Acentronura gracilissima Temminck and Schlegel, 1850

Acentronura gracilissima Temminck and Schlegel, 1850 View in CoL

Figures 8B View Fig , 9B View Fig ; Tables 2–4 View Table 2 View Table 3 View Table 4

Hippocampus gracilissimus Temminck and Schlegel, 1850: 274 View in CoL , pl. 120, fig. 6 ( Japan).

Atelurus germani Duméril, 1870: 584 View in CoL (Cochin, China).

Diagnosis.— See generic diagnosis. Acentronura gracilissima differs from its congeners by the following combination of characters: supraoccipital consisting of a distinct dorsomedial crest, non-segmented, ridge-like in lateral view, followed by an elevated and bony dimple; bony dimple approximately one-third in length of dorsomedial crest.

Description.— Morphometric and meristic characters listed in Tables 2–4 View Table 2 View Table 3 View Table 4 . Superior trunk and tail ridges discontinuous below dorsal-fin base, lateral tail ridge present, inferior trunk ridge ends at anal ring, lateral trunk ridge confluent with inferior tail ridge. Head angled ventrally 25° from the principal body axis, the anterodorsal profile slightly conoid in lateral aspect; supraoccipital, low, not elevated, with a distinct dorsomedial crest, non-segmented, ridge-like in lateral view, followed by an elevated and bony dimple; bony dimple approximately one-third in length of dorsomedial crest; bilateral bony lobed protuberances on the posterolateral margins of the post-temporal bones ( Fig. 10B View Fig ); anterior nuchal plate absent ( Fig. 9B View Fig ); posterior nuchal plate present anterior to cleithrum with dorsomedial crest-like ridge along its dorsum, large gap present between the supraoccipital and posterior nuchal plate; small blunt spine midway between orbit and lobed protuberance on operculum; cleithral ring distinct, discontinuous mid-dorsally; single gill slit between supraoccipital and cleithral ridge; rim of orbit projecting dorsolaterally and slightly ventrolaterally; snout spine absent; interorbital narrow, depressed; opercular ridge distinct, entire, angled dorsally toward gill opening; swelling of gular region ventroposterior to orbit absent; pectoral-fin base without distinct ridges, low, strongly elevated ventrolateral bulge supporting the pectoral-fin absent; trunk deepest anteriorly, principal body ridges distinct; principal body ridge spines absent; caudal fin absent. Acentronura gracilissima exhibits strong sexual dimorphism associated with the presence of a male brood pouch. The brood pouch is formed along the ventral midline of the tail and is present below the anteriormost 14 tail rings ( Fig. 8B View Fig ). µCT scanning of the specimen ( Fig. 9B View Fig ) revealed the brood pouch is enclosed by 14 arcuate bony extensions, which extend ventrolaterally from the anterior ventral plate ridges of the tail and are reduced in size posteriorly. The anteriormost pouch plate is broad and paddle-shaped at ventrocaudal margin and curved posterolaterally relative to other pouch plates. The second pouch plate appears to consist of double arcuate bony extensions merged as one bony extension. The posteriormost pouch plate is diminutive in size.

Distribution.— Acentronura gracilissima is known from the tropical western Pacific from New Caledonia, Japan, Indochina, and the South China Sea ( Dawson, 1985; Rivaton and Richer de Forges, 1990; Randall and Lim, 2000; Nakae et al., 2018; Araki et al., 2019).

Material examined.— Acentronura gracilissima: CAS-SU 6681, 70.4 mm SL, Honshu Island, Sagami Sea, Japan, 35°09 ' 34.0 '' N, 139°29 ' 37.9 '' E, date collected 1900 .