Microcanthus Swainson, 1839

Tea, Yi-Kai & Gill, Anthony C., 2020, Systematic reappraisal of the anti-equatorial fish genus Microcanthus Swainson (Teleostei: Microcanthidae), with redescription and resurrection of Microcanthus joyceae Whitley, Zootaxa 4802 (1), pp. 41-60 : 45-47

Microcanthus Swainson

Microcanthus Swainson 1839: 170 , 215 (as a subgenus of Chaetodon Linnaeus ; type species Chaetodon strigatus Cuvier 1831 ,

by monotypy). Therapaina Kaup 1860: 140 (type species Chaetodon strigatus Cuvier 1831 , by monotypy).? Helotosoma Kaup 1863: 162 (type species Helotosoma servus Kaup 1863 , by monotypy). Neochaetodon Castelnau 1873: 130 (type species Neochaetodon vittatus Castelnau 1873 , by subsequent designation of Jordan

1919: 368).

Diagnosis. Microcanthus is readily distinguished from all other microcanthid genera in having the following combination of characters: dorsal fin with XI spines and 15–17 (usually 16) segmented rays; anal fin with III spines and 13–15 (usually 14) segmented rays; pectoral fin with 15–17 (usually 16) rays; scales ctenoid, with ctenial bases present; lateral line scales partially or heavily obscured by adjacent scales; and body pale in preservation with five horizontal dark stripes reaching the posterior edges of dorsal and anal fins, and base of caudal fin.

Description. Dorsal-fin rays XI,15–17, all segmented rays branched except anteriormost; soft dorsal fin extensively covered in scales, scales reaching almost to distal edge of fin; anal-fin rays III,13–15, all segmented rays branched; basal portion of dorsal and anal fins with scale sheaths; pectoral-fin rays 14–17, all rays branched except for uppermost; dorsal and anal fin spines stiff and pungent; second anal-fin spine blade-like; base of pectoral fin covered in numerous small scales, scales reaching just beyond base of fin rays; inner pelvic-fin ray not attached to body by membrane; pelvic-fin rays I,5, all segmented rays branched; upper procurrent caudal-fin rays 8–9; lower procurrent caudal-fin rays 7–9; principal caudal-fin rays 9 + 8 (8 + 7 branched); total caudal-fin rays 32–35; lateral line complete; tubed scales irregularly obscured by overlapping scales; scales ctenoid with ctenial bases; diagonal rows of scales in lateral series 48–59; circumpeduncular scales 26–30; gill rakers 16–18; branchiostegals 7.

Vertebrae 10 + 15; supraneurals 3; predorsal formula 0/0+0/2/1+1; trisegmental pterygiophores associated with segmented rays of the dorsal (except the anteriormost 3–5 bisegmental) and anal fins (except the anteriormost 3–4 bisegmental); terminal rays in dorsal and anal fins with well-developed stays; ribs present on vertebrae 3 through 10; epineurals present on vertebrae 1 through 10–12 (usually 11); parhypural and hypurals 1–5 autogenous; welldeveloped hypurapophysis on parhypural; epurals 3, anteriormost largest; two uroneurals; uppermost procurrent ray on ventral part of caudal fin with procurrent spur, the ray immediately below foreshortened ( Johnson 1975); haemal spines on preural centrum 2 and 3 autogenous ( Figure 1 ); interarcual cartilage present between uncinate process of epibranchial 1 and pharyngobranchial 2; no toothplates on epibranchial 2 or 3; well-developed suborbital shelf on third infraorbital; posttemporal and preopercle finely serrate; laterosensory canal present in supracleithrum.

Body laterally compressed, moderately tall in lateral view, dorsal-fin origin to pelvic-fin origin 51.9– 59.8% SL; head small 28.1–36.6% SL; snout acute, 7.8–11.0% SL; head profile steeply sloping, slightly concave; eye large, 11.0–14.6% SL; mouth terminal, horizontal to slightly oblique; distal portion of maxilla partly covered by lachrymal, barely reaching anterior edge of orbit; preopercle serrated; lower jaw projecting slightly; jaws with numerous rows of small, setiform teeth anteriorly; preorbital region naked; scales large, ctenoid with ctenial bases, covering the body from postorbital region of operculum and the cheek, posteriorly to base of caudal fin; interorbital region naked, 8.3–11.4% SL in width; pelvic fin free, not bound to body by a membrane, situated well behind vertical through pectoral-fin base; caudal fin emarginate.

Etymology. The generic epithet Microcanthus is a combination of the Greek “mikros” for small, and “akantha” for thorn, alluding to the minute crenulations on the preopercle ( Swainson 1839).

Remarks. The various genera of Microcanthidae are readily separated on the basis of dorsal- and anal-fin counts, scale morphology, body shape, and general coloration. Tilodon and Neatypus can be separated from Atypichthys and Microcanthus in having ten dorsal fin spines (versus 11 in Atypichthys and Microcanthus ) and higher dorsal-and anal-fin ray counts. The four genera can be separated further on the basis of colour patterns, in having either vertical or oblique bars versus horizontal or near-horizontal stripes ( Figure 2 ).

The lateral-line scales of at least Microcanthus , Atypichthys and Tilodon are heavily obscured by overlapping adjacent scales ( Figure 3 ). This appears to be an ontogenetic character, as juveniles have mostly unobscured lateralline scales, but these become increasingly obscured in larger specimens. This character is not found in Neatypus (checked for and examined in syntype of N. obliquus Waite ; AMS I.7034). Whether it is a synapomorphy supporting a relationship between Microcanthus , Atypichthys and Tilodon requires more investigation. We tentatively consider Helotosoma servus Kaup (1863) , type species of Helotosoma Kaup (1863) , as a synonym of Microcanthus strigatus . See Remarks for M. strigatus for discussion.

Figures

FIGURE 1. X-radiograph of Microcanthus strigatus, KPM-NI 24269, 73.8 mm SL, Yakushima Island, Japan. Radiograph by A. Hay.

FIGURE 2. Genera of Microcanthidae. Tilodon and Neatypus are monotypic. A) Tilodon sexfasciatus, in situ photograph from Blairgowrie, Victoria, Australia; B) Neatypus obliquus, in situ photograph from Bunbury, Western Australia; C) Microcanthus joyceae, in situ photograph from Magic Point, Maroubra, New South Wales, Australia; and D) Atypichthys strigatus, in situ photograph from Henry Head, Botany Bay, New South Wales, Australia. Photographs by S. Schulz (A), C. Mark (B), and E. Schlogl (C & D).

FIGURE 3. Lateral-line scales of: A) Scorpis lineolatus (Scorpididae), AMS I.48993-001, 55.3 mm SL; B) Microcanthus joyceae (Microcanthidae), AMS I.48994-001, 64.2 mm SL; C) Atypichthys strigatus (Microcanthidae), AMS I.48992-001, 93.9 mm SL. Drawings on right show portion of lateral line and adjacent scales. Lateral-line scales are show in dark grey; scale bars indicate 2 mm. Photographs by Y.K. Tea; drawings by A.C. Gill.