Xenisthmus chi, Gill & Hoese, 2004

Gill, A. C. & Hoese, D. F., 2004, Three New Australian Species of the Fish Genus Xenisthmus (Gobioidei: Xenisthmidae), Records of the Australian Museum 56, pp. 241-246 : 242-243

Xenisthmus chi n.sp.

Fig. 1

Xenisthmus sp. 2 .—Allen & Russell, 1986: 100 (Rowley Shoals).

Type material. HOLOTYPE: WAM P.28030-033, 20.6 mm, male, Timor Sea , Rowley Shoals , Clerke Reef, 2 km south of Bedwell Island, 17°18'S 119°20'E, rotenone, G. R. Allen & R. C. Steene, 11 August 1983 . PARATYPE: WAM P.28030- 041, 22.0 mm, female, collected with holotype .

Diagnosis. A species of Xenisthmus with the following combination of characters: second dorsal-fin rays I,12; analfin rays I,11; predorsal area broadly scaled to just behind vertical through posterior edge of preopercle, with narrow median wedge of scales extending further forward almost to pore D; posterior naris without flap on anterior rim; and head and body pale with brown reticulate mottling, forming about 11 X-shaped markings along sides between pectoral- and caudal-fin bases.

Description. Dorsal-fin rays VI + I,12, all segmented rays branched; first dorsal-fin pterygiophore formula 3–13110; anal-fin rays I,11, all segmented rays branched; pectoralfin rays 17/17, upper 1 (2) and lower 1 (2) ray(s) unbranched; pelvic-fin rays I,5, inner ray unbranched; segmented caudalfin rays 9 + 8; branched caudal-fin rays 7 + 7; upper unsegmented caudal-fin rays 7 (8); lower unsegmented caudal-fin rays 7 (8); total caudal-fin rays 31 (33); scales in lateral series 57/55 (52 f1; 54 f1); scales in forward transverse series 20/19 (17 f1; 18 f1); scales in backward transverse series 18/18 (16 f1; 19 f1); circumpeduncular scales 27 (26); predorsal scales 21 (18); cheek scales 4; gill-rakers 3 + 9 (2 + 9); pseudobranch filaments 4; vertebrae 10 + 16; epurals 2.

As thousandths of SL: head length 248 (241); predorsal length 340 (332); prepelvic length 238 (245); preanal length 515 (555); first dorsal-fin origin to second dorsal-fin origin 184 (182); second dorsal-fin base length 340 (355); analfin base length 306 (291); pectoral-fin base depth 73 (68); first dorsal-fin origin to pelvic-fin origin 170 (173); second dorsal-fin origin to anal-fin origin 141 (132); snout length 39 (45); orbit diameter 58 (55); head width 131 (145); body width 107 (109); bony interorbital width 15 (18); snout tip to retroarticular tip 97 (106); caudal-peduncle length 170 (168); caudal-peduncle depth 112 (114); length of first spine of first dorsal fin 92 (82); length of third spine of first dorsal fin 102 (100); length of sixth spine of first dorsal fin 63 (68); length of spine of second dorsal fin 87 (77); length of first segmented ray of second dorsal fin 117 (100); length of last segmented ray of second dorsal fin 121 (123); analfin spine length 73 (64); length of first segmented anal-fin ray 83 (95); length of last segmented anal-fin ray 131 (127); pectoral-fin length 209 (227); fourth segmented pelvic-fin ray length 189 (186); caudal-fin length 204 (209).

Body covered with small scales; scales cycloid on anterior and lower abdomen (in front of vertical through middle of abdomen), and narrowly along upper and ventral edges of body (adjacent to bases of dorsal and anal fins), remainder of body and caudal peduncle with ctenoid scales; ventral contour of abdomen fully scaled, except for narrow area beneath branchiostegal membranes; predorsal area broadly scaled to just behind vertical through posterior edge of preopercle, with narrow median wedge of scales extending farther forward almost to pore D; cheeks and upper part of operculum scaled; scales present on pectoral-fin base; narrow band of mostly ctenoid scales on fleshy portion of caudal-fin base; no scales on dorsal- or anal-fin bases.

Head pores A'BC D(S)EFHIJK' M'NOPQ'; lower lip fleshy and protruding, with uninterrupted, free ventral margin; anterior naris in short tube; posterior naris with raised rim, without prominent membranous flap anteriorly; tongue indented anteriorly; gill opening extending anteriorly to about midway between verticals through posterior edge of preopercle and posterior edge of eye.

Upper jaw with 2 or 3 (anteriorly) or 2 (posteriorly) rows of small, conical teeth, outer-row teeth largest and slightly curved; lower jaw with 3 (anteriorly) or 2 (posteriorly) rows of small, conical teeth, outer-row teeth largest and slightly curved; vomer, palatines and tongue edentate.

Preserved coloration. Head and body beige to pale brown with brown reticulate mottling (obvious when first examined in 1992, but now considerably faded); two short brown bars extending from eye, one from below mid-ventral part of orbital rim to just above posterior edge of maxilla, and one from posteroventral part of orbital rim to middle of cheek; upper part of pectoral-fin base with irregular dusky brown spot; second brown spot on middle part of pectoral-fin base (mostly concealed by gill membranes); brown mottling on body aligning to form about 11 X-shaped markings along sides between pectoral- and caudal-fin bases; first dorsal fin hyaline with irregular dusky brown spots basally, and prominent dark grey-brown spot posteriorly on distal part of fin behind sixth spine; second dorsal fin hyaline with two (anteriorly) or three (posteriorly) series of small brown spots arranged along length of each fin ray; caudal fin hyaline to beige with short dusky brown bar lining hypural edge near middle few caudal-fin rays; immediately behind bar, a dark grey-brown spot, overlying bases of lower three caudal-fin rays on upper hypural plate; remainder of caudal fin with three or four narrow wavy bars, best developed on upper part of fin; anal fin hyaline to beige; pectoral fins hyaline, with small, dark brown spot basally on upper third of fin; pelvic fins hyaline.

Live coloration. Not known.

Comparisons. Xenisthmus chi closely resembles X. balius in general coloration pattern and in lacking a flap on the posterior naris. It differs from that species in having fewer segmented dorsal- and anal-fin rays (12 and 11, respectively, versus 13 and 12–13, usually 12, in X. balius ); a different first dorsal-fin pterygiophore pattern (3–13110 versus 3– 22110); fewer scales in lateral series (52–57 versus 60–70); more extensive coverage of ctenoid scales on the body (body behind middle of abdomen with ctenoid scales versus at most only a few scattered ones on caudal peduncle); cheek and operculum scaled (versus naked); and predorsal scales with median series extending to near pore D (versus predorsal area broadly scaled to about vertical through posterior edge of preopercle).

Remarks. The specific epithet is a noun in apposition derived from the Greek letter (chi), and alludes to the Xshaped markings on the body.