Plectorhinchus caeruleonothus Johnson & Wilmer, 2015, sp. nov.

Plectorhinchus caeruleonothus sp. nov.

New English name: Blue Bastard

Figures 1–5, 14; Tables 1–5

Holotype. QM I. 39243, 536 mm, Boyd Bay, SW of Weipa, Queensland, Australia, 12°54.9’S 141°38.6’E, 0.5–3 m, rod & line with artificial fly, B. Bright, 24 Jun 2014.

Paratypes. (n = 16) Queensland, Australia: AMS I.46510-001, 439 mm, same data as holotype, 3 Jul 2014; QM I. 6496, 175 mm, Prince of Wales Island, 10°36’S 142°12’E, T.C. Marshall & E.G. Ogg, 9 Aug 1938; QM I. 39242, 423 mm, same data as holotype, 26 Jul 2014; QM I. 39295, 532 mm, same data as holotype, 8 Jul 2014 (skeleton, otoliths & tissue sample separate). Northern Territory, Australia: NTM S.11263-025, 182 mm, Coral Bay, Cobourg Peninsula, 11°12’S 132°03’E, 2–3 m, rotenone, B.C. Russell, 18 May 1983; NTM 16708-007, 5: 99– 142 mm, Bullocky Point reef, Darwin Harbour, 12°26.14’S 130°49.74’E, rotenone, H. Larson & party, 14 Nov 2008; Western Australia, Australia: BPBM 17384, 245 mm, Kendrew Island, ca 20°29’S 116°32’E, 4 m, spear, J.E. Randall, 12 Oct 1973; BPBM 17385, 363 mm, W side of Kendrew Island, ca 20°29’S 116°32’E, 12 m, spear, J.E. Randall, 13 Oct 1973; CSIRO CA. 1444, 291 mm, Monte Bello Islands, ca 20°30’S 115°30’E, spear, CSIRO, 11 Dec 1979; CSIRO H. 1481-6, 525 mm, N of Cape Preston, 20°33.2’S 116°08.9’E to 20°33.7’S 116°07.1’E, demersal trawl, 30 m, CSIRO, 25 Sep 1988; WAM P.32170-020, 110 mm, Riddell Beach, tidal rock pool, Broome, 17°59’S 122°11’E, rotenone, A. Williams, 13 May 1982; WAM P.33288-006, 169 mm, Montgomery Reef, SW corner, intertidal mid-littoral fore-reef ramp tide pools, 16°00.865’S 124°10.389’E, 0–0.5 m, rotenone, S. Morrison & J. Johnson, 20 Oct 2009.

Non-types. (n = 5) Queensland, Australia: QM I.34724, 36 mm, Sweers Island, east side, rockpool north of Observation Hill, 17°07.9’S 139°36.9’E, rocky overhangs, crevices, brown macroalgae & fine sand, 0.1–1.0 m, rotenone, J. Johnson & A. Gill, 19 Nov 2002. Western Australia, Australia: WAM P.33274-024, 3: 68–101 mm, Adele Island, reef platform at head of Frazer Channel, 15°29.474’S 123°09.798’E, 0–1.0 m, rotenone, S. Morrison & J. Johnson, 15 Oct 2009; WAM P.33291-051, 99 mm, Montgomery Reef, small tide pools, 15°53.7’S 124°20.34’E, 0–0.5 m, rotenone, S. Morrison & J. Johnson, 24 Oct 2009.

Diagnosis. A species of Plectorhinchus with dorsal-fin rays XII, 18–20 (rarely 20); anal-fin rays III, 6–7 (rarely III, 6); pectoral-fin rays 16–17 (usually 17); lateral-line scales 56–61 (modally 59); transverse scale rows above lateral line 15; gill rakers 7–9 + 18–20 = 25–29 (modally 26); pelvic fins reaching anus in juveniles, slightly short of anus in adults; nostrils minute, 0.4–0.8 % SL, 2–3 times in distance from posterior nostril to eye; and fresh colouration in adults including body uniformly silver-grey, cheek and opercles blue-grey, rim of orbit and dorsal edge of maxilla dusky yellow, and posterior margin of opercular membrane silver-grey, non-contrasting with remainder of opercle and adjacent body.

Description. Dorsal-fin rays XII, 18 (18–20); anal-fin rays III, 7 (6–7, rarely 6); all dorsal- and anal-fin rays branched, last to base; pectoral-fin rays 17 (16–17, rarely 16), first ray rudimentary, second and lowermost rays unbranched, others branched; pelvic-fin rays I, 5, all branched; principal caudal-fin rays 9 + 8 = 17, uppermost and lowermost rays unbranched; lateral-line scales 60 (56–61, modally 59), plus about 16 smaller tubed scales on anterior third of caudal-fin base; scales above lateral line to base of first dorsal-fin spine 15; scales below lateral line to first anal-fin spine 19 (18–20); circumpeduncular scales 34 (33–35); gill rakers on first arch 7 + 19 = 26 (7– 9 + 18–20 = 25–29, modally 26); branchiostegal rays 7; vertebrae 11 + 16 = 27.

Body oblong, moderately deep, depth 2.7 (2.5–2.9) in SL, and compressed, width 2.5 (2.4–2.8) in depth; head length 3.3 (3.0–3.4) in SL; dorsal profile of head moderately convex, a line from its apex forming an angle of about 38° (36–48°) from a horizontal at the snout tip, angle becoming more acute with growth, larger specimens developing small bump in snout profile followed above by slight concavity at about level of anterior nostrils (see holotype, Fig. 2 A); snout long, length 2.8 (2.4–3.6) in HL, proportionally longer with growth; orbit diameter 5.4 (3.3–5.8) in HL, proportionally narrower with growth; interorbital width 3.1 (2.9–3.7) in HL; preorbital width 3.0 (3.0–4.6) in HL, proportionally wider with growth; caudal peduncle depth 2.8 (2.6–3.1) in HL; caudal peduncle length 1.3 (1.3–1.5) in HL.

Mouth moderately large, tip of maxilla reaching a vertical midway between anterior and posterior nostrils (reaching between posterior nostril and eye in subadults, to anterior margin of eye or slightly beyond in juveniles), upper jaw length 3.6 (3.4–3.7) in HL; lips thick and fleshy, upper lip protruding slightly beyond tip of lower jaw; teeth conical, outer series distinctly enlarged, but no canines, in band of up to 6 rows wide anteriorly in each jaw; band in upper jaw narrowing gradually to become one or two rows wide for about last 4 teeth in series; band in lower jaw narrowing abruptly at about midsection of jaw to become uniserial posteriorly; teeth in outer series of upper jaw about 20 on each side, those of lower jaw about 18 on each side; numerous small fleshy papillae interdigitating among teeth, fleshy flap behind bands of teeth in each jaw with shorter more flattened papillae that gradually decrease in size posteriorly; vomer and palatines edentate; tongue smooth, with broadly rounded tip; chin with 3 pairs of prominent pores, pair on lower lip closest together, others evenly spaced; gill rakers moderately long, raker below the angle longest, about 3 (2.5–4.0) in orbit diameter.

Nostrils minute, posterior nostril slightly smaller and dorsoposterior to anterior nostril, its width 10.6 in orbit diameter (10.1–14.0); posterior nostril in front of and just below a horizontal from centre of eye, anterior nostril just below horizontal from lower margin of eye (posterior nostril on horizontal from centre to lower third of eye, anterior nostril on horizontal from lower margin of pupil to just below lower margin of eye); internarial distance moderately wide, 2.5 (2.0–2.8) in distance from posterior nostril to eye; distance from posterior nostril to eye wide, 2.0 (1.8–4.7) in orbit diameter (1.8–2.6 in specimens> 250 mm SL); posterior nostril with raised membranous rim and broadly rounded flap anteriorly, flap about half nostril width (flap one-third to subequal to nostril width); anterior nostril with higher raised membranous rim and longer more spatulate flap posteriorly, slightly greater than nostril width.

Opercle lacking exposed spine; posterior edge of preopercle finely serrate (serrations more distinct in juveniles and subadults); margins of subopercle, including angle and interopercle, smooth.

Scales small and ctenoid on body and most of head, cycloid anteriorly on interorbital and suborbital; lateral line continuous, gently curved anteriorly, following dorsal contour of body and becoming straight on peduncle; head scaled except on snout just forward of anterior nostrils and band from anterior margin of eye to snout, bounded above by an oblique imaginary line from just above dorsal margin of eye through dorsal edge of anterior and posterior nostrils; small imbedded scales on preorbital reaching anteriorly to vertical just anterior to anterior nostril; horizontal scale rows from preorbital margin opposite tip of maxilla across cheek to edge of preopercle about 40, vertical rows below middle of eye obliquely to margin of preorbital about 30 (difficult to count accurately due to scales becoming imbedded with growth, numerous small supplementary scales, and irregular size and alignment of scales along rows); basal sheath of scales present on dorsal and anal fins; spinous and soft portions of dorsal fin with 1–2 and about 4–8 scale rows at bases respectively; spinous and soft portions of anal fin with 2–3 and about 4–7 scale rows at bases respectively; minute scales sparsely distributed on soft rays of all fins, except inner base of pelvic fins; axil of pectoral fin with scaly process, fleshy inner base naked.

Dorsal fin scarcely notched, its origin just anterior to vertical from posterior margin of opercle (midway between upper corner and posterior margin of opercle to just anterior to posterior margin of opercle); base of soft dorsal-fin usually subequal to that of spinous portion, 1.0 (0.9–1.3) in its length; interspinous membranes distinctly incised; dorsal-fin spines strong, first spine 5.6 (5.1–7.3) in HL; fourth spine longest, 2.6 (2.2–2.6) in HL; fifth spine next longest, followed by third; 12th (12th or 13th) soft dorsal-fin ray longest, 2.5 (1.7–2.7) in HL (2.2–2.7 in specimens> 250 mm SL); origin of anal fin below base of 4th (3rd to 5th) soft dorsal-fin ray; first anal-fin spine short, 3.9 (3.5–4.8) in second; second anal-fin spine more robust and slightly longer than third (slightly shorter than third in some paratypes), 2.9 (1.7–2.9) in HL; second anal-fin ray longest, 2.0 (1.5–2.1) in HL; anal-fin base short, 1.3 (1.3–1.8) in its height; caudal fin emarginate (truncate in small juveniles), 1.5 (1.3–1.5) in HL; fifth pectoral-fin ray longest, 1.6 (1.4–1.6) in HL, subequal to pelvic fins; origin of pelvic fins posterior to lower base of pectoral fins, on vertical from base of 4th to 5th dorsal-fin spine; pelvic-fins nearly reaching anus (reaching anus in juveniles and some subadults), first ray longest (second subequal to first), 1.6 (1.3–1.6) in HL.

Colour when fresh. Holotype (Fig. 2 B) silver-grey above on head and body, gradually becoming lighter below from middle of sides. Suborbital region with pale bluish reflections (particularly so immediately after specimen taken from the water). Stark white below horizontal from corner of mouth, including lower part of head, exposed branchiostegal membranes, lower pectoral-fin base, chest, belly and lower part of caudal peduncle (in life some paratypes pale grey, rather than white on lower head and body, changing to white on death). Iris generally dusky orange-yellow dorsally and narrowly a more pale orange-yellow ventrally. Orbital margin orange-yellow, with narrow streak of same hue extending about half width of eye anteroventrally from posteroventral margin. Dorsal half of upper lip blue-grey, lower portion stark white. Maxillary groove and dorsal margin of maxilla dusky-yellow, remainder of maxilla stark white. Skin beneath edge of preopercles and upper branchiostegal membrane below posterior tip of opercle narrowly dusky-yellow, latter not visible externally unless opercles are flared (two of three paratypes from type locality lacking any yellowish pigmentation in these areas). Inside mouth white anteriorly, but throat and gill cavity to level of preopercular margin, including base of gill arches, bright orange-red. Spinous dorsal-fin medium grey, with spines lighter silver-grey, contrasting against the membrane. Soft dorsal-fin, anal fin and caudal fin grey, with intervening membrane darker than rays. Pectoral-fin base white with diffuse dusky blotch above on outer face. Pectoral-fin axil and dorsal part of inner face dusky orange-yellow, followed below by irregular dark brown blotch extending to about centre of inner face (paratypes from type locality differ variously in the size, shade and positioning of the irregular brown blotch; in two it extends dorsally to axil, in one blotch is vague and largely replaced by orange-yellow pigmentation, however in all it covers not more than half of inner fin base). Distal third of pectoral-fin pale grey, basal two-thirds whitish. Pelvic fin with distal half pale grey, basal half white.

Paratype, BPBM 17384, 245 mm SL (Fig. 3 A), dull blue-grey on head, body and fins, with numerous very faint narrow pale stripes along sides. Caudal fin covered with vague dark spots. Similar sized individuals, estimated at 30–35 cm TL, photographed underwater off Cape Leveque (Fig. 3 B) and Malus Island, Dampier Archipelago, Western Australia (not collected), silver-grey with 9 narrow whitish stripes along head and body. Upper stripes continuing forward through interorbital to tip of snout after traversing head. Stripe under eye with distinct crescentic ventral curve. Caudal fin with distinct black margin and profusely spotted with irregular dark markings about half pupil diameter.

Juveniles, approximately 20–25 cm TL, photographed underwater at Malus Island and West Lewis Island (Fig. 3 C), Dampier Archipelago (not collected), dark chocolate-brown, with about 10 irregular narrow white stripes along head and body. Stripes on head, including curved one below eye, aligned similarly to those described above, but slightly broader and more well-defined. Numerous oblique white stripes on dark brown background on caudal fin, stripes converging toward centre of rear margin of the fin. Pectoral fins yellow, inner fin base with central dark brown blotch.

Colour in alcohol. Holotype (Fig. 2 A) uniformly greyish brown on upper head and body; fading gradually to pale cream below horizontal from upper pectoral fin base, and on underside of head, branchiostegal membranes, chest and belly. Narrow pale streak about half width of orbit extending anteroventrally from lower margin of eye. Dorsal half of upper lip and dorsal margin of maxilla dark grey, lower portions pale cream. Lower lip uniformly pale cream. Spinous dorsal-fin pale grey with some faint irregular pale cream blotching to membrane and base of fin spines. Soft dorsal-fin, anal fin and caudal fin grey, with intervening membranes darker than rays. Pectoral-fin base with diffuse dusky blotch dorsally on outer face and darker more well-defined blotch extending from axil of fin to upper half on inner face. Distal third of pectoral-fin faintly dusky. Pelvic fin with distal half faintly dusky. Juvenile, QM I.34724, 36 mm SL (Fig. 4 C), with two broad pale cream stripes laterally on head and body, contrasting strongly against dark chocolate-brown ground colouration. Upper stripe on each side originating medially at snout tip above upper lip, traversing interorbital region, passing around upper margin of orbit, through upper corner of gill opening to soft dorsal-fin base, along upper margin of caudal peduncle, and through dorsal quarter of caudal fin. Lower stripe originating on margin of preorbital midway between snout tip and tip of maxilla, curving upward to lower margin of orbit at vertical from anterior third of eye, traversing opercle to midbase of pectoral fin, continuing along sides to lower edge of caudal peduncle and through lower quarter of caudal fin. Third narrower pale stripe originating on midline of forehead at vertical from anterior margin of orbit, bifurcating just anterior to dorsal-fin origin, continuing along spinous dorsal-fin base on each side and terminating at base of the penultimate spine. Soft dorsal-fin with submarginal pale stripe. Anal fin with diffuse dusky basal streak, extending onto last few rays of fin. Pectoral and pelvic fins semitranslucent.

Juveniles, 68–101 mm SL (WAM P.33274–024, Fig. 4 B), 110 mm SL (WAM P.32170–020) and 99–142 mm SL (S.16708-007, Fig. 4 A) with pale longitudinal stripes of variable width, strongly contrasting on dark brown background; stripes gradually splitting to form greater number of narrower stripes with growth; five in 68 mm SL specimen, six in 101 mm SL specimen, eight in 110 mm SL specimen and nine in 142 mm SL specimen. Caudal fin with converging pale diagonal stripes, increasing in number with growth.

Larger juvenile paratypes, 169 mm SL (WAM P.33288-006) and 182 mm SL (NTM S.11263-025), pale grey with numerous poorly-contrasting narrow pale stripes along sides of body and curved pale stripe on preorbital and suborbital, all vague and faded with preservation. Caudal fin in both specimens covered with numerous dusky spots of about one third diameter of pupil and with narrow dusky margin. Soft dorsal, anal and caudal fins with membranes contrasting slightly darker than rays. Subadult paratype, 291 mm SL (CSIRO CA.1444), lacking lateral stripes, but retaining obvious spots on caudal fin. All larger paratypes lacking stripes along body, spotting on caudal fin, or other juvenile markings.

Genetics. The phylogenetic analyses indicate that P. caeruleonothus differs genetically from its closest sampled congener P. albovittatus by only 2.28%, but from all other Plectorhinchus species in this study by an average of 13.19% (Table 2). Interestingly, P. schotaf, the species that is the most physically similar to P. caeruleonothus (see discussion below) is also the most genetically divergent (by 16.02% from form A and 15.89% from form B, Table 2, Fig. 1).

Etymology. From the Latin caeruleo for blue and nothus for bastard. ‘Blue Bastard’ has been the name commonly applied to this species by anglers for many years, so given for its blue sheen in life and difficulty to hook and land on artificial fly.

Distribution and abundance. Found on sand, rubble and reef bottom in western and far northern Australia, between Ningaloo Reef, Western Australia and Lizard Island, Queensland, mostly in the intertidal zone, or shallow reef, but one specimen also trawled in a depth of 30 m (Fig. 14).

The most southerly record on the east coast of Australia is a large individual estimated to be 1 m in total length, identified from an underwater photograph taken by J. Kerry in 2013 in Lizard Island lagoon (ca 14°41’S 145°27’E), at a depth of about 6 m. This individual and two others observed over a three month period were noted by the photographer to be solitary and actively foraging in the open sandy area of the lagoon diurnally. The species appears to be absent from the central and southern sections of the Great Barrier Reef.

Dorsal-fin spines Dorsal-fin rays Pectoral-fin rays

XI XII XIII 17 18 19 20 21 22 16 17 18 P. albovittatus - 1 15 - 14 2 - - - 14 2 P. caeruleonothus - 22* - - 11* 10 1 - 2 20 * - P. polytaenia - 16 8 - - - 3 10 11 2 20 1 P. schotaf 1 24* - - 4 6 14 * 1 17 6 1 P. unicolor - 24* 1 3 6 12 4* - 2 21* 2 Upper gill rakers Lower gill rakers

7 8 9 10 11 12 13 14 16 17 18 19 20 21 P. albovittatus - 6 11 - - - - - - - - 1 6 10 P. caeruleonothus 7* 9 5 - - - - - - - 3 11* 7 - P. polytaenia 4 9 10 1 - - - - - - 2 10 10 2 P. schotaf - - - 2 8 11 3 1 7 17 1 - - - P. unicolor - - - 2 9* 11 - - 1 10* 8 3 - - Total gill rakers

25 26 27 28 29 30 31 P. albovittatus - - - 4 4 8 - P. caeruleonothus 1 8* 3 7 3 - - P. polytaenia - 3 8 5 6 2 - P. schotaf - 1 4 6 11 2 1 P. unicolor - - 2 5* 8 5 2 Lateral-line scales

54 55 56 57 58 59 60 61 P. caeruleonothus - - 1 2 7 5 5* 2 P. schotaf 9* 7 6 2 1 - - - P. unicolor 3 11* 8 1 - - - - Behaviour. Agonistic behavioural interactions have been recorded between individuals of the new species. When approached by conspecifics on open sandy or rubbly flats, individuals have often been observed to interact highly aggressively, engaging in one-on-one conflicts (B. Bright pers. comm., 2014). On noticing a rival in close proximity, the two have been recorded (using video and still images) as coming together near the surface, locking jaws, and engaging in prolonged and violent struggles (Fig. 5). At the end of the struggle, one or both individuals make a hasty retreat, disappearing from view. Observations at Weipa (B. Bright pers. comm.) and Lizard Island (J. Kerry pers. comm.), indicate that large individuals are most often solitary, foraging diurnally over relatively open expanses of soft substrate. Reasons for the intraspecific agonism are uncertain, however it is surmised that individuals recognise other conspecifics as rivals for food, territory, or mating rights, and thereby mount an aggressive response to deter them. This behaviour appears to be unique among species of Plectorhinchus, most of which are relatively gregarious and peaceful among their own species. However, similar but less exaggerated behaviour has been reported in the family among at least three species of Haemulon in tropical western Atlantic region (eg. Böhlke & Chaplin, 1968; McFarland & Hillis, 1982; Mochek & Silva, 1975). Among Haemulon flavolineatum, H. plumieri and H. sciurus, aggressive responses were generally deemed by the above authors to be for defence of territory. The behaviours involved menacing rushes toward other fish, wide opening of the mouth to exhibit the bright red colour of the oral cavity, flaring of the fins, and sometimes a brief coming together of the jaws. Specimens involved in these disputes that were collected and dissected by Mochek & Silva were found to be females. In contrast to Plectorhinchus caeruleonothus sp. nov., the Haemulon species tend to form small to large aggregations, at least during daylight hours.

of similar sized specimens of Plectorhinchus caeruleonothus sp. nov., P. unicolor, P. schotaf and P. sordidus. Discussion. Plectorhinchus caeruleonothus (Fig. 2–4) is most physically similar to P. schotaf (Fig. 10–11), sharing similar morphology, meristic formulae, and colouration in adults, including a mostly uniform silver-grey head and body with yellowish highlights to the orbital margin and dorsal edge of the maxilla. It differs from that species most obviously in having the colouration of the margin of the opercular membrane and skin covering the underlying cleithrum consistent with that of the adjacent head and body (opercular margin and skin covering the underlying cleithrum crimson red when fresh, contrasting strongly against silver-grey colouration of adjacent head and body in P. schotaf). In addition, the rear base of the pectoral fin is dusky orange-yellow from the axil when fresh, usually with an irregular dark brown blotch in the upper half (red, with no brown blotch in P. schotaf). However, colouration of juveniles is strikingly different between the two species. Juveniles of P. caeruleonothus have strongly contrasting pale stripes on a dark background along the head and body (Fig. 3 B–C, 4A–C). The pale stripes become narrower and more numerous with growth, until they fade completely by about 250 mm SL (Fig. 3 A). The caudal fin has alternating pale and dark diagonal stripes, which gradually break up into dark spots and disappear by about 300 mm SL (Fig. 3–4). In contrast, small juveniles of P. schotaf have about six narrow pale blue lines along the head and body on a yellowish background (Fig. 11; see also Randall, 1995, Fig. 538; specimen BPBM 30410, 45 mm SL, labelled as P. s o rd i d us). There are no distinctive markings on the caudal fin at any life stage in P. s c ho t af and the lines on the head and body fade and disappear much earlier, by about 120 mm SL. Although meristics for both species mostly overlap, there are numerous modal differences in counts: soft dorsal-fin rays (18–20, rarely 20, versus 18–21, modally 20 in P. schotaf), pectoral fin-rays (16–17, modally 17, versus 16– 18, modally 16 in P. s c ho t a f), gill rakers (7–9, modally 8 upper rakers, 18–20, modally 19 lower rakers, 25–29, modally 26 total rakers, versus 10–14, modally 12 upper rakers, 16–18, modally 17 lower rakers, 26–31, modally 29 total rakers in P. schotaf), lateral-line scales (56–61, modally 59, versus 54–58, modally 55 in P. s c ho t a f) and transverse scale rows above the lateral line (15, versus 11 in P. schotaf) (Table 4–5). Differences also exist in various proportional measurements at a given length, with the P. caeruleonothus having a wider preorbital depth, longer upper jaw, smaller nostrils, wider distance from the posterior nostril to eye, longer caudal peduncle, longer first and second dorsal-fin spines, higher soft dorsal fin, and longer caudal and pelvic fins (Table 5).

Although P. caeruleonothus is most closely aligned and genetically similar to P. albovittatus based on barcoding results (Fig. 1, Table 2), the latter is readily distinguished morphologically, by colouration and meristic data. Adults of P. albovittatus are dark grey with numerous small pale spots and short irregular lines on the head and body; the soft dorsal-fin and lobes of the caudal fin have large black areas; and the anal, pectoral and pelvic fins are black (no pale spots or irregular lines over the head and body, and no prominent black areas on any fins in P. caeruleonothus). In common with P. caeruleonothus, small juveniles of P. albovittatus have three narrow pale stripes alternating with two much broader dark brown to black stripes along the body, however the lower pale stripe fades or terminates anteriorly at the posterior margin of the eye (narrow pale stripe continues across the suborbital and curves ventrally to the maxilla on the preorbital region in P. caeruleonothus, see Fig. 4 C). In larger juveniles and subadults of P. albovittatus longitudinal stripes remain few and broad (versus stripes gradually splitting with growth to form multiple narrower stripes; up to 10 pale stripes forming in subadults of P. caeruleonothus before disappearing in the adult stage). The number of dorsal-fin spines and total gill rakers are also usually greater in P. albovittatus (XII–XIII, rarely XII and 28–30, median 29, versus XII and 25–29, median 27 respectively in P. caeruleonothus, Table 4).

Plectorhinchus caeruleonothus may be distinguished from the sympatric P. unicolor by fresh colouration (Fig. 2 B, 3A–C, versus 6B, 7A–B, 8A–B, 9A–B), as well as various counts and proportional measurements. The head and body of adults is generally silver-grey to blue-grey in life and slate-grey on death, versus bronze, olive-brown, or tan in life and grey-brown on death in P. unicolor. Colouration of the posterior margin of the opercular membrane and underlying cleithrum is non-contrasting against the adjacent head and body, versus opercular margin dusky and the underlying cleithrum red in P. unicolor. In addition, the caudal fin of adults in life is uniformly grey (slightly darker than that of the upper body), versus dull yellowish, or greenish brown in adults and bright yellow in juveniles of P. unicolor. Juveniles have a distinctive pattern of pale longitudinal stripes on a dark background along the head and body, and alternating pale and dark oblique stripes on the caudal fin, gradually transforming into spots in subadults (Fig. 3 A–C, 4A–C), versus juveniles with head, body and caudal fin uniformly coloured, without stripes, spots, or other distinctive markings in P. unicolor (Fig. 7 A–C). Plectorhinchus caeruleonothus has a lower upper gill raker count (7–9, versus 10–12 in P. unicolor), a lower total gill raker count (25–29, modally 26, versus 27–31, modally 29 in P. unicolor), a higher lateral-line scale count (56–61, modally 59, versus 54–57, modally 55 in P. unicolor), a higher transverse scale count above the lateral line (15, versus 12–13 in P. unicolor) and a higher circumpeduncular scale count (33–35, versus 31–32 in P. unicolor) (Tables 3–5). It may also be separated by proportional measurements, including smaller nostrils, wider distance from the posterior nostril to eye and a longer caudal peduncle, as well as a longer snout and wider preorbital depth in specimens up to 500 mm SL (Table 3).

Among other species of Plectorhinchus with a uniform body colouration, P. caeruleonothus can readily be distinguished from P. ceylonensis and P. gibbosus by dorsal-fin spine and ray counts (XII, 18–20, versus XIV, 19 in P. ceylonensis and XIII–XIV (rarely XIII), 15–17 in P. gibbosus); from P. chubbi by dorsal-fin spine and ray and gill raker counts (XII, 18–20 and 7–9 + 18–20, versus XI–XII (rarely XII), 16–17 and 13–16 + 22–24 in P. chubbi); from P. g r i s e u s by dorsal-fin ray and upper gill raker counts, lateral profile of the forehead, and body depth (XII, 18–20 and 7–9 + 18–20, lateral profile of snout without distinct concavity, depth 2.5–2.9 in SL, versus XII, 21–23 and 11–12 + 17–18, lateral profile of snout with distinct concavity in adults (Fig. 12 B), depth 2.2–2.4 in SL in P. griseus); and from P. sordi dus (fig. 13a–b) by gill raker counts (7–9 + 18–20, versus 9–11 + 15–16 in P. sordidus).

Although adults of P. caeruleonothus can easily be distinguished from P. polytaenia by colouration, most meristic values overlap and juveniles of both species up to about 150 mm SL are marked with a similar pattern of pale longitudinal stripes. Hence juveniles have frequently been confused in museum collections. The juveniles are separated most readily by dorsal-fin ray counts (XII, 18–20 (rarely 20), versus XII–XIII, 20–22 in P. polytaenia) and by the alignment of the pale lines on the suborbital and interorbital regions of the head. In P. caeruleonothus lines on the suborbital are crescentic, strongly curving up from the preopercular margin toward the eye, then down on the preorbital to the maxilla (not meeting across the snout with those from the opposite side), and the lines from the nape pass forward straight through the interorbital region to terminate on or near the snout tip (Fig. 3 C, 4A-C). In contrast, in P. polytaenia the longitudinal lines on the suborbital and preorbital regions are straight to slightly wavy, and meet across the snout with those from the opposite side (Fig. 12A). The lines extending forward from the nape all converge just posterior to or within the interorbital space, and all markings on dorsal part of snout are transverse across the snout.