Tetragonurus pacificus Abe, 1953

Aluwihare, Yasmin C., Fernando, Sara, Gunasinghe, Malsha S., Ranasinghe, Cherika, Ranjula, M. A. S., Weerakoon, Thiranya, Haputhantri, Sujeewa S. K., Jayasinghe, Prabath K., Lisher, Mark W. & Psomadakis, Peter N., 2025, First confirmed record of adult Pacific squaretail, Tetragonurus pacificus (Teleostei: Perciformes: Stromateoidei) from the Indian Ocean, with an expanded description, Zootaxa 5604 (1), pp. 41-51 : 43-49

publication ID	https://doi.org/10.11646/zootaxa.5604.1.4
publication LSID	lsid:zoobank.org:pub:6B0C9970-3214-4C4F-922B-50D394C33D35
DOI	https://doi.org/10.5281/zenodo.15024070
persistent identifier	https://treatment.plazi.org/id/A86E032D-FFD1-FF94-FF65-FCD95D98F887
treatment provided by	Plazi
scientific name	Tetragonurus pacificus Abe, 1953
status

Treatment

Tetragonurus pacificus Abe, 1953 View in CoL

Figures 1–5 View FIGURE 1 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 , Tables 1 View TABLE 1 , 2 View TABLE 2

Tetragonurus pacificus Abe 1953: 45 View in CoL , figs. 5–6 (west of Solomon Islands); Grey 1955: 14, fig. 7; Haedrich 1967: 98; Ahlstrom et al. 1976: 333, fig. 14; Haedrich 1986: 851; Last 2001: 3785; Parin & Piotrovsky 2004: S56; Koeda & Teramura 2019; Anderson 2022.

Material examined. 2023.09.01 NH , 3 specimens: 129–133 mm SL, off Chilaw (7°2.29’N, 79°25’E), Sri Lanka, Indian Ocean, 10 July 2018, R/V Dr. Fridtjof Nansen, station 80, pelagic trawl (10 m depth). GoogleMaps

Description. Proportional measurements and counts are given in Table 1 View TABLE 1 . First dorsal fin 11‒12 spines. Second dorsal fin 11‒12 soft rays.Anal fin 1 spine, 10‒11 soft rays. Pectoral fin 15‒18 rays. Pelvic-fin rays I, 5. Longitudinal scale rows 75 to anterior margin of caudal keels. Branchiostegal rays 6. Gill rakers 8‒12 + 9‒10, total 17‒22. Vertebrae 39.

Body elongate and fusiform, with comparatively long caudal peduncle, 27.6‒29.8 % SL, square in cross-section, each side with 2 lateral keels of modified scales. Snout blunt, its length 27‒31.2% HL. Eye rounded, its diameter subequal to snout length, and 23.2‒27.0% HL. Dorsal surface of interorbital space slightly convex, its least bony interorbital space 20.3‒24.3% HL. Posterior nostril slit-like, located midway between uppermost point of eye and snout tip; anterior nostril oblong, located midway between posterior nostril and snout tip. Mouth moderately large, slightly oblique; posterior margin of maxilla reaching a vertical through middle of orbit. Teeth on jaws uniserial, those in upper jaw curved inward. Lower jaw teeth flattened and blade-like. Palatines and vomer with tiny inwardly directed conical teeth. Posteroventral corner of preopercle with serrations. Scales cover entire body and head except snout. Body scales arranged as slanted, parallelly and spirally keeled-like structures, tightly attached to the body and overlaying each other. Scales on top of head with minute denticles; subequal. Lateral line indistinct, from uppermost point of opercle, following dorsal outline of body and reaching caudal peduncle, but not caudal-fin base. First dorsal-fin origin above anterior third of pectoral fin; second, third or fourth dorsal-fin spines longest (length vary among individuals), spine length thereafter gradually shorter posteriorly. Last 1‒2 dorsal-fin spines short, one-tenth of the longest spines (penultimate or last spine split at base and counted as a single element) ( Fig. 4 View FIGURE 4 ). Base of last dorsal-fin spine equidistant between penultimate dorsal-fin spine and base of first dorsal-fin soft ray (unbranched) or closer to the latter. Second dorsal fin taller than first, its base shorter than first. Pelvic-fin origin beneath front third of pectoral fin. Anal fin origin slightly behind second dorsal fin origin. End of anal-fin base posterior to end of the second dorsal-fin base. Pectoral fins pointed, posterior tip reaching a vertical through base of fifth dorsal fin. Caudal fin forked, both lobes pointed.

Fresh coloration. Body black with silvery green reflection on flanks, head region darker than body. All fins black, membranes paler ( Fig. 2 View FIGURE 2 ).

Preserved coloration. Blackish brown. Head and fins slightly darker ( Fig. 3 View FIGURE 3 ).

Distribution. Tetragonurus pacificus has been recorded from various subtropical and tropical locations across the Indo-Pacific region. These include areas from Sumatra to the Seychelles, the West Australian Basin, and above the Equator Seamount in the Indian Ocean ( Grey 1955; Parin & Piotrovsky 2004); west of the Solomon Islands, southwest of Tarawa ( Kiribati), and Taiwan in the Western Pacific Ocean ( Abe 1953; Koeda & Teramura 2019); north of Samoa and the Hawaiian Islands in the Central Pacific Ocean ( Grey 1955; Ahlstrom et al. 1976); and from a mid-oceanic station (3°47ʹN, 118°30ʹW) in the Eastern Pacific Ocean ( Ahlstrom et al. 1976) ( Fig. 1 View FIGURE 1 ). This study provides the first confirmed record of this species in Sri Lankan waters and reaffirms its presence in the Indian Ocean, based on adult specimens.

Remarks

Koeda & Teramura (2019) re-described T. pacificus based on two specimens (96.2–122.5 mm SL) from the Pacific Ocean, noting that in the Indian Ocean, the species was previously recorded only from early juveniles (9–21 mm SL) (see also Anderson 2022). An exception to this was the record of nine adult specimens (140–204 mm SL) reported by Parin & Piotrovsky (2004), who provided some meristic and morphometric data; however, no detailed descriptions were given, or voucher specimens preserved. Therefore, the specimens analyzed in this study represent the largest T. pacificus individuals examined in detail with vouchers deposited in a scientific institution. Morphological data obtained from the Sri Lankan specimens align with most morphological characters of T. pacificus , including the holotype. The few morphological discrepancies, such as preorbital depth and first anal-fin spine length ( Table 1 View TABLE 1 ), may be attributed to ontogenetic growth, intraspecific variation, body distortion or shrinking caused by preservation, and/or slight differences in measuring methodologies. Our study confirms most of the meristic characters previously reported to distinguish T. pacificus from its congeners. These characters include 75 (72–80) longitudinal scale rows along body axis (vs. 83–95 in T. atlanticus and 97–114 in T. cuvieri ); 39 (39–43) vertebrae (vs. 44–51 in T. atlanticus and 52–58 in T. cuvieri ); 11 or 12 (9–12) first dorsal-fin spines (vs. 13–17 in T. atlanticus and 15–21 in T. cuvieri ) and extends the known upper and lower ranges for other characters: 12 (6–9) gill rakers on upper limb (vs. 5–8 in both T. cuvieri and T. atlanticus ) and 9 or 10 (12–16) gill rakers on lower limb (vs. 9–14 in T. cuvieri and 10–14 in T. atlanticus ). The new data obtained from the three Sri Lankan specimens also variously extend the upper ranges of some body ratios in T. pacificus such as head length, body width, snout length, pectoral fin length, caudal peduncle length, dorsal and anal-fin spines length ( Table 1 View TABLE 1 ). Furthermore, morphological analyses herein (Sri Lankan specimens + literature data) provide new characters that may be useful to distinguish T. pacificus from T. cuvieri : head length (25.3–28.7% SL vs. 18–25% SL); orbit diameter (6.0–9.6% SL vs 2.7–4.5% SL or 26.2–28.7 vs. 14.1–26.0% HL).

Notably, the head length, body depth and pectoral-fin ratios for the holotype of T. pacificus provided by Anderson (2022), are not congruent with the values provided by the original description ( Abe, 1953) (see Table 1 View TABLE 1 ). It is also interesting to note that the Sri Lankan specimens of T. pacificus appear to have a darker fresh coloration relative to the Taiwanese specimen figured in Koeda & Teramura (2019).

Genetics. Pairwise sequence divergence based on the COI gene region for T. pacificus was comparatively high with T. atlanticus (14.3%) and T. cuvieri (8.6%). Further, there was a clear genetic divergence between T. cuvieri and T. atlanticus (pairwise K2P, 8.8%) ( Table 2 View TABLE 2 ). The Maximum Likelihood phylogenetic tree, inferred from the mitochondrial COI nucleotide sequences supports two distinct lineages of Tetragonurus . One lineage contains two sister species namely T. cuvieri and T. pacificus , while the other lineage has a single species T. atlanticus ( Fig. 5 View FIGURE 5 ). The Sri Lankan sample (2023.09.01 NH) clustered with a sequence of T. pacificus ( Japan) published in Teramura et al. (2022) with a pairwise distance of 0.04%. Tetragonurus cuvieri has an intraspecies pairwise distance of 1.3%. This intraspecies variation within T. cuvieri could be explained by its wide distribution as well as larger number of available sequences for the species. Whereas T. atlanticus has an intraspecies variation of 0.63% pairwise distance.