Peacock Rockskipper

Istiblennius meleagris (Valenciennes)

Salarias meleagris Valenciennes in Cuvier and Valenciennes, 1836:332 [terre de Van-Diemen: lectotype designated below: MNHN A.1808].

Blennius vittipinnis Castelnau, 1875:25 [Dampier's Archipelago, West Coast of Australia; 7 syntypes MNHN A.2120 [3] and A.2121 [3], BMNH 1883.7.4.5.6 [1]].

DESCRIPTION.—Dorsal fin (Table 32). XII to XIV,17 to 21 = 29 to 34 (XIII in 94% of specimens; 17 segmented rays in only 1 of several hundred specimens examined for this character); mean number of total elements usually higher for males from any locality than for females from same locality (higher for 15 of 16 localities where means for both sexes are available, statistically significantly higher for 6 of the 15 localities); membrane between spinous and segmented-ray portions notched more than three-fourths length first segmented ray; posterior membrane from posteriormost ray attaching to point ranging from on dorsal edge of caudal peduncle anterior to caudal-fin base to point out on dorsal edge of caudal fin <15% caudal-fin length (attachment usually on caudal peduncle, rarely posterior to vertical at caudal-fin base).

Anal fin (Table 32). II,17 to 21 (rarely 17 or 18); mean number of segmented rays usually higher for males from any locality than for females from same locality (higher for 15 of 16 localities where means for both sexes are available, statistically significantly higher for 4 of the 15 localities); posteriormost ray usually split to base (about 65% of specimens); posterior element of split ray usually well developed, readily discernible externally, not bound by membrane to caudal peduncle in any of 50 specimens examined for character (bound, however, in single available ophioblennius-stage specimen). Mature males with skin covering anal-fin spines and distal half of rays expanded slightly, crinkled slightly along lateral edges.

Pectoral-fin rays 12 to 15 (12 only unilaterally, in 1 of 125 specimens examined for character; 14 bilaterally in 93% of specimens). Pelvic-segmented rays 3.

Caudal fin. Dorsal procurrent rays 4 to 7 (6 or 7 in 97% of specimens examined for character), ventral procurrent rays 5 to 8 (6 or 7 in 95% of specimens), modal combination of 6 dorsal with 6 ventral procurrent rays in 36% of specimens ( in 32%. in 21%); segmented rays 13 to 15, strongly modally 13 (Table 33).

Vertebrae (Table 32). 11 or 12+24 to 28 = 35 to 39 (12 in only 3 specimens, 24 in only 1 specimen of 309 specimens examined for characters; total vertebrae 37 or 38 in 95% of specimens); mean number of total vertebrae tending to be higher for males from any locality than for females from same locality (higher for 11 of 16 localities where means for both sexes are available, statistically significantly higher for 2 of the 11 localities); posteriormost pleural rib on 11th to 13th from anteriormost centrum (on 12th in 98% of specimens examined for character); posteriormost epineural on 15th to 21st from anteriormost centrum (rarely on 15th); mean number of vertebra bearing posteriormost epipneural tending to be higher (more posterior) for males from any locality than for females from same locality (higher for 8 of 14 localities where means for both sexes are available, statistically significantly higher for 3 of the 8 localities).

Cirri. Nape cirrus present or absent (Table 33) bilaterally or unilaterally, varying somewhat depending on population; cirri predominantly simple, occasionally forked or with up to 4 branches. Orbital cirrus tree-like with medial and lateral branches (specimens as small as 26 mm SL with up to 6 branches), varying from slightly shorter than to much longer than orbital diameter, number of branches and relative length increasing with increase in SL. Nasal cirri short, palmate, with 2 to 8 branches (usually less than 6) on rim of each anterior nostril.

Lateral line. Continuous canal anterodorsally with simple pores (no vertical pairs of pores), extending posteriorly to point between verticals from 8th and 13th (rarely only to 8th) dorsal-fin spines, descending to midside and, in specimens >30 mm SL, continuing posteriorly as series of 6 to 26 short, disconnected, horizontally bi-pored canals (tubes) in skin; number of tubes usually more than 10, tending to increase in number with increase in SL; posteriormost tube in area between vertical from base of 2nd segmented dorsal-fin ray and caudal-fin base, usually posterior to vertical from base of 9th segmented ray; position tending to be more posterior in larger specimens.

Mandibular pores 4 to 6 (6 pores bilaterally in 88%, unilaterally in 9% of specimens examined for character; 4 pores, unilaterally, in only 1 specimen).

Five to 8 sensory pore positions between 1 o'clock and 5 o'clock on postorbital margin (6 in 93% of specimens examined for character); 0 to 2 positions with paired pores (0 in 7%, 1 in 74%, 2 in 19% of specimens examined for this character).

Posterior canines absent.

Ventral margin of upper lip crenulate; dorsal margin of lower lip usually entire, often weakly, irregularly scalloped, usually restricted to corners of mouth (only 2 of several hundred specimens examined with well-developed crenulae along entire lip margin).

Males with fleshy, blade-like crest dorsally on head; earliest indication of beginning crest a darkly pigmented, longitudinal line dorsally on head, indication variably present in males as small as 28 mm SL; however, definite ridge or crest first apparent only in specimens >30 mm SL, but usually not before 35 mm SL; total absence of crest or ridge may occur in rare males up to 59 mm SL, but almost all males >45 mm SL have noticeable crest. Crest or ridge-like precursor absent in females <50 mm SL, rarely indicated in females <70 mm SL; females >70 mm SL usually show indications of ridge formation, but definite crests present only in females ≥89 mm SL; however, indications variably absent in females as large as 106 mm SL.

Table 32.—Frequency distributions for certain meristic characters of specimens of Istiblennius meleagris from various localties (arranged by sex and more-or-less linearly around the northern Australian coast from Scarborough, WA, to Sydney, NSW). Underlining indicates significant differences (p ≤ .05) between means of sexes from same locality.

Color pattern (in preservative; Figures 43, 44). Head variably dusky with dark spot posterior to eye; dark area just posterior to corner of jaws. Body pale to dark dusky, often with indications of up to 6 dark, irregular, more or less paired, broken bands; bands contrasting more strongly with body ground color in females than in males; dark dorsal extensions of bands positioned on dorsal body contour, entering dorsal fin basally; numerous small, pale spots on body (spots often absent in long-preserved specimens) arranged, more or less, in unmatched rows. Spinous- and segmented-ray portions of dorsal fin faintly to darkly dusky, often bearing dark, slender, oblique stripes (particularly males) or dark spots (particularly females); fin dusky with pale, slender, oblique stripes in some males. Anal fin faintly to darkly dusky, variably with pale spots in interradial membranes. Caudal fin faintly to darkly dusky, with vertical pair of darker dusky spots centrally at base, more distinct in females than males; darker caudal fins with small, pale spots in interradial membranes. Pectoral fins variably dusky. Pelvic fins uniformly pale or dark dusky.

Table 33.—Frequency distributions for occurrence of nuchal cirri and number of segmented caudal-fin rays in specimens of Istiblennius meleagris from various localities (arranged more-or-less linearly around the northern Australian coast Scarborough, WA, to Sydney, NSW).

Freshly preserved specimens with pale spots very conspicuous, variably present on the head. Randall et al. (1990:385, middle fig.) provide color photograph of fresh male specimen, but color seems faded. Grant (1987:333, fig. 713) provides color photograph of fresh female specimen, very similar to preserved specimens, except: pale spots on head and body white or pale blue; dark spot distally in interradial membranes between first 2 dorsal-fin spines; anal fin and segmented-ray portion of dorsal fin darkest just subdistally; Grant describes life color as, “head, face and lips are peacock-purple…shading through to the dull purple of the body.”

Ophioblennius stage. Only 1 ophioblennius-stage specimen, 16.1 mm SL (Figure 4d), available: 4 tiny canines anteriorly in lower jaw; enlarged, posterolaterally projecting canine on only 1 side of lower jaw; head with small melanophores dorsally; few faint bands beginning to form dorsally on body below anterior spinous dorsal fin; pectoral fin (Figure 6e) with column of elongate melanophores in interradial membranes at about mid-length of fin and distal band of much smaller melanophores. Specimens ≥16.3 mm SL are all metamorphosed.

Size. Largest male 118 mm SL, largest female, 106 mm SL.

GEOGRAPHIC VARIATION.—Specimens from different localities exhibit noticeable variation in mean numbers of meristic elements (Table 32), numbers of segmented caudal-fin rays, and in presence of nuchal cirri (Table 33).

Means for meristic elements appear to be highest along the western and eastern coasts of Australia, and lowest along the northermost coast. Nape cirri appear to show a progression from cirri being absent on one or both sides in Western Australian specimens, to occasionally being present on both sides in Northern Territory specimens, to usually being present on both sides in Queensland and New South Wales specimens. The presence of 14 or 15 segmented caudal-fin rays occurs only in some specimens from Western Australia and Northern Territory, with slight indication that presence is more common in the former area than the latter.

The transition area, where nape cirri change from being usually absent to being usually present, is relatively narrow. It occurs somewhere between the eastern Gulf of Carpentaria and Prince of Wales Island off the northwest tip of the York Peninsula, northern Queensland. Northern Australia was connected to southern New Guinea during the last glaciation, about 18,000 years ago, when sea level was over 100 m lower (Myers, 1989, fig. 8; Springer and Williams, 1990). This connection probably isolated populations of I. meleagris on either side and permitted them to diverge. Gradual rise in sea level has inundated the connection and allowed the separated populations to become reproductively connected once again.

The partial barrier formed by the Cape York Peninsula, perhaps, still limits interbreeding between populations on either side, and this is manifested by the relatively narrow area where intergrades in nape cirri occur.

COMPARISONS AND RELATIONSHIPS.—We have no intuitive suggestion as to what the sister group of Istiblennius meleagris might be, although our phylogenetic analysis places it in a weakly supported polytomous clade with I. edentulus, I. rivulatus, and I. dussumieri (Figure 60; see Phylogenetic Analysis section). It is easily distinguished from all other Istiblennius species in having the following combination of characters: 14 pectoral-fin rays; crenulate upper lip; highly complex (tree-like) orbital cirrus; usually more than 10 (up to 26) bi-pored lateral-line tubes, which usually extend posteriorly beyond vertical from 10th segmented dorsal-fin ray (as far as caudal-fin base); dorsal fin rarely attached posterior to caudal peduncle; no posterior canines in lower jaw.

DISTRIBUTION (Figure 69).—Known only from Australia, ranging from Scarborough (vicinity of Perth), Western Australia, north around the top of the continent, then east, and south to Sydney, New South Wales. Occurs close to shore at very shallow depths and, perhaps, under brackish or freshwater conditions. One lot of specimens (WAM P.15850-001) was taken near the mouth of the Murchison River, and two others (WAM P.5918-001, P.6581-001) were taken about 15 km upstream in that river. No other species of Istiblennius is reported to occur so far from the sea, and few, if any of the others are known to occur under brackish conditions. Grant (1987:333) reported that groups of up to 20 individuals may occur together beneath flakes of rock and dead coral clinker well above low-tide area. According to Grant, I. meleagris often occurs closely together with I. edentulus.

NOMENCLATURAL