Naso tergus, Ho & Shen & Chang, 2011

Ho, Hsuan-Ching, Shen, Kang-Ning & Chang, Chih-Wei, 2011, A New Species Of The Unicornfish Genus Naso (Teleostei: Acanthuridae) From Taiwan, With Comments On Its Phylogenetic Relationship, Raffles Bulletin of Zoology 59 (2), pp. 205-211 : 206-210

Naso tergus , new species

( Figs. 1–4 ; Table 1)

Naso thynnoides (non-Cuvier, 1829): Chen et al., 2010:542 (in part, the figured specimen is misidentification of present species).

Material examined. – Holotype: NMMB-P10808 (adult male, 335 mm SL), off Nanfangao, Ilan, NE Taiwan, NW Pacific Ocean, 70–80 m, bottom gill net, 15 Dec.2010.

Paratypes: NMMB-P10809 (1, adult female, 325) ; NMMB-P10810 (1, fully ripe female, 320) ; NMMB-P10811 (1, adult male, 347) ; NMMB-P10812 (1, adult female, 321) ; NMMB-P10813 (1, adult female, 320), otolith and gut taken ; NMMB-P10814 (1, adult female, 303), otolith and gut taken ; NMMB-P10815 (1, adult female, 306), otolith and gut taken ; NMMB-P10816 (1, adult male, 342), otolith and gut taken ; NMMB-P10817 (1, adult female, 302), otolith and gut taken ; QM I.38837 (1, adult male, 315); all collected together with the holotype . NMMB-P2373 (1, adult male, 280), Houbihu fish market, Pingtung, S. Taiwan, N. South China Sea, 6 Feb.2002 . NMMB-P10480 (1, adult female, 275), Houbihu fish market, Pingtung, S. Taiwan, N. South China Sea, 1 Dec.2002 .

Diagnosis. – A species of Naso differs from congeners in having VI dorsal-fin spines; an acute head, without rostral projection or tuberosity in adults; two bony plates of caudal peduncle; a relatively large eye (6.3–7.2% SL); a median body depth, 2.8–2.9 in SL; 100–120 teeth on upper jaw and 90–110 on lower jaw; a uniformly brownish body coloration; a relatively small adult size which might not exceed 350 mm SL; and a different genome structure.

Description. – The following proportions are given for the holotype, followed by range of all types in parentheses, if different from the holotype. Dorsal-fin rays VI, 28 (VI, 26–30; mainly VI, 28); anal-fin rays II, 27 (II, 26–28; mainly II, 27); pectoral-fin rays 16 (1 unbranched+14 branched and 2 unbranched); pelvic-fin rays I, 5; gill rakers 4+13=17 (4+11– 13=15–17). Head length 3.9 (3.8–4.1) in SL; body depth 2.9 (2.7–2.9); body width 6.8 (6.6–7.5); predorsal length 3.6 (3.5–4.0); prepectoral length 4.0 (3.6–4.3); prepelvic length 3.4 (3.1–3.4); preanal length 2.6 (2.4–2.6). Snout length 1.9 (1.8-1.9) in HL; orbit diameter 2.7 (2.6–2.8); caudal peduncle length 3.0 (2.6–3.1); caudal peduncle depth 4.8 (4.8–5.6); caudal peduncle width 3.8 (3.8–4.6); upper jaw length 4.8 (4.5–5.2); first dorsal-fin spine length 2.7 (2.0–4.7); sixth dorsal-fin spine length 2.9 (2.7–3.0); pectoral-fin length 1.8 (1.7–1.8); pelvic-fin spine length 2.9 (2.4–3.9); first anal-fin spine 4.3 (3.3–6.8); 2 nd anal-fin spine 3.6 (3.2–4.5); caudalfin length 1.0 (0.9–1.1).

Two elliptical bony plates on each side of caudal peduncle, the anterior plate usually larger than the posterior one and about equal size in adult males; a keel-like spine projecting laterally in adults, more in males than females; shape of peduncular spines semi-circular and slightly pointed with the tip projecting forward in adult males ( Fig. 2).

Body moderately elongated and compressed, the lateral profile spindle-like, deepest at midpoint of body; head relatively large, dorsal profile gradually sloping with a small convexity usually present at about level of eye; interorbital arched well above eyes; mouth relatively small, upper jaw slightly projecting; gape horizontal to slightly dorsally inclined; teeth uniserial, slender, lanceolate, tips slightly curved posteriorly, 100–120 in upper jaw, 90–110 in lower jaw (usually 10 more in upper than lower), the numbers do not vary relative to size among the type series; lips narrow; tongue broadly rounded.

A deep oblique groove on snout from in front of middle of eye to nearly two-thirds distance to front of mouth; nostrils very small, just above deep groove and anterior to middle of eye by a distance about one-third orbital diameter; anterior nostril with a low membranous rim and a small triangular

posterior flap which barely extends posteriorly; posterior nostril rounded in shape.

Scales very small, close-set, the margins not apparent without magnification, each with an elevated dense patch of posteriorly directed spinules (thus the texture is smooth when stroked posteriorly, but finely abrasive in the forward direction); body and head completely scaled except for lips, opercular membrane, and a narrow zone around anterior nostril; membranes of fins scaleless; spines, and to a lesser extent soft rays, scaled laterally (pelvic and first dorsal and anal spines scaled anteriorly as well). Lateral line on upper side of body approximately following contour of back; no slit-like pores above lateral line (as in N. caesius ) can be observed.

Origin of dorsal fin well posterior to a vertical at upper end of gill opening, as well as posterior margin of operculum; dorsal and anal spines slender but transversally broad basally, especially the first in each fin; all dorsal spines about equal in length; first dorsal-fin ray slightly longer than sixth dorsal-fin spine; first two rays unbranched; successive rays progressively slightly longer to about fourteenth ray; the last ray shortest and forked to base. Origin of anal fin at a vertical from base of fourth dorsal-fin spine; first anal-fin spine usually shorter than the second spine; lengths of anal-fin rays almost similar to those of dorsal fin; caudal fin emarginated (apparently not truncate even in adults); origin of pectoral fin at a vertical of posterior margin from operculum; pectoral fin short, first two and last rays unbranched, the third to fifth rays longest; origin of pelvic fin opposites that of dorsal fin, the appressed fin not reaching the origin of anal fin; pelvic fin short, the first soft ray longest, about equal to the spine in length. Anus situated above second third of appressed pelvic fins.

Coloration. – When fresh, uniformly light brownish or greenish above and pale below. When preserved, uniformly light gray-brownish, paler ventrally, with no obvious dark or light markings on head or body; caudal fin uniformly brown; pectoral fin light brown, the membranes pale distally; membranes of dorsal and anal fins blackish, without any markings; pelvic fin pale; tongue and gill rakers creamy white.

Distribution. – Known from the type series, collected from off northeastern and southern Taiwan by bottom gill-net and hook and line at depths between 70 and 80 meters.

Size. – The mean body size of males is slightly larger than that of females. All four males have large, white, band-like testes and the largest specimen is 347 mm SL. All females have large ovaries with eggs in or approaching the final stage, the largest specimen is 325 mm SL.

Otolith. – Sagittal otolith oblong-ovate with moderate thickness, convex in mesial and slightly concave in lateral faces. Dorsal margin, slightly raised with irregular sculpture; ventral margin, slightly rounded and crenate; posterior margin, flattened to rounded and irregular. Rostrum, elongate, moderately broad and sharply rounded; antirostrum, small and pointed; excisura, wide, notch very deep or shallow with groove, angle acute. Sulcus acusticus, ostial and heterosulcoid; ostium, very elongate and narrow, lying mainly on rostrum, deepest in middle; cauda, slightly sinuate, strongly flexed, very deep posteriorly undercutting ventral area; ostium/cauda ratio, 1/0.60–0.64 ( Fig. 3 ).

Etymology. – The species name, tergus , meaning ‘to hide’, refers to the typical appearance of this species that makes it resemble subadults of many other Naso species.

Feeding habit. – Benthic crustaceans and bivalves in the stomach and intestine of N. tegurs indicate that this fish feeds on marine organisms living on the sea floor.

Comparison. – Naso tergus is morphologically similar to N. caesius but differs in having a relatively short caudal peduncle (7.8–9.9%, vs. 13.6–14.2% SL in N. caesius ); a relatively large eye (6.3–7.2% vs. 5.0–5.6% SL); a relatively wide interorbital distance (8.9–10.0% vs. 7.9–8.6% SL); caudal fin emarginated in adults (vs. truncate); a relatively large head (24.6–26.6% vs. 23.8–24.3% SL); a relatively shallow body depth (34.4–36.8% vs. 36.2–39.0% SL); a relatively wide body (13.3–15.2% vs. 12.1–13.2% SL); a relatively long prepelvic distance (29.3–32.2% vs. 26.9–27.7% SL); a relatively long preanal distance (38.6–41.3% vs. 34.7–35.9% SL); modally 16 pectoral-fin rays (vs. 17); and a relatively small adult body size (up to 347 mm SL, vs. up to 625 mm SL in Randall, 2002). Randall & Bell (1992) mentioned there was a curious series of about 20 vertical slit-like pores in a row a few mm above and parallel to the lateral line, however, it is not observed in N. tergus .

Naso tergus is also similar to N. hexacanthus (data of 2 specimens is provided in Table 1) but differs in having a uniformly light brown colour (vs. irregular lines on dorsal and anal fins, yellowish ventrally, black borders on opercle and subopercle and a white lower lip in N. hexacanthus ); modally 16 pectoral fin rays (vs. 17); a relatively large eye (6.3–7.2% SL vs. 5.3–5.4% SL); modally 12 gill rakers on lower limb of first gill arch (vs. 8–10); body depth 2.8–2.9 in SL (vs. 2.5–2.6); relatively small prepectoral, prepelvic and preanal length; relatively long dorsal-fin spines; and a pale tongue and gill rakers (vs. both blackish).

Moreover, the presence of two caudal plates separates N. tergus from N. caeruleacauda Randall, 1994 , N. minor ( Smith, 1966) and N. thynnoides , which have only one plate; the absence of a rostral protuberance in adults separates it from N. annulatus (Quoy & Gaimard, 1825) , N. brachycentron (Valenciennes in Cuvier & Valenciennes, 1835), N. brevirostris (Valenciennes in Cuvier & Valenciennes, 1835) and N. unicornis (Forsskål, 1775) ; the absence of markings or a distinctive colour pattern on body or fins separates it from N. brevirostris , N. elegans (Rüppell, 1829) , N. lituratus ( Forster, 1801) , N. lopezi Herre, 1927 , N. maculatus Randall & Struhsaker, 1981 , N. reticulatus Randall, 2001 and N. vlamingii (Valenciennes in Cuvier & Valenciennes, 1835); the relatively shallow body separates it from N. brachycentron , N. elegans , N. lituratus , N. tuberosus Lacepède, 1801 , N. unicornis and N. vlamingii ; the absence of a hump on the back separates it from N. brachycentron , N. tonganus (Valenciennes in Cuvier & Valenciennes, 1835) and N. tuberosus ; the absence of filamentous caudal fin rays separates it from N. elegans , N. fageni Morrow 1954 , N. unicornis , and N. vlamingii ; the absence of a tuberosity on the snout separates it from N. fageni , N. mcdadei Johnson, 2002 , N. tonganus and N. tuberosus ; and the relatively small adult body size separates it from most congeners, except for N. minor , N. thynnoides and N. brevirostris . Our new species is also different from two possible undescribed species mentioned in Chen et al. (2010) in lacking black spots on the sides of the body.

Phylogenetic reconstruction. – Three DNA genes of four N. tergus specimens were analysed and compared with the molecular data of same sequences provided by Klanten et al. (2004). The new species is well separated from other species of Naso by the molecular data ( Fig. 4 ). According to the maximum likelihood phylogenetic tree, N. tergus is in the N. maculatus sub-clade (sensu Klanten et al., 2004: 228) but with a very low bootstrap support. Maximum likelihood phylogenetic tree also has low boostrap support among N. brevirostris , N. elegans and N. maculatus sub-clades and even between N. annulatus and other sub-clades ( Fig. 4a ). Bayesian posterior probabilities, however, have better support for all major sub-clades and suggest the new species could be regarded as the fifth sub-clade for Naso genus based on the same clade classification criteria ( Klanten et al., 2004).

Two issues arise from the phylogenetic reconstruction reported in this study. Firstly the low bootstrap support for many of the clades suggests the history of this genus was characterized by episodes of rapid evolutionary divergence. Secondly distinct morphologies associated with foraging and feeding can arise independently in each of the clades. This is confirmed by the similarity between N. caesius / N. hexacanthus and N. tergus , all of which share the streamlined scombiform morphology but occur in distinct clades.

A number of distinctive morphological and structural features arise independently in each of the main clades. Our result on the species relationships within Naso differs from that proposed by Borden (1998) on the basis of morphology and demonstrates a conflict by using molecular and morphological characters for the identification of evolutionary groupings in this genus.

Figures

Fig. 1. Naso tergus, new species: a, Holotype, NMMB-P10808, adult male, 335 mm SL; b, Paratype, NMMB-P10813, adult female, 320 mm SL.

Fig.3. Scanning microscopy image of the mesial face of right sagittal otolith taken from Naso tergus, NMMB-P10816, adult male, 342 mm SL. Scale bar = 1 mm.

Fig. 4. Phylogenetic trees of Naso species using combined DNA data (ETS2, 16S and Cyt b): a, Topology by ML analysis and bootstrap values at the branches; b, Topology by Bayesian analyses and posterior probabilities at the branches. Numbers refer to the sub-clades: 1 for the N. annulatus sub-clade, 2 for the N. brevirostris sub-clade, 3 for the N. elegans sub-clade, 4 for the N. maculatus sub-clade, and 5 for the N. tergus sub-clade. Black and open squares indicate the foraging modes.