Plectranthias nanus Randall

Gill, Anthony C., Pogonoski, John J., Moore, Glenn I. & Johnson, Jeffrey W., 2021, Review of Australian species of Plectranthias Bleeker and Selenanthias Tanaka (Teleostei: Serranidae: Anthiadinae), with descriptions of four new species, Zootaxa 4918 (1), pp. 1-116 : 95-96

Plectranthias nanus Randall

Figures 3B , 11 , 39 ; Tables 1–16, 24

Common name: Dwarf Perchlet

Plectranthias nanus Randall 1980: 159 , fig. 22, tabs 13–14 (type locality: Cocos Island, Guam, Mariana Islands; paratypes from Cocos-Keeling and Christmas Islands).— Allen & Steene 1988: 180, fig. 134 (checklist, photo, Christmas Island).— Paxton et al. 1989: 507 (checklist).— Allen & Smith-Vaniz 1994: 9 (checklist, Cocos (Keeling) Islands); Allen 2000: 87 (checklist, Christmas Island); Allen et al. 2006: 991 (checklist).— Allen & Erdmann 2012: 289 (diagnosis, photo, Christmas Island).— Hobbs et al. 2014a: 194, tab. 1 (checklist, Cocos (Keeling) Islands and Christmas Island).— Hobbs et al. 2014b: 210, tab. 1 (checklist, Cocos (Keeling) Islands and Christmas Island).— Anderson 2018: 31 (checklist). — Parenti & Randall 2020: 28 (checklist).

Diagnosis. The following combination of characters distinguishes P. nanus from all other congeners: dorsal rays X,13–16; fourth dorsal spine longest; pectoral rays 13–15, rarely 13, all rays unbranched; lateral line incomplete, with 15–21 tubed scales; greatest body depth 29.4–33.3 % SL.

Remarks. A relatively small species (maximum size 39.8 mm SL, Randall 1980), P. nanus is known in Australia from the Cocos (Keeling) Islands, Christmas Island, Great Barrier Reef and Coral Sea ( Figure 11 ). Elsewhere, it ranges widely throughout the Indo-West Pacific, as far west as the Red Sea and as far east as the Pitcairn Islands and Hawaii ( Randall 1980; Heemstra 1996; Heemstra & Randall 2009). It closely resembles P. longimanus in coloration, and shares with it apomorphic modification of the caudal skeleton (hypurals 1 and 2 undifferentiated, versus autogenous in other studied Plectranthias species; Figure 5 ). Characters distinguishing the two species are discussed under Remarks for P. longimanus .

In their table 1 and key for western Indian Ocean Plectranthias, Heemstra & Randall (2009) gave a count of 15 branched caudal-fin rays for this species. In their account for the species, however, they gave a range of 13–15 branched caudal rays. The latter count range agrees with the original description: “13 to 15, usually 13” ( Randall 1980: 162). All specimens examined by us had 13 (7+6) branched caudal rays.

We add the following new observations: scales with peripheral cteni; vertebrae 10+16; supraneurals 2; predorsal formula 0/0/2/1+1; dorsal pterygiophores in interneural spaces 9–13 1/1/1+1/1/1+1 or 1/1/1+1/1+1/1; no trisegmental pterygiophores associated with dorsal fin; terminal dorsal pterygiophore in interneural space 17–18; no trisegmental pterygiophores associated with anal fin; terminal dorsal pterygiophore in interneural space 5–6; ribs present on vertebrae 3 through 10; epineurals present on vertebrae 1 through 10–12; hypurals 1 and 2 represented by undifferentiated plate, other hypurals and parhypural autogenous; well-developed hypurapophysis on parhypural; epurals 3; single uroneural (posterior uroneural absent); ventral tip of cleithrum with well-developed posteroventral process; proximal tip of first anal-fin pterygiophore near distal tips of parapophyses on vertebra 10.

Morphometric data are summarised in Table 24.

Material examined. Australia. AMS I.19454-088, 2: 23–24 mm SL (radiographs only), Queensland, Great Barrier Reef, channel between Yonge and Carter Reefs , 14°35′S, 145°36′E, 1–18 m , AMS team, 11 Nov 1975 ; AMS I.20430-005, 3: 15.6–22.1 mm SL, Christmas Island, Flying Fish Cove , 5–10 m, G . R. Allen & R.C. Steene , 12 Jun 1978 ; AMS I.20442-005, 25.8 mm SL, Christmas Island, Rocky Point , 10–12 m, G . R. Allen & R. Steene , 29 May 1978 ; AMS I.25112-116, 3: 20.4–29.0 mm SL (subsequently cleared and stained), Coral Sea, west edge of Osprey Reef , 0.75 km north of pass, 13°54′S, 146°33′E, 1–15 m, rotenone , AMS party, 8 Nov 1984 ; WAM P.28541-039, 7: 16.4–25.1 mm SL, Coral Sea, Holmes Reef , 16°31′S, 147°50′E, 12–14 m, G . R. Allen , 20 Nov 1985 ; WAM P.29637- 027, 23.9 mm SL, Coral Sea, Lihou Reef and Cays , Georgina Cay , 17°37′S, 151°26′E, 18–29 m, G . R. Allen et al., 12 Nov 1987 . French Polynesia. AMS I.21646-036, 3: 23.0– 27.5 mm SL (radiographs only), Society Islands, Moorea, near Irioa Island , 17°S, 149°W, 15–20 m, B. Goldman, 18 Jun 1976 .

Additional specimens identified for distribution information. Australia. AMS I.22616-035, 26.0 mm SL, Great Barrier Reef , Escape Reef North, 15°49′S, 145°50′E, coral and sand, 5–18 m , AMS team, 2 Nov 1981 ; AMS I.22634-006, 31.0 mm SL, Great Barrier Reef , Escape Reef North, 15°49′S, 145°50′E, reef dropoff, 18 m, W.A. Starck, 4 Dec 1981 ; AMS I.25107-032, 2: 18.0– 23.5 mm SL, Coral Sea , Osprey Reef, west edge dropoff (13°56′S, 146°34′E), 10–25 m , AMS team, 6 Nov 1984 ; AMS I.28993-016, 5: 17.3–27.5 mm SL, Cocos (Keeling) Islands , west side near Wreck Point, 11°19′S, 96°49′E, G . R. Allen , 23 Fed 1989 ; AMS I.33747-023, 22.5 mm SL, Coral Sea , Boot Reef, 09°58.70′S, 144°42.52′E, coral, sand and rubble, 23–30 m , AMS team, 27 Jan 1993 .

Figures

FIGURE 3. Lower part of left cheek in ventrolateral view showing serrations on interopercle (IOP), preopercle (POP) and subopercle (SOP) in: A) Plectranthias longimanus, WAM P.28534-021, 22.8 mm SL (note that mouth is open); B) P. nanus, WAM P.28541-039, 24.3 mm SL. Dotted line shows junction between interopercle and subopercle; arrows indicate antrorse serrations on preopercle; scale bars = 2 mm. Illustrations by A.C. Gill.

FIGURE 5. Caudal skeletons of A) Plectranthias inermis, AMS I.45300-250, 28.0 mm SL, showing autogenous hypurals 1 and 2; B) P. longimanus, AMS I.22613-052, 20.3 mm SL, showing undifferentiated hypurals 1 and 2. Abbreviations: EP1–3, epurals 1–3; HSP, haemal spines; HYP1–5, hypurals 1 through 5; HYPAP, hypurapophysis; NSP, neural spines; PARHYP, parhypural; PU2–3, preural centrum 2 and 3; UR, uroneural. Cartilage shown in dark grey; arrows indicate outer principal caudal rays; scale bars = 1 mm. Illustrations by A.C. Gill.

FIGURE 11. Distribution records for Australian specimens of Plectranthias azumanus (triangle), P. melanesius (open circles) and P. nanus (closed circles).

FIGURE 39. Plectranthias nanus, specimens from Holmes Reef, Coral Sea, Australia: A) photo of aquarium specimen (not retained); B) WAM P.28541-039, 25.1 mm SL, preserved. Photos by Y.K. Tea (A) and A.C. Gill (B).