Acanthoplesiops naka, Randall D. Mooi & Anthony C. Gill, 2004

Randall D. Mooi & Anthony C. Gill, 2004, Description of a new species of the fish genus Acanthoplesiops Regan (Teleostei: Plesiopidae: Acanthoclininae) from Tonga., Zootaxa 432, pp. 1-10 : 3-8

Acanthoplesiops naka sp. n.

Tongan Spiny Basslet

Figures 1-2, Table 1

Holotype. USNM 327794 , 9.9 mm SL, Tonga, Ha’Apai Group, Ofolanga Island , 19°36'15”S 174°28'15”W, deep reef slope and wall off SW side of island, near vertical coral wall with sandy channel at base and steep dropoff at end, 70-105 ft (21-32 m) , J.T. Williams, B.B. Collette, G.D. Johnson, D.G. Smith, C.C. Baldwin, E.A. Powers, et al., rotenone and dipnet, 12 Nov 1993 ( field number JTW 93-30 ).

Diagnosis. A species of Acanthoplesiops with the following combination of characters: dorsal-fin rays XVIII,4; anterior dorsal-fin pterygiophore formula?/?/I+I/I/I/I/I//I/I/I; 3 dentary pores; 2 intertemporal pores; head and body generally brownish with darker pigment spots (from shrunken melanophores), excepting a pale stripe from the anterior half of the symphyseal flap to origin of dorsal fin continuous with a pale first dorsal spine, a pale spot on the pectoral-fin base, and a pale caudal peduncle continuous with pale bases of last 3 segmented dorsal- and anal-fin rays, median fins slightly darker than body colour with pale tips to their rays, pectoral fin hyaline with darker outlines to rays, pelvic-fin spine dark with pale tip, first pelvic-fin ray dark on proximal two-thirds and pale on distal third, second pelvic-fin ray dark except for extreme distal tip; regular ctenoid scales; no membranous attachment of last dorsal- or anal-fin rays to caudal fin.

Description. Dorsal-fin rays XVIII,4, first two segmented rays branched; number of supraneurals not discernible from x-rays, anterior dorsal-fin pterygiophore formula?/?/ I+I/I/I/I/I//I/I/I; anal-fin rays VII,4, first two segmented rays branched; pectoral-fin rays 16; pelvic-fin rays I,2, inner segmented ray unbranched; segmented caudal-fin rays 8 + 8, and 2 dorsal and 2 ventral procurrent rays. Caudal fin not connected by membrane to last ray of dorsal and anal fins. Vertebrae 13+14 = 27; ribs present to at least the 12th precaudal vertebrae; epineural bones present to at least 12th precaudal vertebrae. Individual elements of caudal skeleton, other than rays, not discernible on x-ray.

Cephalic sensory pore openings (Fig. 2; all pores bilaterally paired unless otherwise indicated). Nasal pores 2, one pore just posterior to upper lip, second pore just above posterior nostril; anterior interorbital pores 1; supraotic pores 1; posterior otic pores 1; suborbital pores 3 on left side, 4 on right; preopercular pores 6 on left side, 7 on right; dentary pores 3; intertemporal pores 2; parietal pores 1; anterior temporal pores 1; posttemporal pores 1.

Gill membranes fused together ventrally but free from isthmus; branchiostegal rays 6; gill rakers not counted. No lateral-line scales yet apparent. Olfactory capsule with two openings; anterior opening a short tube, positioned about midway between posterior opening and edge of lip; posterior opening with slightly raised rim, positioned near anterodorsal rim of orbit (Fig. 2). Opercle with secondary opercular spine ventral to the primary spine and overlapping the subopercle (Fig. 2). Head and anterior body scaleless, posterior body with ctenoid scales. Teeth numerous, conical and small.

Morphometrics (as % of SL, from x-radiograph, excepting interorbital and pectoral length; also reported as % of HL where appropriate). Head length (HL) 35; head depth at posterior margin of eye 20 (58% HL); eye diameter 9.7 (28% HL); snout length 6.8 (19% HL); bony interorbital 4.5 (13 % HL); upper jaw length 15 (43 % HL); lower jaw length 20 (58 % HL); dorsal-fin base 63; length of dorsal-fin spines, 1st 8.4, 2nd 9.9, 3rd 12.5, 4th 13, 5th 14, 6th 14.7, 16th 17.7, 17th 18, 18th 18 (although broken); length of dorsal-fin rays, 1st 20, 2nd 17, 3rd 14, 4th 8.5; anal-fin base 23; anal-fin spine lengths, 1st 9.7, 2nd 13, 3rd 15, 4th 16, 5th 17, 6th 18, 7th 19; anal-fin ray lengths, 1st 20, 2nd 19, 3rd 14.3; 4th 10.2; pectoralfin length 17; pelvic-fin spine length 18; 1st pelvic-fin ray length 39; middle caudal-fin ray length 28; snout tip to dorsal-fin origin 38; snout tip to pelvic-fin base 35; snout tip to origin of anal fin 70; body depth at anal-fin origin 21; dorsal-fin origin to pelvic-fin base 24; dorsal-fin origin to anal-fin origin 43; dorsal-fin origin to anal-fin insertion 63; pelvic-fin base to anal-fin origin 35; anal-fin origin to dorsal-fin insertion 62; dorsal-fin insertion to anal-fin insertion 14; hypural depth 11; peduncle length 9.5; peduncle depth 13; anal-fin insertion to upper hypural 13; dorsal insertion to lower hypural 13.

Live coloration. Unknown.

Preserved coloration (Fig. 1). Head and body generally brownish with darker pigment spots (from shrunken melanophores), excepting a pale stripe from the anterior half of the symphyseal flap to origin of dorsal fin continuous with a pale first dorsal spine, a pale spot on the pectoral-fin base, and a pale caudal peduncle continuous with pale bases of last 3 segmented dorsal- and anal-fin rays, median fins slightly darker than body colour with pale tips to their rays, pectoral fin hyaline with darker outlines to rays, pelvic-fin spine dark with pale tip, first pelvic-fin ray dark on proximal half and pale on distal half, second pelvic-fin ray dark except for extreme distal tip.

Habitat and distribution. The only specimen was collected from the SW side of Ofolanga Island of the Ha’Apai Group of Tonga at 19°36'15”S 174°28'15”W.

It was found between 21 and 32 m off of a deep reef slope with a near vertical coral wall having a sandy channel at its base followed by a steep dropoff.

Etymology. The epithet ‘ naka ’ is derived from the first letters of our respective children ’s names, Aaron and Adam (RDM) and Nat and Kelly (ACG). The minute size of the holotype is reflective of the still comparatively small sizes of our children.

Comparisons

Acanthoplesiops naka resembles A. indicus in having a broad pale band on the caudal peduncle and posteriormost rays of the dorsal and anal fins (Fig. 1). As a result, it was initially catalogued as A. indicus . However, A. naka is distinguished from A. indicus in having only XVIII dorsal-fin spines (vs XIX -XX), three dentary pores (vs four), two intertemporal pores (vs one; the smallest A. indicus available, SAIAB 17291 12.2 mm SL, has only one intertemporal pore on right side but might have two on the left, some damage

makes it difficult to interpret), two dorsal-fin pterygiophores inserted into the 3rd interneural space (vs one), ctenoid scales (vs modified “cycloid”, see Smith-Vaniz & Johnson, 1990: 235) and no membrane joining the last dorsal- and anal-fin rays to the caudal rays. We could not determine the number of supraneurals in A. naka , but we note that although A. indicus is reported as having no supraneurals, one of 13 specimens (SAIAB 17293) we examined had one in the first interneural space.

Acanthoplesiops naka differs from all other congeners by the broad pale band on the caudal peduncle, two intertemporal pores, and the uniquely low dorsal-fin spine count (XVIII vs XIX -XXI). A. naka also exhibits a lower number of total dorsal- and anal-fin elements, and lower total vertebral number (Table 1). A. psilogaster does share the condition of two dorsal-fin pterygiophores in the 3rd interneural space and an unscaled belly anteriorly (this latter similarity might be due to lack of scale development in the apparently juvenile holotype of A. naka ). However, even the smallest specimen of A. psilogaster (11.8 mm SL, USNM 288813) shows no trace of a pale caudal peduncle and has only one intertemporal pore on each side. We also found two specimens of A. hiatti with two dorsal-fin pterygiophores in the 3rd interneural space (USNM 135783, 16 mm SL and USNM 257874, 15.3 mm SL); at first, this suggested misidentification with A. psilogaster , but dorsal- and anal-fin counts, vertebral counts and belly scales indicate that there is occasional variation in the dorsal-fin pterygiophore formula within A. hiatti .

Remarks

Smith-Vaniz and Johnson (1990) hypothesized that A. psilogaster and A. echinatus were most closely related based on the putatively derived conditions of three dentary pores and a reversal to teeth present on the 2nd infrapharyngobranchial. A. naka also has only three dentary pores, and although the condition of the dentition on the 2nd infrapharyngobranchial will have to await the collection of more specimens, it appears that the new species should be considered related to A. psilogaster and A. echinatus . Among these three species, A. naka is most likely to be the sister taxon to A. psilogaster of Japan, Taiwan and extreme northern Philippines. They share two features previously thought to be unique to A. psilogaster within the genus: two dorsal-fin pterygiophores inserted in the 3rd interneural space and belly unscaled anteriorly. The homology of the former is perhaps challenged by our discovery of the variable dorsal-fin pterygiophore formula found in A. hiatti (2 of 22 specimens with two pterygiophores in the 3rd interneural space), and the latter is of dubious value given that the A. naka specimen is small with incomplete scale development. However, without any additional evidence, we tentatively insert the new species as the sister to A. psilogaster in the phylogeny of Acanthoplesiops presented by Mooi and Gill (in press) resulting in the following relationships: A. hiatti ( A. indicus ( A. echinatus ( A. naka + A. psilogaster ))).

It is difficult, if not futile, to make conclusions regarding biogeography based on a single specimen whose phylogenetic relationships are uncertain. However, the distribution of Acanthoplesiops has been extended eastward to Tonga and there now appear to be three allopatric West Pacific species that form a clade; A. echinatus in the Moluccas and Sulu Sea is the sister to A. naka in Tonga and A. psilogaster in Japan, Taiwan and extreme northern Philippines. Acanthoplesiops indicus from the Indian Ocean is sister to this clade, but the history of the genus becomes more complicated with the basal position of A. hiatti which is again West Pacific, being sympatric with A. echinatus in the west as well as being found in Fiji and onto the Pacific plate in the Marshall Islands. This does not impact the broader conclusions of Mooi and Gill (in press) that a sister relationship of Acanthoplesiops and Notograptus follows a general pattern where Indo-West Pacific taxa are sister to an Australian clade.

We also note that there are some apparent geographical differences in meristics within at least two species (Table 2). Acanthoplesiops echinatus exhibits higher dorsal-fin spine counts in Moluccan specimens, and perhaps a trend to higher segmented anal-fin ray counts in the Philippines. These might deserve some attention when further specimens are available, particularly as only Moluccan specimens are known to have the pointed papillae covering much of the head for which the species is named (Smith-Vaniz & Johnson, 1990). Acanthoplesiops hiatti from Marshall Islands of the Pacific plate, and the single Fijian specimen, have lower dorsal-fin spine counts and tend to have lower anal-fin spine counts than western populations. A more thorough survey of characters among these populations might reveal additional differences that would more clearly indicate separate evolutionary histories and endemism.