Omobranchus subgen. Bleeker

Omobranchus zebra (Bleeker)

Petroscirtes zebra Bleeker, 1868:279 [Singapore in mari].

Petroscirtes bhattacharyae Chaudhuri, 1916:107 [Barkul Point, Chilka Lake}.

DESCRIPTION (see also Table 5).—Dorsal fin XII, 18–20 = 30–32: anal fin II, 20–22; both anal-fin spines of males discernible externally; segmented caudal-fin rays 13; dorsal + ventral procurrent caudal-fin rays 11–14; vertebrae 10 + 27–28 = 37–38; epipleural ribs 12–15; prenasal pores present; interorbital pores 3; circumorbital pores 7–8 (8 in 16 of 19 specimens); lateral-line tubes 1–4; lateral-line tubes extending posteriorly to below level of dorsal-fin spines 2–7 (2–5 in all but 1 specimen); gill opening either restricted to area dorsal to level of dorsalmost pectoral-fin ray (19 of 20 specimens) or extending ventrally to opposite dorsalmost ray (1 specimen); lower-lip flap present; circumorbital bones 4; lower jaw teeth 16–22; upper jaw teeth 15–24 (Figure 46); no fleshy bladelike crest on top of head of either sex.

COLOR PATTERN.—Males: Head: Four broad, dark bands present on head; anterior band running from anteroventral margin of orbit across mouth, midway between corner of jaws and anterior contour of head, to chin; band interrupted dorsally by eye, continuing dorsally for short distance from dorsoposterior margin of orbit; 2nd band angled slightly posteroventrally from posteroventral margin of orbit to ventral midline of head; 3rd band encircling head with dorsal one-third directed anteroventrally from center of nape to point just posterior to eye, then slightly posteroventrally to ventral midline; 3rd band bears dark intensification on anterior margin just posterior to eye; 4th band encircling head just anterior to dorsal-fin origin; dorsal half of 4th band angled slightly anteroventrally, ventral half slightly posteroventrally; 3rd and 4th bands usually fainter ventrally, especially on cheek and branchiostegal region; all 4 bands join corresponding bands of opposite side on underside of head, but are slightly fainter at ventral midline, particularly 1st band. Short, posteroventrally slanted, diffuse, dusky line usually present anteriorly on free opercular membrane. Snout and interorbital area dusky, blending with dorsal end of 1st band, forming confluent dark dusky area with well-defined posterior margin across top of head.

Trunk: Background pale to pale dusky, slightly darker dorsally, with dark markings. Bleeker’s (1868) description and unpublished illustration of the holotype (our Figure 34) indicate 8 evenly spaced, broad, dark bands present on the body, each almost reaching from the dorsal to the ventral body contour (holotype now completely faded). Our material, however, exhibits only the anterior 2–4 bands, and even these may be faint. Where the bands are not prominent our specimens may exhibit a series of large, circular, dark spots, each appearing where a band would cross the lateral body midline; the last two bands are usually not represented in our specimens; the first two bands are usually developed, at least dorsally, and slant slightly posteroventrally.

Pectoral Fin: Rays pale with even spread of fine, dark specks of pigment, membranes transparent. Posteroventrally slanted, broad, dark band traversing fleshy pectoral-fin base, anteriorly connected to faint dusky line on free opercular membrane dorsally and continued slightly onto abdomen ventrally.

Pelvic Fin: Pale with even spread of fine, dark specks of pigment.

Dorsal Fin: Pale with even spread of fine, dark specks of pigment and dark dusky to dark markings. Series of paired dark blotches (diffusely dusky and unpaired in some specimens) present along fin base anteriorly above dorsal ends of body bands; blotches becoming faint, then absent, midway along fin; each blotch covering membrane between bases of 2 adjacent rays; pairs of blotches separated by single, unpigmented membrane space. Broad, dark, marginal stripe present on spinous portion of fin; pigment on anterior distal edge of 1st spine and membrane between distal tips of first 2 spines intensified, often the only prominent dark markings in dorsal fin of small specimens.

Anal Fin: Dusky, ray tips abruptly pale.

Caudal Fin: Pale with even spread of fine, dusky specks of pigment on rays and membranes bordering rays. Basal margin of fin dusky; 2 longitudinal, diffuse, dusky streaks on fin, 1 just above and 1 just below lateral midline of fin.

Females: Similar to males except: series of spots on lateral midline of body very faint to absent; dark markings on dorsal fin slightly fainter, anal fin slightly paler.

GEOGRAPHIC DISTRIBUTION (Figure 6).—Bay of Bengal coast of India to Singapore.

Menon and Talwar (1972) reported O. zebra from Galathea Bay, Great Nicobar Island. Their description appears to apply to O. zebra but we have not seen their specimen and have not, therefore, included the record on Figure 6.

Dutt and Visveswara Rao (1961) reported O. zebra (as Petroscirtes bipunctata) nesting in holes in mangrove stems in the Godavari River estuary (16°28′N, 82°03′E) in India. We became aware of this reference too late to include the record on Figure 6.

HABITAT.—Mangrove areas, sand and mud bottom, brown stained water, low salinity areas. Bhattacharya (1916) reported larval stages, as P. bhattacharyae, 3.2–15.9 mm total length, from Chilka Lake waters of densities 1.006–1.007, corrected to 15° C. Converted to salinity, these densities equal a range of about 8.2–10.8 o/oo.

COMPARISONS.—Omobranchus zebra differs from all other species of Omobranchus except O. ferox, with which it is broadly sympatric, in having 4 circumorbital bones (4 bones is an uncommon variant in the other species). It differs from O. ferox most obviously in having the gill opening almost always restricted to the area dorsal to the level of the dorsalmost pectoral-fin ray, rarely extending ventrally to opposite the dorsalmost ray (versus opposite the 3rd to 6th ray in O. ferox), in having well-defined bands on the head, and in having a longer head (see key couplet 13).

Aside from the number of circumorbital bones, O. zebra may be most readily separated from the other species with which it appears to be broadly sympatric as follows: from all in having a longer head (key couplet 13); from O. elongatus and O. germaini in lacking those characters of the elongatus species group (see “Comparisons” under O. elongatus) and in having 12 dorsal-fin spines (versus 12–14, modally 13 spines); from O. meniscus in lacking a fleshy crest in males, in having lateral-line tubes, and in having the gill opening restricted to the area dorsal to the level of the 2nd pectoral-fin ray (versus tubes absent and gill-opening extending ventrally to opposite 3rd ray); and from O. punctatus in having 1–4 (rarely 4) lateral-line tubes, which extend posteriorly to below the level of dorsal-fin spines 2–6 (rarely 5–6; versus 3–8 tubes, rarely 3—4, which extend posteriorly to below dorsal-fin spines 6–11, rarely 6–7).

NOMENCLATURAL