Lethrinus obsoletus (Forsskal)

Wilson, G. G., 1998, A description of the early juvenile colour patterns of eleven Lethrinus species (Pisces: Lethrinidae) from the Great Barrier Reef, Australia, Records of the Australian Museum 50 (1), pp. 55-83 : 75-76

Lethrinus obsoletus (Forsskal)

PI. 2E-H, Fig. 5B,C,E

Material examined. (72 specimens: 17-81 mm). AUSTRALIA Great Barrier Reef region , AMS 1.20772 - 027 (1: 81 mm), AMS 1.34911 - 008 (4: 18-27 mm), AMS 1.34911 - 009 (3: 36-39 mm), AMS 1.34911 - 010 (1: 33 mm), AMS 1.34912 - 009 (1: 33 mm), AMS 1.34913 - 003 (1: 25 mm), AMS 1.34913 - 004 (1: 25 mm), AMS 1.34913 - 005 (1: 27 mm), AMS 1.34913 - 006 (1: 43 mm), AMS 1.34919 - 002 (2: 31-35 mm), AMS 1.34919 - 003 (1: 31 mm), AMS 1.34926 - 001 (1: 18 mm), AMS 1.34927 - 001 (1: 58 mm), USNM 336685 (1: 20 mm), USNM 336686 (3: 26--34 mm), USNM 336687 (4: 37-40 mm), not retained (45: 17-63 mm) .

Diagnosis. Body moderately deep, depth 2.5-3.0 in SL; 5.5 dorsal scale rows, 15-16 ventral scale rows; maxillary serrations 5-7 at 16-18 mm, 1-4 at 25-29 mm, 0-2 at 30- 39, absent thereafter; cheek scales present at 16-30 mm, variable at 31-39 mm, absent at less than 39 mm; dorsal spine 4 or 5 the longest at less than 35 mm.

Diagnostic colour notes. Body and head cream to pale grey-tan; banding intense, shoulder blotch cuboidal or vertically-rectangular, seldom with pale halo, not crossing lateral-line; dorsal margin of lateral-line silvery cream; 2-4 orange-tan primary stripes, pectoral stripe of greatest width and intensity; dark mid-lateral stripe frequently displayed (20-80 mm), intense, width approximately 5.0 in body depth; fins mostly hyaline.

Colour notes. Minimal change in colour patterns throughout juvenile ontogeny, frequently displays dark intermittent patterns, able to switch colour patterns in 1-3 seconds. At 17-19 mm, recently-settled: body cream, abdomen silver, lateral-line cryptic; 4-7 indistinct grey bands along dorsum, may join faint ventral bands at lateralline; shoulder-blotch dark grey, often inconspicuous, cuboidal (Fig. 5B), never crossing lateral-line, without pale halo, rapidly switched on or off within 2-3 seconds; 2-3 indistinct primary stripes, pale orange-tan, dorsal and ventral stripes often indistinguishable, pectoral stripe marginally widerthan orbital stripe; nape and snout yellowtan, cheek silver without dark markings; fins hyaline.

At 20-80 mm ( PI. 2E-G): body pale grey-tan, belly off-white, lateral-line usually distinct, dorsal margin offwhite; banding and shoulder blotch distinct and intense when alive ( PI. 2E,F), fading after death ( PI. 2G), medial dorsal bands usually joining 2-3 cuboidal blotches below lateral-line ( PI. 2E,F), ventral bands extending diagonally between blotches and belly, shoulder blotch distinct, mostly vertically-rectangular (Fig. 5C,E, PI. 2F,G), occasionally cuboidal (Fig. 5B, PI. 2E), seldom bordering lateral-line, rarely with pale halo; dark mid-lateral stripe frequently seen in field, dark brown to black, width approximately 5.0 in body depth; primary stripes orange-pink to orangetan, dorsal stripe seldom conspicuous, pectoral stripe slightly wider than others (as in PI. 2G); head similar to body, nape and snout uniform or mottled tan, black nearvertical band often bisecting orbit in field, grey blotch below orbit from 22 mm (inverted v-shaped band from 45 mm) and dark grey to brown blotches along rear margin of preoperculum and operculum in captivity; fins mostly hyaline, pale pink distally in field, anterior half of spinous fins off-white with scattered brown patches.

Colour in alcohol. At 17-20 mm, body off-white, dark banding and shoulder blotch dulled, primary stripes lost, fins hyaline; at greater than 25 mm, body and head olive to grey-tan, belly off-white, banding and dark head markings distinct though dulled, primary stripes lost; fins hyaline. After preservation, L. obsoletus juveniles (18-80 mm) are often indistinguishable from L. lentjan and L. nebulosus , preventing identification beyond L. obsoletusl lentjanlnebulosus .

Ecology. Common, mostly solitary, occasionally joins small (approximately 10-15 individuals) multi-species schools with similarly-sized congeners ( L. atkinsoni , L. genivittatus , L. harak , L. variegatus ), Stethojulis strigiventor , Parupeneus barberinus and P. indicus , and particularly Siganus fuscescens ; settles at 16-18 mm into shallow seagrass cover (0.2-4 m; Halodule , Thalassia , Cymodocea ) on fringing and lagoonal reefs, an ontogenetic shift apparent to adjacent reef-flat habitats at 70-90 mm, individuals 90-140 mm only occasionally encountered within shallow areas; forages diurnally off seagrass leaves (18-30 mm) or on benthic items (at greater than 30 mm), no aggressive interactions noted, individuals 18-25 mm rapidly seek shelter within the seagrass canopy when disturbed.

Field identification. Generally an easy species to identify in the field, although can be confused with L. genivittatus , L. lentjan and L. nebulosus (Table 1). Small specimens (18-25 mm) can be distinguished from L. nebulosus and L. lentjan by their intense shoulder blotch (Fig. 5) and orange to orange-tan primary stripes. At this size, L. obsoletus and L. genivittatus can be reliably separated by their primary stripes, dorsal bands, and shoulder blotch. At greater than 25 mm, L. obsoletus can be distinguished from other species by the prominent orange-pink to orangetan pectoral stripe. During display of the dark mid-lateral stripe, the off-white dorsal border of the lateral-line, in combination with a moderately-deep body profile (cf. pale dorsal border with slender body in L. variegatus ), is diagnostic. At all sizes, individuals may need to be observed for several minutes before the full range of colour patterns can be ascertained. At less than 40 mm, the collection of specimens for examination of freshly-killed colours may also be necessary. Further notes on separating these species in the field are given in the "Field Identification" sections of L. genivittatus and L. lentjan .

Lethrinus obsoletus appears unique among GBR lethrinids in continuing to display a dark mid-lateral stripe pattern throughout the adult stage ( PI. 2H).

Previous descriptions. Wheeler (1961) included a colour sketch of a captive 88 mm TL specimen (as Lethrinus ramak ) along with brief descriptions of the stress, resting and freshly-killed colouration displayed by "very young" specimens (sizes not given). Masuda & Kobayashi (1994) provided a colour plate of a specimen 35 mm total length.

Identification notes. Carpenter & AlIen (1989) note the presence of a "knob" on the outer surface of the maxilla in L. obsoletus to be useful for separating this species from other lethrinids. This character is indistinguishable at less than 80 mm. Alternative meristic and morphological measurements (particularly the number of dorsal scale rows, presence of cheek scales and number of maxillary serrations), along with the shoulder blotch and primary stripes, represent the easiest means of distinguishing smaller L. obsoletus from other close species.