A new wriggler of Eleotrid (Teleostei: Xenisthmidae) from Taiping Island, South China Sea Author Chang, Chih-Wei 0000-0002-0659-9582 Marine Ecology and Conservation Research Center, National Academy of Marine Research, Kaohsiung, 806614, Taiwan, R. O. C. & National Museum of Marine Biology and Aquarium, Pingtung, 94450, Taiwan, R. O. C. changcw@namr.gov.tw Author Shen, Kang-Ning 0000-0002-6338-6049 Marine Ecology and Conservation Research Center, National Academy of Marine Research, Kaohsiung, 806614, Taiwan, R. O. C. knshen@namr.gov.tw Author Chen, I-Shiung 0000-0002-4190-7720 Institute of Marine Biology, National Taiwan Ocean University, Keelung, 202301 Taiwan, R. O. C. & Center of Excellence for the Oceans, National Taiwan Ocean University, Keelung, 202301 Taiwan, R. O. C. iscfish@gmail.com text Zootaxa 2025 2025-12-30 5738 1 32 37 https://doi.org/10.11646/zootaxa.5738.1.7 journal article 314631 10.11646/zootaxa.5738.1.7 22fab97c-c453-4dcd-8294-a1996c1d7685 1175-5326 19138681 4CB16768-E076-42FC-A51E-4DC0309A5B68 Xenisthmus aureus sp. nov. ( Îĭ塘º ) ( Figures 1 , 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 . 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. TABLE 1. Morphometry of Xenisthmus aureus from Taiping Island, South China Sea.

Type status	Holotype
Size (mm SL)	17.0
% inSL
Head length	23.6
Predorsal length	33.6
Snout to 2nd dorsal origin	53.0
Snout to anal fin origin	56.1
Prepelvic length	25.5
Caudal peduncle length	12.0
Caudal peduncle depth	11.2
First dorsal fin base	13.0
Second dorsal fin base	23.6
Anal fin base	31.4
Caudal fin length	21.5
Pectoral fin length	21.4
Body depth of anal fin origin	13.3
% inHL
Snout length	24.7
Eye diameter	18.4
Postorbital length	54.6
Cheek depth	15.5
Lower jaw length	41.8
% in caudal peduncle length
Caudal peduncle depth	93.9

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 ) 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. FIGURE 1. Head lateral-line system of Xenisthmus aureus sp. nov. , holotype, 17.0 mm SL, Taiping Island, South China Sea, Taiwan, R.O.C. Bar = 1 mm. The arrow indicates the vertical end of gill-opening. FIGURE 2. Xenisthmus aureus sp. nov. , holotype, 17.0 mm SL, Taiping Island, South China Sea, Taiwan, R.O.C. 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.