Gymnoxenisthmus flavicinctus, Gill & Bogorodsky & Mal, 2019

Gill, Anthony C., Bogorodsky, Sergey V. & Mal, Ahmad O., 2019, Description of a second species of Gymnoxenisthmus from the Red Sea (Teleostei: Gobioidei: Xenisthmidae), Zootaxa 4590 (1), pp. 177-183 : 178-179

Gymnoxenisthmus flavicinctus , new species

Yellow-barred naked wriggler

( Figures 1 A–C, 2–3)

Holotype. SMF 35866, 20.2 mm SL, male, Red Sea , Saudi Arabia, Jeddah, Sharm Obhur, 21°42'30"N, 39°05'44"E, coral-reef slope, 14–16 m, coll. T.J. Alpermann & S.V. Bogorodsky, 16 February 2017.

Diagnosis. Gymnoxenisthmus flavicinctus is distinguished from its congener, G. tigrellus , in having entirely unbranched pectoral-fin rays (versus only upper two rays and lower ray unbranched), 17 (versus 15) pectoral-fin rays and 13 (versus 12) segmented anal-fin rays.

Description. Dorsal-fin rays V + I,13, all but first segmented rays branched; anal-fin rays I,13, all segmented rays branched; pectoral-fin rays 17/17, all rays unbranched; pelvic-fin rays I,5, all segmented rays unbranched, inner ray vestigial; segmented caudal-fin rays 9 + 8; branched caudal-fin rays 7 + 6; upper unsegmented caudal-fin rays 6; lower unsegmented caudal-fin rays 6; total caudal-fin rays 29; no developed gill rakers (two tiny rudiments present on upper part of ceratobranchial 1); pseudobranch filaments 5.

As percentage of SL: head length 23.4; predorsal length 36.1; prepelvic length 27.2; preanal length 58.4; first dorsal-fin origin to second dorsal-fin origin 15.8; second dorsal-fin base length 36.1; anal-fin base length 30.6; pectoral-fin base depth 8.4; first dorsal-fin origin to pelvic-fin origin 17.8; second dorsal-fin origin to anal-fin origin 13.9; snout length 5.4; eye diameter 8.9; head width 16.8; body width 15.2; bony interorbital width 1.0; snout tip to retroarticular tip 11.4; caudal-peduncle length 13.9; caudal-peduncle depth 8.9; length of first spine of first dorsal fin 10.9; length of third spine of first dorsal fin 11.9; length of spine of second dorsal fin 8.4; length of first segmented ray of second dorsal fin 9.9; length of last segmented ray of second dorsal fin 13.4; anal-fin spine length 6.4; length of first segmented anal-fin ray 8.4; length of last segmented anal-fin ray 13.9; pectoral-fin length 20.8; fourth segmented pelvic-fin ray length 25.2; caudal-fin length 20.3.

Scales absent; laterosensory head pores absent; distribution of superficial neuromasts on head as shown in Fig. 2 ; lower lip fleshy and protruding, with uninterrupted, free ventral margin; anterior naris in short tube; posterior naris without raised rim or membranous flap; tongue tip bluntly rounded; gill opening extending anteriorly to vertical through about midpoint between edge of preopercle and eye.

Upper jaw with 1 or 2 rows of conical teeth, those of outer row larger and curved; lower jaw with 2 rows of conical teeth, those of outer row larger and curved; vomer and palatine edentate.

Live coloration (based on colour photographs of holotype when alive and freshly dead; Figs. 1A & B ): head and body translucent bluish grey; head with bright yellow stripe extending from mid-upper part of upper lip to eye, then from behind eye to upper edge of opercle; short bright yellow bar on upper part of preopercle; yellow bar and stripe on postorbital head slightly to heavily dotted with melanophores; tip of lower lip and chin yellow; iris dark grey, dotted with gold to greenish yellow flecks; narrow dark grey stripe extending along dorsal edge of vertebral column from beneath pectoral fin to anterior half of caudal peduncle; posterior part of abdomen beneath grey stripe pale orange to bright yellow, densely dotted with melanophores; remainder of body beneath grey stripe pale yellow; a faint vertically elongate orange spot extending from origin of first dorsal fin, but not reaching grey midlateral stripe; a series of seven inverted v- or wedge-shaped yellow to orange marking extending from dorsal contour of body to vertebral column, first from posterior part of first dorsal fin, second from base of first segmented dorsal-fin ray, third short and diffuse from base of third segmented dorsal-fin ray, fourth from base of fifth segmented dorsal-fin ray, fifth from base of seventh segmented dorsal-fin ray, sixth from base of tenth segmented dorsal-fin ray, and seventh from base of thirteenth dorsal-fin ray; posterior half of caudal peduncle semi-translucent (abruptly white when freshly dead), this edged posteriorly on caudal-fin base with very narrow dark grey bar, which in turn is edged dorsally, ventrally and posteriorly in orange; first dorsal fin hyaline with orange-brown spot at base of first two spines, and large (almost eye-sized) blackish spot over submarginal part of fin covering half of membrane between first two spines and entire membranes between second to fourth spines; second dorsal fin hyaline with irregular blackish submarginal stripe distally, and basal part of first, third, fifth, seventh, tenth and thirteenth segmented rays dull orange, these connecting ventrally with yellow bars on body and dorsally with blackish oblique bars that extend posteroventrally from blackish submarginal stripe; anal fin hyaline with faint orange-brown spot at base of each segmented ray and dark grey stripe through middle of fin; caudal fin hyaline, narrowly pale orange-grey on base, with dark grey submarginal marking extending around distal part of upper 12 segmented rays; rays of caudal fin and portion of fins within dark markings on both dorsal fins and anal fin pale orange; pectoral fin whitish hyaline, with orange-yellow spot over basal part of upper five rays; pelvic fin white.

Preserved coloration ( Fig. 1C ): head and body generally pale beige, paler on posterior part of caudal peduncle, and greyish brown on lower abdomen; melanophores within yellow markings on head and body remain; dark midlateral stripe remains, though indistinct; dark grey markings on fins remain.

Etymology. The specific epithet is from the Latin flavus, yellow, and cinctus, belted, and refers to the yellow bars on the upper body.

Habitat and distribution. Known only on the basis of the holotype collected in 14–16 m, at Sharm Obhur, Jeddah, Saudi Arabia ( Fig. 3 ). Habitat of the new species is steep coral-reef slope down to about 30 m depth, upper half of the slope characterised by short steps covered with hard and soft corals; the specimen was collected at the base of soft coral.

Comparisons. As noted above, G. flavicinctus differs from its congener, G. tigrellus , in having entirely unbranched pectoral-fin rays (versus only upper two and lower ray unbranched). It also probably differs in having 17 (versus 15) pectoral-fin rays and 13 (versus 12) segmented anal-fin rays, though we note that there is intraspecific variation in these characters in the related genus Xenisthmus (see Gill et al. 2017). The two species further differ in live coloration ( Fig. 1 ). Most notably, G. tigrellus is more strongly banded, with 14 bars or chevronshaped bars extending the full height of the body (versus bars or inverted v-shaped markings on the upper part of the body only). It also lacks the prominent white bar on the posterior half of the caudal peduncle.

The live coloration of G. flavicinctus is similar to that of Rotuma lewisi Springer (1988) . However, G. flavicinctus is readily distinguished by the following: second dorsal- and anal-fin rays I, 13 in each fin, with all but the first or all segmented rays branched, respectively (versus I,9 with all segmented rays unbranched), and the ceratohyal consisting of separate anterior and posterior ossifications (versus a single ossification).

Remarks. Xenisthmidae are a small family of very cryptic gobiid fishes, which are usually only collected with the use of ichthyocides. In addition to their cryptic habitat, most species are small-sized, not exceeding 3 cm total length, thus easily overlooked in ichthyocide stations. In spite of their poor representation in museum collections, it appears that most species have restricted distributions. Of the five species currently known from the Red Sea, three are endemics: Xenisthmus oligoporus Gill, Bogorodsky & Mal (2017) and both species of Gymnoxenisthmus . Based on unpublished genetic data, it is likely that X. polyzonatus ( Klunzinger 1871) is also a Red Sea endemic. The fifth Red Sea species, X. balius Gill & Randall (1994) , in not endemic, but nonetheless has a relatively restricted distribution, occurring in the Arabian Gulf and Red Sea (see Gill et al. 2017).

This is the fourth new Red Sea endemic fish species to be collected from the same area in Sharm Obhur. The others are Acanthoplesiops cappuccino Gill, Bogorodsky & Mal (2013) , Callogobius pilosimentum Delventhal, Mooi, Bogorodsky & Mal (2016) , and Trimma quadrimaculatum Hoese, Bogorodsky & Mal (2015) . Two other undescribed species of Holocentridae ( Sargocentron sp.) and Gobiidae ( Hetereleotris sp.) were also collected there and are under study by the second author.

Figures

FIGURE 1. Gymnoxenisthmus species. A–C: G. flavicinctus, holotype, SMF 35866, 20.2 mm SL, male, Sharm Obhur, Jeddah, Saudi Arabia, Red Sea, A: alive; B: freshly dead; C: preserved in 70% ethanol. D: G. tigrellus, holotype, SMF 34903, 15.2 mm SL, gravid female, Farasan Archipelago, Red Sea. Photos by S.V. Bogorodsky (A, B), Y.K. Tea (C) and S. Traenkner (D).

FIGURE 2. Superficial neuromasts of Gymnoxenisthmus flavicinctus, holotype, SMF 35866, 20.2 mm SL, in (A) dorsal and (B) lateral views. Nares shown in dark grey. Arrow shows anterior extent of gill opening.

FIGURE 3. Distribution records for Gymnoxenisthmus tigrellus (open circle) and G. flavicinctus new species (closed circle).