margin in individuals <36 mm SL); posterior margin of preopercle convex, rounded; no predorsal scutes; prepelvic scutes 1–4 (modally 4); pelvic scute without spine; dorsal fin with three unbranched and 12–14 (13) branched rays; anal fin with three unbranched and 18 or 19 (18) branched rays, its origin located just below base of ninth to eleventh dorsal-fin ray; 1UGR 20–24 (22), 1LGR 30–34 (32), 1TGR 50–56 (53); 2UGR 15–18 (11), 2LGR 26–30 (29), 2TGR 42–48 (46); 3UGR 12–13 (12), 3LGR 15–17 (17), 3TGR 27–30 (29); 4UGR 9–10 (9), 4LGR 12–14 (13), 4TGR 21–24 (22); gill rakers 5–7 (6) on posterior face of third gill arch; transverse scales 8; pseudobranchial filaments 16–21 (18); total vertebrae 38 or 39 (39); pelvic fin short, 10.2–11.3% of SL, tip of depressed pelvic fin reaching posteriorly to vertical through 3rd to 6th dorsal-fin ray origin; pectoral fin short, 15.9–16.3% of SL; predorsal fin length short, 50.1–53.6% of SL (mean 52.3%); caudal peduncle rather long, 16.8–18.6% of SL (17.5%); body slender, 16.8–19.0% of SL (18.1%), distance between dorsal-fin origin and pelvic-fin insertion 20.1–23.4% of SL (22.0%), between origins of dorsal and anal fins 19.1–20.3% (19.7%); pairs of dark patches on parietal and occipital areas; no dark lines on dorsum; black spots on tips of snout and lower jaw, but not on suborbital area; melanophores scattered on 2nd to 8th uppermost pectoral-fin rays.

Description. Data for holotype given in parentheses (if different). Body laterally rather compressed, elongate, deepest at dorsal-fin origin. Dorsal profile of head and body gently elevated from snout tip to dorsal-fin origin, thereafter gradually lowering to uppermost point of caudal-fin base. Ventral profile of head and body gently lowering from lower-jaw tip to below pectoral fin, thereafter nearly straight (parallel to body axis) to anal-fin origin, gradually elevating along anal-fin base, thereafter nearly straight (parallel to body axis) to lowermost point of caudal-fin base. Abdomen rounded, covered with one to four (three) spine-like prepelvic scutes anterior to pelvic fin insertion. Pelvic scute without spine. Postpelvic and predorsal scutes absent. Anus just anterior to anal-fin origin.

Snout round, projecting, length less than eye diameter. Eye large, round, covered with adipose eyelid, positioned laterally on head dorsal to horizontal through pectoral-fin insertion, visible in dorsal view. Pupil round. Orbit elliptical. Nostrils close to each other, anterior to orbit. Mouth large, inferior, ventral to body axis, extending backward beyond posterior margin of eye. Maxilla short, its posterior tip just reaching to or slightly beyond anterior margin of preopercle (just reaching in holotype; just short of anterior margin in paratypes <36 mm SL). Lower jaw slender.

Single row of conical teeth on each jaw, palatines, and inner edges of pterygoids. Several conical teeth on vomer. Small fine teeth patch on pterygoids. No teeth on upper edge of hyoid arch. Single row of blunt poorly-formed conical teeth on upper edges of basihyal and basibranchial.

Posterior margins of preopercle, subopercle and opercle rounded, smooth. Gill membrane without serrations. Interorbital space flat. Interorbital width less than eye diameter. Pseudobranchial filaments present, length of longest filament less than eye diameter. Gill rakers long, slender, rough, visible from side of head when mouth open. Isthmus muscle long, reaching anteriorly to posterior margin of gill membranes. Urohyal hidden by isthmus muscle (not visible without dissection). Gill membrane on each side joined distally, most of isthmus muscle exposed (not covered by gill membrane).

Dorsal-fin origin posterior to vertical through base of last pelvic-fin ray, slightly posterior to middle of body. Dorsal and anal fins with three anteriormost rays closely spaced and unbranched. First dorsal- and anal-fin rays minute. Dorsal profile of dorsal fin elevated from dorsal-fin origin to third fin ray tip and lowering to last dorsal-fin ray tip. Anal-fin origin just below base of ninth to eleventh (tenth) dorsal-fin ray. Posterior tip of depressed anal fin not reaching caudal-fin base. Uppermost pectoral-fin ray unbranched, inserted below midline of body. Posterior tip of pectoral fin not reaching vertical through pelvic-fin insertion. Dorsal, ventral, and posterior contours of pectoral fin nearly straight. Pelvic fin shorter than pectoral fin. Posterior tip of depressed pelvic fin reaching to vertical through third to sixth (fourth) dorsal-fin ray origin. Caudal fin forked, tips of both lobes pointed.

Body scales deciduous (completely lost in all specimens, except for prepelvic scutes). Head scales absent. Fins scaleless, except for broad triangular sheath of scales on caudal fin.

Colouration of preserved specimens. Body uniformly pale, light ivory longitudinal band running from just behind upper opercular margin to caudal-fin base. Pairs of dark patches on parietal and occipital regions. No dark lines on dorsum either anterior or posterior to dorsal fin. Melanophores scattered along posterior margin of dorsal scale pockets, tips of snout and lower jaw, and lateral surfaces of post-temporal region, and upper part of gill slit, but not on suborbital area. Fin rays transparent, colourless. Melanophores scattered along bases of dorsal and anal fins, and along fin rays of dorsal and caudal fins and anterior part of anal fin. Melanophores scattered on 2nd to 8th (2nd) pectoral-fin rays from uppermost ray ( Fig. 2A ), gill rakers and gill filaments on first gill arch, gill arch, pseudobranch, and inner side of hyoid arch. Fresh colouration unknown.

Distribution. The new species is currently known only from Babeldaob Island, Palau. All of the type specimens were collected from around river estuaries (see collection localities).

Etymology. The specific name ‘ astrum ’ is given in reference to the starry black spots on the pectoral fins of the new species.

Comparisons. The new species is assignable to the genus Stolephorus , as defined by Whitehead et al. (1988) and Wongratana et al. (1999), having a long isthmus muscle reaching anteriorly to the posterior margin of the gill membrane, the urohyal covered by the isthmus muscle, possession of prepelvic scutes, and the anal fin originating just below the middle of the dorsal-fin base, in addition to the absence of postpelvic scutes.

The short maxilla of the new species (posteriorly just reaching or slightly beyond the anterior margin of the preopercle; just short of anterior margin in individuals <36 mm SL) separates the former from the majority of congeners, except for Stolephorus advenus (Wongratana, 1987) , Stolephorus balinensis ( Bleeker, 1849) , Stolephorus belaerius Hata, Lavoué & Motomura, 2021 , Stolephorus commersonnii Lacepède, 1803 , Stolephorus horizon Hata & Motomura, 2023 , Stolephorus indicus ( van Hasselt, 1823) , Stolephorus leopardus Hata & Motomura, 2021 , Stolephorus meteorum Hata, Lavoué, Bogorodsky, Alpermann & Motomura, 2023 , Stolephorus pacificus Baldwin, 1984 , Stolephorus scitulus ( Fowler, 1911) , and Stolephorus teguhi Kimura, Hori & Shibukawa, 2009 ( Whitehead et al., 1988; Wongratana et al., 1999; Kimura et al., 2009; Hata & Motomura 2018a –e, 2021a–c, 2022a, b, 2023, 2024a, b; Hata et al., 2019, 2020a, b, 2021, 2022b, c, 2023; Gangan et al., 2020). Among them, S. astrum can be easily distinguished from S. advenus , S. balinensis , S. belaerius , S. commersonnii , S. horizon , S. indicus , S. meteorum , and S. scitulus by its higher numbers of gill rakers (1LGR 30 or more in S. astrum vs. 28 or fewer in the others) and the pelvic fin reaching posteriorly beyond vertical through the dorsal-fin origin (vs. not reaching) ( Wongratana, 1987a; Hata et al., 2021, 2023; Hata & Motomura, 2023; this study).

On the other hand, the new species is differentiated from the remaining three species ( S. leopardus , S. pacificus , and S. teguhi ), which all have higher gill-raker numbers (1TGR 50–56 in S. astrum vs. 57 or more in the others; Table 3). Moreover, the new species differs from the other three species in its narrower body [body depth 16.8–19.0% of SL (mean 18.1%) vs. 17.5–22.2% (19.6%) in S. pacificus , <19.5% in S. leopardus and S. teguhi ; D–P2 18.3–20.5 % of SL (19.7%) vs. 20.1–23.4% (22.0%) in S. pacificus , <22% in S. leopardus and S. teguhi ; D–A 19.1–20.3% of SL (19.7%) vs. 19.8–23.0% (21.6%) in S. pacificus , <21% in S. leopardus and S. teguhi ; Fig. 3A–C ], shorter predorsal-fin length [50.1–53.6% of SL (mean 52.3%) vs. 56.4–57.4% (56.6%) in S. leopardus ; 52.9–56.9% (mean 54.3%; less than 50% in 2 of 27 specimens) in S. pacificus ; 50.6–57.4% (54.5%) in S. teguhi ; Fig. 3D ], and longer caudal peduncle [16.8–18.6% of SL (mean 17.5%) vs. 13.8–17.4% (15.6%) in S. pacificus ;> 16.5% in S. leopardus and S. teguhi ; Fig. 3E ]. Stolephorus astrum further differs from S. leopardus in having a shorter snout (3.8–4.4% of SL in S. astrum vs. 4.7–4.8% in S. leopardus ; Fig. 3F ), anal-fin base (19.8–22.3% of SL vs. 22.4–22.5%; Fig. 3G ), D–P1 (31.9–34.1% of SL vs. 34.6–35.6%; Fig. 3H ), postorbital length (11.9–13.9% of

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SL vs. 13.9–14.6%; Fig. 3I ), distance from supramaxilla end to maxilla end (12.2–18.0% of maxilla length vs. 8.1–10.8%; Fig. 3J ), more branched dorsal-fin rays [12–14 (modally 13) vs. 11 or 12; Table 4] and fewer branched anal-fin rays [18 or 19 (18) vs. 19–21 (19); Table 5], dorsum without dark lines (vs. single dark line on dorsum from occipital region to upper margin of caudal-fin base), and no pigmentation on lateral surface of both upper and lower jaws (vs. melanophores scattered on both jaws).

Furthermore, S. astrum can be distinguished from S. pacificus by having a shorter pectoral fin (15.9–16.3% of SL in S. astrum vs. 16.5–18.3% in S. pacificus ; Fig. 3K ), with melanophores scattered on 2nd to 8th uppermost rays (vs. pectoral fin without melanophores; Fig. 2 ). The new species differs from S. teguhi in having the anal fin with fewer branched rays [18 or 19 (modally 18) in S. astrum vs. 19–21 (20) in S. teguhi ; Table 4] and shorter base (19.8–22.3% of SL vs. 22.7–25.3%; Fig. 3G ), shorter pectoral fin (15.9–16.3% of SL vs. 16.5–18.8%; Fig. 3K ), greater distance from supramaxilla end to maxilla end (12.2–18.0% of maxilla length vs. 6.3–12.2%; Fig. 3J ), the dorsum without dark lines (vs. single dark line on dorsum from occipital region to upper margin of caudal-fin base), and no pigmentation on lateral surface of both upper and lower jaws (vs. melanophores scattered on both jaws) ( Hata & Motomura, 2021c; this study; Table 6).

Figures

Fig. 1. A, lateral, B, dorsal, and C, ventral views of the holotype (CAS 230413, 48.1 mm SL, Babeldaob Island, Palau), and paratypes (D, CAS 248277, 53.3 mm SL, and E, CAS 248277, 54.8 mm SL (both Babeldaob Island, Palau) of Stolephorus astrum, new species (preserved specimens).

Fig. 2. Pectoral-fin rays of preserved specimens of A, Stolephorus astrum, new species (CAS 230408, 49.8 mm SL, Babeldaob Island, Palau; melanophores scattered on 6 uppermost fin rays; left-right inverted) and B, S. pacificus (USNM 263476, 57.8 mm SL, Talofofo Bay, Guam; no melanophores on pectoral fin).

Fig. 3. Relationships of selected measurements relative to standard length (SL) versus SL or maxilla length in Stolephorus astrum, new species (red circles), S. leopardus (yellow diamonds), S. pacificus (blue triangles), and S. teguhi (green squares). A, body depth (as % of SL); B, distance between dorsal-fin origin and pelvic-fin insertion (D–P2; as % of SL); C, distance between origins of dorsal and anal fins (D–A; as % of SL); D, predorsal-fin length (as % of SL); E, caudal-peduncle length (as % of SL); F, snout length (as % of SL); G, anal-fin base length (as % of SL); H, distance between dorsal-fin origin and pectoral-fin insertion (D–P1; as % of SL); I, postorbital length (as % of SL); J, distance between ends of supramaxilla and maxilla [SMX–MX; as % of maxilla length (ML)]; K, pectoral-fin length (as % of SL).