Xenisthmus aureus, Chang & Shen & Chen, 2025

Xenisthmus aureus sp. nov.

( Îĭ塘º)

( Figures 1 View FIGURE 1 , 2 View FIGURE 2 )

Holotype. NTOUP-2024-04-151, 17.0 mm SL, April 24, 2024; coll. Y.C. Yang et al., Taiping Island , South China Sea, Taiwan, ROC.

Dignosis

The new species can be well distinguished from other congeners by the following unique combination of features: (1) fins: second dorsal fin rays I/11; anal fin rays I/10; pectoral fin rays15; (2) squamation: longitudinal scale series 60–62; perdorsal scales 0; (3) vertebral count 10+16=26; (4) preopercular canal present with 4 pores γ, δ, ε, and ε1; and (5) specific colouration: lateral side of trunk with an upper horizontal series of 20–22 yellow spots; a median series of oblique golden yellow stripes or becoming smaller, irregular golden yellow marks; the orange red cross mark on eye, radiating into mouth and posterior area horizontally, and extending up to top eye margin to downward vertically behind the maxillary on cheek; pectoral fin base with a upper, large oblique golden yellow mark; and caudal fin base with a very small median black spot.

Description

Body very slender, subcylindrical anteriorly and somewhat compressed posteriorly. Anterior head somewhat depressed. All following body proportions are listed in Table 1 View TABLE 1 . Head with very narrow interorbital region. Eye large. Lower jaw more prominent. Both anterior and posterior nostrils as short tubes which pointed to opposite direction on dorsal view. Both jaws with 5–6 rows of small, conical teeth which largest in outer rows. Gill-opening rather large, extending ventrally beyond the rear vertical of preopercle. Vertebral count 10+16 = 26.

Fins. D1 VI, D2 I/11; A I/10; P 15; V I/5. All D1 rays about equal without any filamentous ray. D2 rays low with rather long fin base. The rear tips of D2 rays when depressed extending to the procurrent rays of C. Origin of A inserted below middle vertical between first and second branched rays of D2. P moderate large and oblong, its rear tip not reaching vertical line through anus in male. V well separate, no frenum, connecting membrane. C elliptical, rear edge rounded.

Scales. Body mostly with rather small cycloid scales, predorsal area entirely naked. Upper part of operculum scaled. Cycloid scales present on pectoral-fin base. LR 60–62; TR 19; PreD 0. Prepelvic region naked.

Head lateral-line system. ( Figure 1 View FIGURE 1 )

Canals: Nasal extension of anterior oculoscapular canal with terminal pore σ located in between anterior and posterior tube-like nostrils. Anterior interorbital sections of oculoscapular canal with paired pore λ. A single pore κ in near rear of interorbital region. Pore ω present behind orbit. Lateral section of oculoscapular canal with lateral pore α. Preopercular canal with rather more, 4 pores γ, δ, ε, and ε1.

Sensory papillae: Row a extending beyond vertical midline of orbit. Rows c short. A single cp papilla. Row f paired. Anterior edge of row oi well separated to lower region of row ot. Row ot interrupted with 2 sections

Coloration while fresh

Body and head semi-translucent with snow white to pale white background.

Lateral side of trunk after pectoral fin base with an upper horizontal series of 20–22 yellow spots, then with a median series of oblique golden yellow stripes or becoming smaller, irregular golden yellow marks. On dorsum with 4 yellow spots below first dorsal fin base and also 11 yellow spots beneath the second dorsal fin base. The orange red cross mark on eye, radiating into mouth and posterior area horizontally, and extending up to top eye margin to downward vertically behind the maxillary on cheek. Opercle with large orange blotch on upper part. Posterior region of interorbitals with three major orang blotches. Both lips pale white and anterior tip crossing with orange red bar radiating from orbit.

First dorsal fin translucent and spotless. Second dorsal fin translucent with a series of smaller yellow spots on middle area of fin membrane. Pectoral fin translucent and its base with a large oblique golden yellow mark beneath in having tiny melanophores on upper region. Pelvic fin translucent and spotless. Anal fin translucent with a horizontal row of tiny basal yellow spots or bars. Caudal fin base with a very small median black spot, and its upper and anterior regions surrounding with the very thin orange curves. Caudal fin membrane translucent with about three vertical rows of yellow spots.

Colouration in preservative

Similar to live pattern, but the all brilliant, conspicuous orange, golden yellow, yellow marks or spots on body and all fins entirely faded. Some tiny darker melonophores or dots below the orange or yellow marks still present but less distinct.

Etymology

The specific name, aureus , is referred to the unique feature of its specific colouration pattern: golden-yellow (Latin: aureus ) belly marks and also smaller golden yellow marks or short bars on lateral body among all remaining congeners.

Distribution

So far, the new species merely can be found from the type locality—Taiping Island, South China Sea. It needs for fish biodiversity survey to nearby countries to realize the real distribution range.

Remarks

The new species, Xenisthmus aureus , is more similar to the Australian endemic species, Xenisthmus chi by overall preserved colouration than any other nominal congeneric species in the Indo-Pacific region. However, this new species can be well distinguished from the close related species by the following morphological features: (1) fin ray counts: second dorsal fin rays I/11 (vs I/12), anal fin rays I/10 (vs. I/11), pectoral fin rays 15 (vs. 17); (2) scale counts: longitudinal scale rows 60–62 (vs. 52–57); and (3) specific colouration: lateral side of trunk with an upper horizontal series of 20–22 yellow spots; a median series of oblique golden yellow stripes or becoming smaller, irregular golden yellow marks; the orange red cross mark on eye, radiating into mouth and posterior area horizontally, and extending up to top eye margin to downward vertically behind the maxillary on cheek; pectoral fin base with a upper, large oblique golden yellow mark; and caudal fin base with a very small median black spot (vs. 11 X-shaped markings along body sides). The rare new wriggler is still need to explore more in the near future not only morphological but also molecular phylogenetic aspect.