Last update: 15 May 2017

Common names:

* proposed name

Synonyms:

Etymology:
'Oxuderces' comes from the Greek 'oxuderkês' (quick-sighted), which probably refers to the quick burying behaviour, elicited by visual stimuli

'nexipinnis' means 'with connected fins' in Latin, which refers to the membrane connecting D1 and D2

Maximum recorded length:
81 mm SL (Jaafar & Parenti, 2016)

Live colouration (Murdy, 1989; Takita et al., 1999; GP pers. obs.: Peninsular Malaysia):
ground colour dorsally greyish blue to pale brown, ventrally whitish to pale grey; 6 dark and irregular saddle-like bars and a series of dark blotches along lateral midline may be visible; many irregular tiny brown speckles on nape and dorsum; shiny bluish scales can be present on flanks, on posterior half of trunk; D1 translucent; D2 transparent except for a faint dusky medial stripe and a large black ocellus with orange margin near distal tips of last 4 rays; caudal fin transparent to dusky or orange; anal, pectoral and pelvic fins translucent, orange in some specimens; large blackish blotch on upper base of pectoral fin; upper lip and tips of anterior nostrils lined with black; upper lip posteriorly and ventrally orange. Eyes bright green to blue (see photo D)

Colouration on preservation (Jaafar & Parenti, 2016; GP pers. obs.: Peninsular Malaysia):
ground colour pale to dark brown; dark spots, bars and blotches can be retained, especially on head and nape; tips of anterior nostrils and upper lip lined with black or dark brown; dark brown blotch at the base of the pectoral fins; all fins transparent except for the dusky ocellus at the posterior portion of D2

Diagnosis (Jaafar & Parenti, 2016):
first hemal spine extending ventrally to a point dorsal to mid-length of the first anal-fin pterygiophore (versus extending ventrally to mid-length of the first anal-fin pterygiophore in O. dentatus); conspicuous dermal invagination posterior to the point of attachment of pelvic-fin base; mouth superior; head length 27-30%SL.

The genus is diagnosed by (1) a fleshy external trough (deep groove or slit) running along the medial longitudinal axis in a dorsal interorbital position (see photo G); the groove is supported internally by two ridges of the medially fused frontal bones (interorbital bridge) that curve towards the midline, and is lined with thick epidermis and thin dermis; (2) highly thickened epidermis over the eye; (3) neural spine of the fourth vertebra broad, spatulate and posteriorly directed; (4) portion of the anterior ceratohyal posterior to the insertion of the fourth branchiostegal ray elongate and notched (versus not elongate, or elongate and not notched; see drawing D); and (5) the metapterygoid-symplectic-quadrate strut and the anguloarticular form an acute angle

Description (Jaafar & Parenti, 2016):
Head wider than deep: head length (HL) 27-30%SL, head width 33-44%HL, head depth 30-43%HL. Eye diameter 9-16%HL; interorbital distance 3-5%HL. Snout length 12-16%HL, jaw length 41-60%HL. Body depth at anus 9-13%SL, body width at anus 4-7%SL. Predorsal length 28-32%SL. Length of base of D1 plus D2 55-59%SL; length of base of anal fin 36-40%SL; pelvic-fin length 14-18%SL; pectoral-fin length 14-19%SL; caudal-fin length 21-25%SL. D2 with 25-26 elements; anal fin with 24-26 elements; pectoral rays 21-24. Lateral longitudinal scale count 50-54; ventral region of head, cheek, operculum, isthmus, dorsal region anterior to D1 and pectoral-fin bases scaleless, except in largest specimens; all scales small and embedded.

All congeners also have: depressed anterior portion of the head and pointed snout profile. Distinct notch in middle of upper lip between two medial premaxillary teeth; thick lips, with posterior lip distally protruding. Gape wide, extending 3-4 eye lengths posterior to the orbit; preopercle thin and crescent-shaped, five thin branchiostegal rays. Roof of mouth with fleshy ellyptical palp with pointed tips, studded with papillae. Oral jaw teeth in single row both on premaxilla and dentary, all caninoid; premaxillary teeth decreasing in length posteriorly, dentary teeth more uniform in size; 1-2 canine teeth on both sides of premaxillary symphysis, longer than other teeth, extending anteroventrally and projecting beyond lower jaw when mouth closed; no canine teeth on both sides of mandibular symphysis; no vomerine and palatine teeth. Gill opening restricted, beginning from a point anterior to midpoint of pectoral-fin base, then coursing anteroventrally to a point dorsal to the pelvic fin origin. Eyes not meeting medially, without dermal cup. Supraorbital pore C in the anterior portion of the fleshy external interorbital trough; anterior oculoscapular pores; large posterior nostril, anteroventral to eye; anterior nostril at tip of pendulous short tube overlapping lower jaw. D1 and D2 connected by membrane for entire height; D2 or anal fin and caudal fin not connected by membrane; D1 with six spines; all elements of D2 and anal-fin segmented rays, last two rays sharing same pterygiophore; pelvic fins fused in a pelvic disc, not reaching genital papilla; caudal fin lanceolate with 17 segmented rays; dorsal procurrent rays 5; ventral procurrent rays 4; 2 epural bones. Male genital papilla triangular and conical, with posterior tip pointed; female genital papilla rectangular and bulbous

Diet:
no published study is available

Reproduction:
no published study is available

Ecological notes (Polgar & Bartolino, 2010: as O. dentatus):
locally abundant on lower mudflats, moving in areas of mud covered by a thin film of water; larger individuals (>7-8 cm) can be caught by trawl nets on lower mud flats at high tide (Murdy, 1989; GP pers. obs.: Peninsular Malaysia). O. nexipinnis stores air in burrows (Ishimatsu et al., 2000). In Malaysia, it is a common prey of homalopsid snakes (Bitia hydroides; Jayne et al. 1995; and Cerberus rynchops; Jayne et al. 1988). O. nexipinnis can be ecologically partitioned between larger individuals, more abundant at lower levels of the mudflat, and smaller ones, more abundant at higher levels; a similar pattern was observed in different habitats of the same ecosystem in Boleophthalmus boddarti and Periophthalmus gracilis, while P. variabilis apparently exhibited an inverse pattern (Polgar & Bartolino, 2010)

left: Tanjung Piai, Peninsular Malaysia: lower mudflat at low tide; Ox. nexipinnis was here abundant

right: Kukup Island, Peninsular Malaysia: a shot of the mangrove forest and of the adjacent tidal flat at low tide; the mud layer was less than 30 cm deep, with shell lag deposits (Placuna sp.) beneath

(photo: G. Polgar, 2006; 2007)

Distribution:
Indo-West Pacific region, from Bay of Bengal to the Andaman Sea, Java Sea, Gulf of Thailand and southern South China Sea; type locality: Sea of Penang, Malaysia (Jaafar & Parenti, 2016)

Remarks:
The type material of Apocryptichthys nexipinnis Cantor, 1850 (some fragile skins conserved at the BMNH) was examined by Jaafar & Parenti (2016). See also Remarks on the congener O. dentatus

Photographs of Oxuderces nexipinnis:

A: freshly dead specimen (photo: E.O. Murdy, Muar, Peninsular Malaysia, 1985); B: two specimens in aquarium (photo: G. Polgar; collected in Tanjung Piai, Peninsular Malaysia, 2006); C: in aquarium, out of water (photo: G. Polgar, collected in Kukup Island, Peninsular Malaysia, 2007); D: an underwater shot of the specimen in C (photo: G. Polgar, 2007); E: another close-up of the same individual (photo: G. Polgar, 2007); F: detail of the diagnostic dermal invagination (red arrow) posterior to the point of attachment of pelvic-fin base (ZRC 54696, West Bengal, India; photo: G. Polgar, 2017); G: dorsal view of the interorbital trough (T) diagnostic for Oxuderces spp., and of the position of the supraorbital pore C; black scale bar = 1 mm (photo: G. Polgar, 2007; freshly dead specimen collected in Kukup Island, Peninsular Malaysia) - * with permission from the author

Drawings of Oxuderces nexipinnis:

A	B C
D

A: cephalic sensory and nasal pores of Oxuderces spp.: an = anterior nostril; ao = anterior oculoscapular canal pore; f = caninoid teeth; pio = posterior interorbital pore; pn = posterior nostril (modified from Murdy, 1989)*; B: Apocryptichthys cantoris (Day, 1871) (Day, 1889, Fig. 94; illustration of a specimen from Madras or Andamans); C: Apocryptichthys cantoris (Day, 1871) (Koumans, 1953); D: lateral view of the hyoid arch of Ox. dentatus, rotated about 45° clockwise; the portion of the anterior ceratohyal that is not present in other oxudercine genera is coloured in red: Ch = ceratohyal; an = anterior ceratohyal; po = posterior ceratohyal; 1-5 = branchiostegals (modified from Murdy, 1989)* - * with permission