Istiblennius rivulatus (Rüppell 1830)

Oviparous, distinct pairing (Ref. 205).

Dorsal spines (total): 12 - 14; Dorsal soft rays (total): 20 - 23; Anal spines: 2; Analsoft rays: 20 - 23

Common in very shallow water (as shallow as 10 cm) on rocky shores (Ref. 9962).

Adults are common in very shallow water (as shallow as 10 cm) on rocky shores (Ref. 9962). Oviparous. Eggs are demersal and adhesive (Ref. 205), and are attached to the substrate via a filamentous, adhesive pad or pedestal (Ref. 94114). Larvae are planktonic, often found in shallow, coastal waters (Ref. 94114).

Istiblennius rivulatus (Rüppell)

Salaris [sic] rivulatus Rüppell, 1830:114 [Tor 9SW coast Sinai Peninsula); holotype or lectotype SMF 1843; see nomenclatural discussion section below].

Salarias oryx Ehrenberg in Cuvier and Valenciennes, 1836:335 [mer Rouge; lectotype designated in present study from among MNHN A.1804].

DESCRIPTION.—Dorsal fin. XII to XIV,20 to 23 = 33 to 35 (XII in only 8, and XIV in only 1 of 111 specimens examined for character; 20 in only 5, and 23 in only 2 of 103 specimens; 33 in only 2 of 53 males and 3 of 49 females); mean number of total elements significantly higher for males than for females (34.3 vs. 34.1, p = .02); membrane between spinous and segmented-ray portions notched about one-third to two-thirds length of first segmented ray; membrane from posteriormost ray usually attaching to point on dorsal edge of caudal fin posterior to caudal-fin base in specimens over 45 mm SL, attachment rarely extending out along edge more than 20% caudal-fin length (at most 25%).

Anal fin. II,20 to 23 (rarely 20 or 23); mean number of segmented elements significantly higher for males than for females (21.6 vs. 21.2, p = .05); posterior element split to base in about 25% of specimens; posterior element of split ray usually visible externally; fin not bound by membrane to caudal peduncle. Skin covering anal-fin spines and distal half of rays not modified in males.

Pectoral-fin rays 12 or 13 (13, only unilaterally, in 1 of 75 specimens examined for this character).

Pelvic-fin segmented rays 3.

Caudal fin. Dorsal procurrent rays 5 to 7 (rarely 5, modally 6), ventral procurrent rays 5 to 8 (rarely 5 or 8, modally 6), procurrent rays and in 89% of specimens; segmented rays 13.

Vertebrae. 11+27 to 29 = 38 to 40 (rarely 11+27), mean 39.0 in both sexes; posteriormost pleural rib on 12th or 13th from anteriormost centrum (on 12th in 30 of 34 specimens examined for character); posteriormost epineural on 15th to 19th from anteriormost centrum (on 17th in 50% of specimens).

Cirri. Nape cirri simple, occasionally absent unilaterally. Orbital cirri usually simple, occasionally with single branch, length greater than (up to almost twice) orbital diameter in males ≥40 mm SL, slightly less than to slightly more than orbital diameter in females; nasal cirri simple, occasionally with single branch.

A single female (BMNH 1967.5.13.18), abnormally, lacked both nape cirri and had orbital cirri with numerous small side branches and palmately branched nasal cirri; 1 nasal and 1 orbital cirrus of each pair appeared malformed, but others, except for branching, appeared normal; nasal and orbital cirri darkly pigmented (cirri uniformly pale in normal specimens).

Lateral line. Continuous canal anterodorsally with simple pores (no vertical pairs of pores), extending posteriorly to point between verticals from bases of 7th and 10th dorsal-fin spines (usually posterior to base of 8th spine), thence continuing posteriorly as series of 1 to 8 (usually 4 to 6) short, disjunct, horizontally bi-pored tubes (canals) in skin; posteriormost tube on body in area between verticals from bases of 9th dorsal-fin spine and 3rd segmented ray (usually between bases of 12th spine and 1st segmented ray).

Mandibular pores 4 or 5 (4 only unilaterally, in 4 of 48 specimens checked for character).

Six or 7 sensory pore positions between 1 o'clock and 5 o'clock on postorbital margin (6 in 90% of 62 specimens examined for character); 0 to 2 positions occupied by pair of pores (rarely 0 pairs in specimens over 50 mm SL).

Mandibular posterior canine teeth absent.

Ventral margin of upper lip and dorsal margin of lower lip entire, without crenulae.

Well-developed fleshy crest on head present or absent in males 31 to 40 mm SL, present in larger males; no crest noticeable in most females, but ridges or low crests common in females >57 mm SL; 2 largest females, 88 to 93 mm SL, lacked crests.

Color pattern (in preservative; Figures 34, 35). Males and females essentially the same. Head with dusky band posterior to eye, pale band below eye extending across corner of mouth; other dusky markings on upper lip and head posteriorly; crest of male with dusky band at each end, separated by broader, pale area. Body varying from unmarked to bearing up to 9 irregular to regular dusky bands, middle 2 or 3 bands represented by paired elements; bands extending dorsally well out on dorsal fin, breaking up into slender, dusky, diagonal streaks; posterior body variably with several pairs of faintly dusky, wavy pinstripes with slightly more faintly dusky areas between members of each pair (reminiscent of pattern in I. lineatus). Dorsal fin with black mark distally on membrane between first two spines, occasionally continuing to between third and fourth spines, then continuing as fainter, dusky, distal or subdistal margin on remainder of spinous dorsal fin. Anal fin faintly dusky with darker dusky subdistal margin; ray tips noticeably pale. Pectoral, pelvic, and caudal fins plain.

Size. Largest male 86 mm SL, largest female 93 mm SL; smallest specimen 25.3 mm SL, sex indeterminate. Ophioblennius-stage specimens unknown.

COMPARISONS AND RELATIONSHIPS.—Istiblennius rivulatus is unique in Istiblennius and Blenniella in having 12 pectoral-fin rays; all other species have strongly modally 14 rays. Also noteworthy are its simple cirri, particularly the long orbital cirrus, and the relatively shallow notch in the dorsal fin. Our phylogentic analysis of Istiblennius (Figure 60; see discussion in Phylogenetic Analysis section) was unsatisfactory in fully resolving the relationships of I. rivulatus. Although placed in a polytomous clade of 4 species, the clade is only weakly supported. Possible evidence of its relationships to other species in the genus is the presence of a configuration of striping on the body that it shares with the Lineatus complex of species, which, interestingly, is widely distributed in the Indo-Pacific, but absent from the Red Sea, except for a record from just inside its entrance. The presence of a dark spot anteriorly in the dorsal fin of both sexes of I. rivulatus is, perhaps indicative of a relationship with I. dussumieri, in which, however, only the male exhibits the dark spot.

Distribution (Figure 71).—Southern Gulf of Suez; Gulf of Aqaba; northern Red Sea south only to Mersa Ar-rakiyai, Sudan (~20°12′N). Extremely common in very shallow water (as little as 10 cm) on rocky shores.

NOMENCLATURAL

Istiblennius rivulatus is a species of combtooth blenny found on coral reefs in the western Indian ocean. Males of this species can reach a maximum of 8.6 cm (3.4 in) in standard length, while females can reach a maximum of 9.3 cm (3.7 in) in standard length.

Common in very shallow water (as shallow as 10 cm) on rocky shores (Ref. 9962)