A NEW RECORD OF THE LONG MORAY, GYMNOTHORAX LONGINQUUS (ACTINOPTERYGII: ANGUILLIFORMES: MURAENIDAE), FROM SOUTHERN VIETNAM, SUPPORTING THE UNCERTAIN RECORD IN THE GULF OF THAILAND

A Abstract. Five specimens of Gymnothorax longinquus (Whitley, 1948) were collected from a fish market and a fish landing site in Tac Cau and Ha Tien, southern Vietnam. These specimens represent the first record of G. longinquus in Vietnam, and further support a previously uncertain record in Thailand. The specimens collected are described and illustrated. Gymnothorax longinquus may prefer muddy shallow water from northern Australia to the Gulf of Thailand.


INTRODUCTION
Vietnam, a peripheral country of the coral triangle with diverse marine habitats along the coastline of over 3260 km, has the potential to maintain massive marine bioresources. The actual number of the muraenid species (Muraenidae), however, recorded in Vietnam and amounting to about 40 species (Nguyen and Nguyen 2006, Prokofiev 2010a, Huang et al. 2018) is much lower compared to 75 muraenid species recorded in Taiwan (Ho et al. 2018, Huang et al. 2019. The lower species number implies that the moray diversity of Vietnam may still be underestimated despite that several new muraenids and new records have been reported in the last decade (Prokofiev 2010b, Hibino et al. 2016, Huang et al. 2018, Smith et al. 2018, Prokofiev 2020. More efforts are needed in the moray taxonomic research in Vietnam. Gymnothorax longinquus (Whitley, 1948) is a rare moray with only few specimens and reports available since its description. All known specimens were collected from the northern part of Australia except for one from a Thailand fish market with uncertain capture locality (Böhlke and McCosker 2001). The holotype was collected from Queensland, Australia (Whitley 1948). In this study, five specimens of G. longinquus were acquired from Kien Giang Province in southern Vietnam, located beside the Gulf of Thailand. The existence of G. longinquus in southern Vietnam may further support the previous uncertain record in Thailand and the statements that G. longinquus is distributed through northern Australia to the Gulf of Thailand (Böhlke and McCosker 2001). The mitochondrial cytochrome oxidase subunit I (COI) gene of these five specimens was also sequenced to prove the identification and to provide molecular inference for future research.

MATERIAL AND METHODS
A total of five specimens of Gymnothorax longinquus were collected from Kien Giang Province during the fish faunal survey in September 2017. Three specimens were from a fish market in Tac Cau and two specimens were from a fish landing site in Ha Tien. A piece of muscle tissue was cut and preserved in 95% ethanol until DNA extraction. Specimens were then fixed in 10% formalin, transferred to 70% ethanol for permanent preservation deposited in the Department of Oceanography, National Sun Yatsen University, Kaohsiung (museum number prefix: DOS). Morphometric measurements followed Böhlke et al. (1989), expressed as proportion in total length (TL) or head length (HL). Vertebral counts followed the terminology of Böhlke (1982), expressed as predorsalpreanal-total vertebrae. The dentition was determined under a stereomicroscope; the dental terminology followed that used in Böhlke and Randall (2000). The sex was determined by examining the gonads.

RESULTS
Brief description of specimens in this study. Body moderately stout, tail tapering ( Fig. 1), depth at gill opening 13.7-17.9 and at anus 17.6-22.1 in TL (Table  1). Anus slightly posterior to mid-body, preanal length 1.9-2.0 in TL. Dorsal fin moderately high, origin anterior to gill opening, predorsal length 1.3-1.5 in HL. Anal fin low, originating immediately behind anus. Gill opening slightly below midline of body. Head moderately elongated, 7.5-7.8 in TL. Eyes slightly closer to corner of mouth rather than tip of snout, diameter 10.9-13.8 in HL. Snout short and stout, 5.9-6.6 in HL. Jaws subequal, 2.3-2.5 in HL, lower jaw slightly longer in our specimens, teeth not visible when mouth closed. Anterior nostril at snout tip, tubular, shorter than eye radius; posterior nostril as pore with raised rim, above and posterior to anterior margin of eye.
Color of fresh specimens (Fig. 1): whole body uniformly dark brown with irregularly scattered pale blotches. Pale blotches light gray to cream, depending on specimens. Fin margin darker, especially at posterior part of tail. Head pores with fine dark rims. Inner mouth same color with body. Gular folds slightly darker. Corner of mouth and gill opening not dark (although not significant in the figure). Iris of eye yellowish. Phylogenetic analysis. The topology of ML tree revealed that all sequences generated in this study clustered with two Gymnothorax longinquus sequences from Cape York (HM376361) and Channel Island (FOAO2098-19) in northern Australia, confirming the morphological identification of the Vietnamese specimens (Fig. 3). All species are monophyletic with the smallest inter-species genetic distance 19.3% between G. longinquus and G. tile. Gymnothorax longinquus is a sister group of G. tile, G. pseudothyrsoideus, and S. dorsalis.

DISCUSSION
Gymnothorax longinquus is a poorly known moray with only eight specimens recorded in previous studies since its description. Before the presently reported study, seven out of eight specimens were from the northern areas of Australia. The only record out of Australian water was reported by Böhlke and McCosker (2001) from a fish market in Thailand with an emphasis on uncertain capture location. Imamura (2013) has already recorded a 540 mm G. longinquus specimen (Museum ID: KAUM-I. 33142) off Chantha Buri in Thailand but it was misidentified as G. pseudothyrsoideus. Gymnothorax longinquus was also reported as Gymnothorax sp. by Hibino (2017) (Khongchai et al. 2003). Therefore, G. longinquus may prefer to inhabit muddy shallow water.
The majority of morphometric measurements and meristic counts conform to those in Böhlke and McCosker (2001) and Böhlke and Smith (2002), except for smaller eye diameter (10.9-13.8 vs. 9.4-11 in references) and the anus slightly posterior to mid-body (vs. anus at mid-body). The differences may attribute to individual variations among geographic regions or differences in measurers. No mature specimens have been identified in either previous or this study (Böhlke and McCosker 2001). Böhlke and McCosker (2001) indicated that the muscles of all preserved specimens they examined were unevenly contracted, giving a curious "lumpy" surface to the body. However, all our preserved specimens have soft and smooth surfaces.
Four morphologically similar species, G. pseudothyrsoideus, G. taiwanensis, G. tile, and S. dorsalis characterized by gray to dark brown body with pale blotches or obscure reticular pattern, short anterior nostril, and blunt snout, may be confused with G. longinquus. Gymnothorax pseudothyrsoideus is most similar to G. longinquus, with overlapping of most morphometrics and meristics. However, G. longinquus  2.5-6.9 Interorbital width 11.9-14.6 a 6.8-8.