Comprehensive Description

provided by Smithsonian Contributions to Zoology
Omobranchus elongatus (Peters)

Petroscirtes elongatus Peters, 1855a:249; 1855&:440 [Mossambique].

Petroskirtes kallosoma Bleeker, 1858:227 [Biliton occidentalis, in mari].

Petroscirtes dispar Fowler, 1937:258 [Bangkok, Siam; junior primary homonym of Petroscirtes dispar Günther, 1861, = Omobranchus punctatus (Valenciennes)].

DESCRIPTION (see also Tables 5 and 6).—Dorsal-fin XII–XIV (XIII in 88.9% of specimens), 17–20 (rarely 17) = 30–33 (rarely 30 or 33); anal fin II, 20–23 (rarely 20); both anal-fin spines of males discernible externally; segmented caudal-fin rays 12–14 (13 in 98.2% of specimens); dorsal + ventral procurrent caudal-fin rays 10–16 (rarely 10 or 16); vertebrae 10–11 (10 in 96.5% of specimens) + 27–30 (rarely 30) = 37–40 (rarely 40); epipleural ribs 11–15 (rarely 11 or 15); prenasal pores present; interorbital pores 1–4 (3 in 97.6% of specimens); circumorbital pores 7–9 (8 in 96.5% of specimens); lateral-line tubes 0–9 (rarely 8 or 9); lateral-line tubes, when present, extending posteriorly to below level of dorsal-fin spine 1–10 (rarely 10); gill opening varying from restricted to area dorsal to level of dorsalmost pectoral-fin ray to extending ventrally to opposite 6th ray (rarely restricted dorsal to dorsalmost ray or extending ventral to 5th ray); lower-lip flap present; circumorbital bones 5; lower jaw teeth 18–26 (Figure 39); upper jaw teeth 15–24 (Figure 38); no fleshy bladelike crest on top of head of either sex.

COLOR PATTERN.—Males: Head: Two diffusely dusky to dark bands on side of head below eye; anterior band extends from anteroventral margin of eye across corner of mouth to ventral surface of chin, where it joins corresponding band of opposite side; second band (frequently absent) extends from posteroventral margin of eye to ventral surface of head, where it may join corresponding band of opposite side. Ventral third of head posterior to bands, and prepelvic area, with numerous dark spots, smaller than pupil of eye. Large, dark smudge present between circumorbital and preopercular series of cephalic sensory pores at level of lower two-thirds of eye; smudge domeshaped in some specimens, occasionally with anterior margin slightly intensified as spot. Operculum occasionally with anterodorsally directed, pale dusky, oval spot, partially to completely encircled by distinct, narrow, pale margin; vertical, darkened area present between spot and posterior preopercular margin; opercular membrane dusky posterior to spot. Snout with middorsal dark patch extending posteriorly to top of head. Ill-defined, horizontal marks dorsolaterally on head posterior to eye in freshly preserved specimens.

Trunk: About 11 or 12 dark bands laterally; 2nd through 7th bands arranged somewhat in pairs; lower half of bands vertical or slanted slightly posteriorly, reaching nearly to ventral midline of body anteriorly; posteriorly lower half of bands shorter, often faint or absent, occasionally ventral portions of all bands poorly developed; upper half of anterior 5 bands inclined dorsoanteriorly; upper half of succeeding bands decreasingly inclined dorsoposteriorly; upper end of posterior bands often elongate; bands not reaching onto upper one-fifth of body; posteriormost 1 or 2 bands usually represented only as narrow, horizontal, dark spot or spots. Body bands narrow, broadest medially, tapering dorsally and ventrally, often surrounded by poorly defined dusky area; bands usually indistinctly separated from dusky areas. Pale background spaces between bands with dorsoanteriorly slanted, very narrow, paler band extending from just below dorsal fin to near ventral body midline; posterior paler bands with dorsal half inclined dorsoposteriorly; paler bands not evident in faded specimens. Two longitudinal series of paired, elongate, dark spots dorsally on body; dorsal series at junction of dorsal fin and body; ventral series just ventral to dorsal series, with each spot directly under corresponding spot of dorsal series; dorsal series united into narrow stripe in some specimens. Trunk portion of gill opening with dusky to dark bar at posterior border of opercular membrane.

Pectoral Fins: Transparent with uniform spread of dark specks of pigment. Fleshy base with 0 to 4 small, dark spots, 1 or 2 occasionally extending onto basal portion of fin membrane.

Pelvic Fins: Pale with uniform spread of fine, dark specks of pigment.

Dorsal Fin: Generally transparent with series of dark stripes; stripes on spinous portion moderately broad, curving ventroposteriorly; stripes on segmented-ray portion narrow, more numerous, curving dorsoposteriorly. Large, intensely dark spot (ocellus), encircled by transparent or pale margin, located slightly above center of fin between 7th and 14th segmented rays (usually extending from 9th to 11 th rays). Series of dusky marks basally on fin associated with adjacent series of body spots. Some specimens with fin more or less uniformly dusky to dark, except for narrow, pale, distal margin.

Anal Fin: Dusky to dark, darkest distally, with narrow, pale, distal margin. Freshly preserved specimens with narrow, curved, oblique, pale lines directed ventroposteriorly from lower ends of paler bands on body; pale lines terminate proximal to distal third of fin, become faint to absent posteriorly on fin. Dark spot just posterior to proximal end of each oblique pale line (after approximately every 3rd ray base), obscured in specimens with dark fins.

Caudal Fin: Pale with uniform spread of dark specks of pigment; dorsal and ventral margins dark. Caudal peduncle with moderately large, dark spot just above and below lateral midline at caudal-fin base: pigment streak from each spot continued posteriorly onto fin.

Females: Color pattern as in males except: overall background paler; dark markings on trunk and posterior body better defined; dusky patches surrounding body bands poorly developed or absent; bands often undeveloped ventrally; narrow, paler bands in pale spaces between dark bands often inapparent; dorsal and anal fins largely transparent, unmarked; ocellus in dorsal fin absent; anal fin dusky only along proximal edge of pale, distal border.

Geographic Variation: Although the color pattern variations of specimens from any particular area appear to encompass the variations found in the species as a whole, there is a prevalence of specimens with reduced head and fin markings from southern Africa and Madagascar and specimens with the same markings intensified from the Gulf of Thailand and Philippine Islands.

Gulf of Thailand: Both dark bands on head well developed; spots on underside of head small, distinct, numerous; dusky smudge posterior to eye usually intensified anteriorly; narrow, paler body bands (in spaces between dark body bands) not apparent in many specimens; fleshy pectoral-fin base usually with 4 distinct spots; dorsal fin with distinct, dark stripes in males; dorsal-fin ocellus of males located between 7th and 10th segmented rays, occasionally reaching posteriorly to 11th.

Madagascar: Anterior dark band on head faint; 2nd band usually faint or absent; spots on underside of head large, indistinct, faint, few in number; dusky smudge posterior to eye usually without anteriorly intensified area; narrow, paler body bands usually distinct; fleshy pectoral-fin base often without spots, occasionally with narrow, pale bar at proximal edge of fin; dorsal fin of males with stripes obliterated by even, dusky pigmentation at anterior and posterior ends of fin; dorsal-fin ocellus of males located between 9th and 14th segmented rays, occasionally reaching anteriorly to 8th ray.

GEOGRAPHIC VARIATION (see also “Color Pattern”).—Variation in meristic characters (Table 6) occurs in some populations of O. elongatus. The Gulf of Thailand population has a higher average number of lateral-line tubes and lower average numbers of total dorsal- and anal-fin elements and vertebrae than most of the other populations. The Mozambique population was notable for having the lowest numbers of total procurrent caudal-fin rays and lateral-line tubes. It was the only population in which some specimens lacked lateral-line tubes altogether.

Although covariance comparisons of numbers of jaw teeth were not feasible for all populations of O. elongatus, significant differences do occur among those populations that were compared (Table 7).

GEOGRAPHIC DISTRIBUTION (Figure 5).—Tropical Indian Ocean, excluding the Red Sea and Persian Gulf, east to the Philippine Islands and Ambon in the Pacific Ocean.

A completely faded specimen from Sumbawa Island, Indonesia, questionably referred to O. rotundiceps obliquus could possibly be O. elongatus (see “Geographic Distribution” under O. r. obliquus).

Menon and Rama Rao (1963) recorded O. kallosoma from Madras and Mandapam, India, and the Andaman Islands (actually Nicobar Islands). Their specimens from India are O. elongatus, whereas their specimen from the Nicobar Islands is O. rotundiceps obliquus. Of the Indian specimens, we were able to examine only the Mandapam specimen (ZSIC F6916/2), which arrived too late to be included in Table 6. We have entered the Madras record for O. elongatus on Figure 5 based on the color pattern description given by Menon and Rama Rao. Meristics for the Mandapam specimen are as follows: dorsal fin XIII,20; anal fin II,23; total procurrent caudal-fin rays 12; vertebrae 11 + 29; epipleural ribs 13; lateral-line tubes 2, extending posteriorly to below fourth dorsal-fin spine; interorbital pores 3; circumorbital pores 8; gill opening extending ventrally to opposite second pectoral-fin ray.

Chu, et al. (1962) reported and illustrated O. elongatus (as O. kallosoma) from Hainan, and Jones and Kumaran (1965) described and illustrated O. elongatus from Minicoy, Laccadive Islands. We have also included these records on the distribution map.

Munro (1958) mistakenly included O. elongatus in his checklist of New Guinea fishes. His error was the result of his reporting the type-locality of Petroscirtes germaini Sauvage, which species he wrongly considered to be a synonym of O. elongatus, as New Guinea, instead of New Caledonia. Munro (1967) continued the error, twice illustrating O. elongatus, based on published illustrations of extralimital records.

HABITAT.—Shallow (0–4 m) coastal waters and tide pools with rocks and little coral. Occasionally recorded from brackish water and stream mouths and from rocks with oysters.

COMPARISONS.—Omobranchus elongatus belongs to a group of species also comprising germaini, rotundiceps, and loxozonus. This group of species is united by the following unique specializations: Males may have one or more caudal-fin rays strongly exserted and/or filamentous and a dark ocellus or group of dark spots usually present near the middle of the segmented-ray portion of the dorsal fin. In addition, all the species have a distinctive pattern of bands on the body: the anterior bands are inclined anteriorly from their ventral origin and the posterior bands are inclined posteriorly. Some males of O. fasciolatus may have a similar dark blotch in the dorsal fin, but the pattern of bands on the body is different and the caudal-fin rays are never exserted or filamentous. Most male O. fasciolatus have a fleshy crest on the head, a structure not present in the elongatus species group. Omobranchus ferox males may also have a dark blotch in the dorsal fin but it is positioned at the posterior end of the fin. The pattern of bars on the body of O. ferox is usually very faint or absent, but when present it adumbrates that of the elongatus group; however, the narrow pale and dark bars adjacent to the posterior margin of the orbit (Figure 29f) readily separate O. ferox from the elongatus group.

With the exception of O. germaini, which overlaps the distribution of O. elongatus and O. rotundiceps, the species of the elongatus group are predominantly allopatric; however, O. elongatus, O. germaini, and O. rotundiceps have all been collected at the small island of Ambon in Indonesia, where each occurred at the mouth of a different stream. Where two or more of these species occur together, all or most of the specimens of each species are readily separable. Overlaps in the overall ranges for characters of the species are the result of combining individual population variations within a species. Problems in identification occur with specimens that have poorly developed color patterns and counts in the overlap portions of the ranges for the characters of the population under consideration.

A pattern of small, dark spots on the underside of the head (Figures 13b, 14b) found in many male and some female specimens of O. elongatus will distinguish those specimens from specimens of the other species in the elongatus group. The spotted pattern is adumbrated by some specimens of O. r. rotundiceps from eastern Australia (Figure 29b), where O. elongatus does not occur. Within the elongatus group, O. elongatus is distinctive in lacking a well-defined dark spot posterior to the eye.

bibliographic citation
Springer, Victor G. and Gomon, Martin F. 1975. "Revision of the blenniid fish genus Omobranchus, with descriptions of three new species and notes on other species of the tribe Omobranchini." Smithsonian Contributions to Zoology. 1-135. https://doi.org/10.5479/si.00810282.177