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Two new cutthroat eel species are described from Vietnam. Dysomma intermedium sp. nov. has a relatively long trunk, being about half of head length and anal-fin origin more than twice pectoral-fin length behind the pectoral-fin tip; pectoral fin well developed; dorsal-fin origin over or slightly in front of base of pectoral fin; two intermaxillary teeth; four or five compound teeth on ethmovomer; single row of seven or eight teeth on lower jaw; total lateral-line pores 70-76; and 21 pre-anal and 118-124 total vertebrae. Dysommina brevis sp. nov. differs from congeners by having a trunk shorter than head length, its length 11.1%-11.8% TL; a short pre-anal length 24.6%-25.6% TL, eye diameter 11.8%-12.3% head length; three large and one or two small teeth on ethmovomer; and fewer teeth on the upper and lower jaws. In addition, a specimen representing the first record of Dysommina orientalis in Vietnamese water is documented., (© 2024 Fisheries Society of the British Isles.)

Two new species of the elongate snake eel genus Bascanichthys are described from the northwestern Pacific. Bascanichthys kabeyawan sp. nov. is described based on a single specimen collected from estuary of southern Taiwan. It is characterized by having head 4.6% TL; tail 52.3% TL; body depth at gill opening 1.1% TL; predorsal-fin length 58.4% HL; snout length 10.9% HL; body bicolored, head without bands; lateral-line pores anterior to anus 104; vertebral formula 4-103-224. Bascanichthys ryukyuensis sp. nov. is described based on two specimens collected from the shallow water of Okinawa-jima Island, Ryukyu Islands of southern Japan. It is characterized by having head 3.7-4.3% TL; tail 43.3-44.2% TL; predorsal-fin length 40.7-45.4% HL; snout length 11.3-13.1% HL; body pale brown, head without distinct dark bands after preservation; lateral-line pores anterior to anus 114-118; total vertebrae 207-216, mean vertebral formula 2-116-212; and dorsal-fin origin before middle of head.

A new species of the congrid eel genus Bathycongrus is described on the basis of three specimens collected from the deep waters of eastern Taiwan. Bathycongrus melanostomus sp. nov., belongs to the few vertebrae species complex and is distinct in having a short and broad snout; a much reduced caudal fin; abdomen, mouth cavity and gill chamber blackish; small conical blunt teeth on vomer forming an elongate patch; total vertebrae 133-135, and total lateral-line pores 108-109.

A new Conger eel species is described based on four specimens collected from Petuaghat fishing harbour, West Bengal, India. The combination of morphological characters and molecular data are discordant with the seven congeners currently recognized. Rhynchoconger smithi sp. nov. can be distinguished by having head smaller than trunk; pre-anal length more than three times in total length; a small eye, diameter 2.0-2.2 in snout length; rictus ending at a vertical through posterior margin of pupil; ethmovomerine teeth patch small, with 58-74 blunt teeth arranged in seven to eight irregular rows; vomerine teeth patch small, with 18-28 granular teeth arranged in four to six irregular rows, distinctly separated by narrow spaces from the ethmovomerine and maxillary teeth; three supraorbital pores and one supra-temporal pore; and 159+ to 164 total vertebrae. Moreover, R. smithi differs significantly from four congeners, R. nitens, R. flavus, R. ectenurus and R. gracilior, with Kimura two-parameter (K2P) distances 14.6%-20.3%., (© 2022 Fisheries Society of the British Isles.)

A new species of Coloconger is described from a large female collected off eastern Taiwan. It can be distinguished from congeners by having a large black blotch on rear portion of tail; posterior end of tail white; and combination of characters: head small 15.7% TL; 4 supratemporal pores, middle 2 in pair; 7 supraorbital pores, 2 pores over posterior nostril; 12 or 14 infraorbital pores; 14 preoperculomandibular pores; 226 dorsal-fin rays and 122 anal-fin rays; preanal vertebrae 77 and total vertebrae 145; rictus through a vertical of posterior margin of pupil; snout shorter than eye diameter; distance between origins of pectoral and dorsal fins 69.4% pectoral-fin length; and body brownish gray dorsally, light gray ventrally, and dorsal fin dark gray. Data and variations of Coloconger japonicus found in Taiwan are provided and discussed.

Meadia minor sp. nov., a scaless ilyophine eel, is described on the basis of 10 specimens collected from off Quy Nhon, central coast of Vietnam. It can be distinguished from its congeners in having a relatively long trunk (21.3-25.0% TL) which is longer than head length; a short and blunt snout (21.4-23.7% HL); gill opening close to pectoral-fin base; interbranchial space broad (20.7-26.2% HL); dorsal-fin origin above posterior third of pectoral fin; body depth 24-28 times in TL; total vertebrae 118-122; mean vertebral formula 7-33-121; and a small body size, reaching 330 mm TL. The generic status of the new species is discussed. Short descriptions of two congeners are provided.

A new species of the synaphobranchid genus Atractodenchelys is described based on 31 specimens collected off Quy Nhon and Nha Trang, southern Vietnam. The new species can be distinguished from the only two congeners in the genus by having much fewer total vertebrae (155‒158, vs. 168‒172 in A. phrix and 186‒199 in A. robinsorum), a relatively long head (head length 10.1‒12.3% TL), a relatively short snout (30.0‒35.8% HL), head longer than trunk length (vs. about equal in length in both species); and 5‒6 compound teeth on vomer (vs. 7‒8 in A. robinsorum).

Muraenichthys longirostris sp. nov. is described based on a single specimen 206 mm total length (TL) collected from off Dong-gang, southwestern Taiwan. The species is characterized by a long and slender snout (18% of head length), unique within the genus. It is also characterized by its position of the dorsal-fin origin (positioned at 2/5 of trunk; horizontal distance from the origin to a vertical through mid-anus 113% of head length), long tail (67% TL), and vertebrae 23-41-131. In addition, Muraenichthys thompsoni Jordan Richardson, 1908 is redescribed based on the holotype and 63 specimens from Fiji to Madagascar, north to Japan. Muraenichthys malabonensis Herre, 1923 is recognized as a junior synonym of M. thompsoni.

Two new species of elongate unpatterned moray eels related to Gymnothorax prolatus are described. Gymnothorax pseudoprolatus sp. nov. is described from one specimen collected off southwestern Taiwan and Gymnothorax vietnamensis sp. nov. is described from seven specimens collected off central Vietnam. Both differ from other species in the group in the number of vertebrae, position of anus, and dentition. Gymnothorax pseudomelanosomatus Loh et al., 2015 is reported from southern Taiwan for the first time; it was previously known from the type series collected off the northeastern tip of Taiwan. Gymnothorax angusticauda (Weber de Beaufort 1916) is reported for the first time in Taiwan and the Philippines.

Dysommina orientalis, a new species of Ilyophine eel from off Taiwan and Japan is described and illustrated. The species had long been recognized as Dysommina rugosa in the western Pacific and is distinguished from D. rugosa by a lower number of predorsal vertebrae, a higher number of total vertebrae, shorter head length, smaller eye size, reduced vomerine dentition, and an increased number of both mandibular and maxillary teeth, as well as significant differences in DNA sequence in COI and 16S.

A new species of the cutthroat eel genus Synaphobranchus is described from Taiwan on the basis of 12 types and 11 non-type specimens. The new species can be distinguished from its congeners in having most of its head and abdomen naked, except for a patch of scales behind the eye, dorsal surface of trunk naked or covered by scattered scales; scales oval in shape; dorsal fin slightly before to about opposite to origin of anal fin; mean vertebral formula 28-28-131, precaudal vertebrae 50-56, total vertebrae 124-135. Comments of the congeners occurred in adjacent regions are provided.

Examination of the congrid genus Bathyuroconger in the northwestern Pacific reveals six species are present. Bathyuroconger albus sp. nov. differs from congeners in having a pale coloration, a moderately reduced gill opening that is close to but not touching the pectoral-fin base, trunk length 2.0-2.9 times HL, preanal vertebrae 45-54, preanal lateral-line pores 42-48. Bathyuroconger dolichosomus sp. nov. has a moderately reduced gill opening that is close to but not touching the pectoral-fin base, trunk length 3.2 times HL, preanal vertebrae 63, and preanal lateral-line pores 61. Bathyuroconger fowleri sp. nov. has a small gill opening distant from the pectoral-fin base, trunk length 1.7-2.1 times head length, dorsal-fin origin over middle of pectoral fin, predorsal vertebrae 10-14, preanal vertebrae 48-52, total vertebrae 173-178. Bathyuroconger hawaiiensis sp. nov. has a slightly reduced gill opening, separated from the pectoral-fin base, dark color, total vertebrae 201-210. Bathyuroconger parvibranchialis (Fowler, 1934), redescribed herein, has a small gill opening distant from the pectoral-fin base, dorsal-fin origin over base of pectoral fin, trunk length 1.4-1.7 times in HL, preanal vertebrae 43-48, total vertebrae 181-189, and a uniformly dark color in adults. Bathyuroconger cf. vicinus has a full-size gill opening in contact with pectoral-fin base, dark color, and total vertebrae 196-201. A key to the species in the northwestern Pacific is provided.

The eel fauna (orders Anguilliformes and Saccopharyngiformes) of Taiwan is increased to 14 families, 79 genera and 232 species. Previous studies (Ho et al., 2015b, c) showed Taiwan had the highest diversity of eels in the world: this is further supported in the present updated work. Elsewhere in this volume, 16 species are newly described and 13 species are newly added to the Taiwanese ichthyofauna, mainly in the families Congridae and Synaphobranchidae. In addition, one new genus and four new species are described from adjacent waters in the Pacific Ocean. A total of 58 new species of the two eel orders are described from Taiwan; 52 of them are valid, and 37 are only found in Taiwan. Four names previously recorded in Taiwan are described as new in present special issue and are removed from the fauna of Taiwan accordingly. This work provides a foundation for the study of eel diversity in Taiwan.

The leptocephali of the Taiwanese duckbill conger are described in detail for the first time based on specimens collected from near the type locality. Morphological and molecular features are employed to compare the leptocephali, juveniles and adults. The morphology of leptocephali can be divided into three stages, with clear metamorphosis of the jaw structure and morphometric proportions. Detailed description and comparison of each leptocephalic stage, juveniles and adults, and the ontogenetic changes of leptocephali are provided.

The congrid eel genus Conger in Taiwan is reviewed. Five species are recognized, including C. jordani, C. cinereus, C. macrocephalus, C. myriaster and a newly collected species, C. philippinus. A mature male specimen of Conger myriaster is described for the first time. Specimens previously identified as C. japonicus (= C. myriaster) were misidentifications of above mentioned species. The diagnostic characters are discussed. Detailed data on all five species and a key to species in Taiwan are provided.

A review of the congrid eel genus Ariosoma in Taiwan is provided. Eight species are recognized, including A. anago (Temminck Schlegel, 1846), A. dolichopterum Karmovskaya, 2015, A. fasciatum (Günther, 1872), A. majus (Asano, 1958), A. megalops Fowler, 1938, A. meeki (Jordan Snyder, 1900), A. shiroanago (Asano, 1958) and a new species which is described based on types and non-types collected from off southwestern Taiwan. These species can be distinguished from each other by the coloration, numbers of head pores, lateral-line pores and vertebrae. A key to all Taiwanese Ariosoma species is provided.

The rare congrid eel species Congrhynchus talabonoides Fowler, 1934 was described from three specimens collected in the Philippines during the Albatross expedition in 1908-1909. The only mention of the species in the literature since its description has been in a type catalog (Smith, 1994), an identification guide (Smith, 1999), and a checklist of the eels of Taiwan (Ho et al., 2015, where it was erroneously placed in the Muraenesocidae). The first author discovered the Taiwan specimen in 2009 during a visit to the National Museum of Marine Biology and Aquarium in Taiwan, the first new specimen reported since its description more than 100 years ago. Several specimens were recently collected in the Andaman Sea. Based on these new specimens and a re-examination of the original three type specimens, we provide a full redescription of the species below.

Ten species of the congrid eel genus Bathycongrus are recognized from Taiwanese waters. Diagnoses and full descriptions of eight species are provided in present work. Three species are described as new. Bathycongrus bimaculatus sp. nov. is a small species with a pale body color, two black patches on dorsal fin, and 109-111 total vertebrae. Bathycongrus graciliceps sp. nov. is a moderately elongate species with a small, slender head and long trunk, and 163 total vertebrae. Bathycongrus castlei sp. nov. is a slender species with a relatively short trunk, dorsal half of body deep brownish, and 160-162 total vertebrae. Two rare species, B. bleekeri Fowler, 1934 and B. macroporis (Kotthaus, 1968), are newly reported from Taiwan. A key to all species in Taiwan is provided.

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Two new species of the congrid eel genus Bathycongrus are described from the deep waters of eastern Taiwan. Bathycongrus albimarginatus sp. nov., which belongs to the species group with two enlarged teeth on the vomer and more total vertebrae, is described from 16 specimens and can be distinguished by vertical fins with broad white margins, more pores on head and predorsal lateral line, total vertebrae 195-201. Bathycongrus brunneus sp. nov., which has no enlarged teeth on the vomer, is described from one specimen and can be distinguished by 12 subequal teeth on the vomer and 174+ total vertebrae.

Three new species of eels of the genus Dysomma are described from the western North Pacific Ocean off Taiwan. Dysomma formosa sp. nov., described from 34 specimens, differs from the congeners in having single row of 11-14 large compound teeth followed by 3-10 smaller teeth on lower jaw, 17-33 lateral-line pores, and 128-133 total vertebrae. Dysomma brachygnathos sp. nov., described from two specimens, differs from the congeners in the lack of a pectoral fin, having an anterior position of anus (preanal length 17.8-ca. 18.6% SL), 23-32 lateral line pores, a short lower jaw, and 131+‒136 total vertebrae. Dysomma robinsorum sp. nov., described from four specimens, differs from the congeners in having an anterior position of anus; preanal length 15.5-16.9% SL, no intermaxillary teeth; multiple rows of teeth on upper jaw; four compound teeth on vomer; teeth on lower jaw multiserial, those on inner row slightly enlarged, and 122-124 total vertebrae. Components of the variation of Dysomma taiwanense originally described from Taiwan are provided based on an additional 52 recently identified specimens.

Three rare snake eel species, Allips concolor McCosker, 1972, Neenchelys daedalus McCosker, 1982, and Apterichtus    moseri (Jordan Snyder, 1901), were newly collected from Taiwan. The distribution ranges of the first two species are now extended to the South China Sea off southern Taiwan and that of the third is extended southward to off northwestern Taiwan. Supplemental data for three recently described species, Neenchelys diaphora, N. gracilis and N. similis, from Taiwan are provided.

Although Taiwan is a small island, it occupies a strategic biogeographic position between the faunal regions of Japan to the north and the Philippines to the south. As a result, it has an interesting mixture of northern and southern species. In this paper, we report new records of two species of congrid eel, one known previously from Japan and China, and the other known only from the Philippines. The first species, Promyllantor nezumi Asano, 1958, was described by Asano (1958) and reassigned to a newly established genus Acromycter  by Smith Kanazawa (1977). Zhang (1989) reported the species from east of Hainan Island, China. The second species, Gnathophis asanoi Karmovskaya, 2004, was described from the Philippines based on two types. Ho et al. (2015) reported one specimen collected from Taiwan but provided one additional specimen was collected more recently.

A new ophichthid worm-eel, Neenchelys gracilis sp. nov., is described from a specimen collected from southwestern Taiwan. It differs from its congeners by having: a minute pectoral fin, many filamentous cirri on the anterior nostril rim; a very slender body; a very small gill opening; and a vertebral formula of 30-78-200.

A new genus and species of ophichthid eel, Sympenchelys taiwanensis is described. The genus is distinguishable from all other myrophine genera by having neural and haemal spines with well-developed paddle-like tips, and from two closely similar genera, Mureaenichthys Bleeker 1853 and Skythrenchelys Castle & McCosker 1999, by the following combination of characters: an unconstricted gill opening; a minute flap just posterior to the gill opening; a slender cleithrum; three preopercular pores; a slender and distinctly tapered maxilla; an undeveloped supraoccipital crest; and large and recurved teeth on jaws and vomer. Sympenchelys taiwanensis is described from five specimens collected from Taiwan, the northwestern Pacific Ocean.

The congrid eel genus Parabathymyrus from the western Pacific Ocean is reviewed and three species are recognized: Parabathymyrus macrophthalmus from Japan, Taiwan, Vietnam and Australia, P. brachyrhynchus from Taiwan, Vietnam, the Philippines, Vanuatu and Solomon Islands, and a new species, P. philippinensis sp. nov., is described based on two specimens collected from the Philippines. These three species can be distinguished from each other by the number of total vertebrae (128-137 in P. macrophthalmus, 162-173 in P. brachyrhynchus and 140-141 in P. philippinensis sp. nov.), preanal vertebrae (39-46, 49-58, and 42), preanal lateral-line pores (36-44, 46-54, and 40-42), total lateral-line pores (121-131, 159-169, and 140-141) and supraorbital pores (4, 4 and 3).

The eel fauna (orders Anguilliformes and Saccopharyngiformes) of Taiwan is one of the richest in the world. Recent genetic and morphological studies have improved the taxonomic resolution and increased the known diversity of the eels of Taiwan, and the overall diversity is comparable to that of adjacent marine zoogeographic regions with rich biodiversity, such as Australia and the Philippines. In this special issue, we verified the historical records and examined numerous recently collected specimens, and conclude that the eel fauna of Taiwan is represented by 207 species in 75 genera and 14 families, with several undescribed species still likely to be discovered. The Muraenidae (71 species), Ophichthidae (60), Congridae (29) and Synaphobranchidae (17) are the most abundant and species-rich in Taiwanese waters. We add 42 species to the Taiwanese fish fauna, including one new genus and 13 new species. Two further species are newly described, one from the Philippines and one from Indonesia.

Three new species of false moray eels belonging to the genus Chlopsis, family Chlopsidae, are described and illustrated. Chlopsis sagmacollaris, from Western Australia, is characterized by its distinct color pattern with a unique dark saddle on the nape and paler body pigmentation than all other congeners. Chlopsis nanhaiensis, from Taiwan, has a bicolored body with its dorsal-fin origin behind a vertical through posterior margin of gill opening. These characters are shared with C. apterus, C. bicolor, C. bidentatus and C. orientalis sp. nov., but C. nanhaiensis has fewer total vertebrae than other bicolored species (ca. 120 in C. nanhaiensis vs. 125-140 in other four species). Chlopsis orientalis, from Vietnam, is distinguished from all other congeners except C. bicolor by its simple bicolored body, head length (ca. 10% of total length), position of dorsal-fin origin (distinctly behind a vertical through posterior margin of gill opening), and counts of preanal and total vertebrae (35 and 135). Although C. orientalis resembles C. bicolor in meristics and morphometrics, it differs in its pigmentation and possession of stouter intermuscular bones. These new species represent the first records of the genus Chlopsis from the northwestern Pacific Ocean and the eastern Indian Ocean.

Species of the synaphobranchid eel genera Dysomma and Dysommina from Taiwan are reviewed. A total of eight species in Dysomma and one species in Dysommina are recognized. A new Dysomma species is described based on four specimens collected from off Taiwan. It differs from its Taiwanese congeners in having 137-139 total vertebrae, pectoral fin well-developed, 2 intermaxillary teeth, 4 compound teeth on vomer, 40-48 total lateral-line pores, the last at about middle of body, and a uniform brownish color. A key to all species in these two genera in Taiwan is provided.

An annotated checklist of eels, orders Anguilliformes and Saccopharyngiformes, occurring in Taiwanese waters is presented. The checklist is the result of a series of systematic studies conducted by the authors in the past few years. The eel fauna of Taiwan is one of the richest in the world with a total of 206 species in 74 genera and 13 families in Anguilliformes and a single species in Saccopharyngiformes. The most species-rich families are the Muraenidae with 71 species, followed by the Ophichthidae with 60 species, the Congridae with 29 species, and the Synaphobranchidae with 17 species. Moreover, three genera and 42 species have been described based on at least one type specimen collected from Taiwan. Of these, 36 species are recognized as valid and 23 species are known only from Taiwanese waters at present. Historical records of all Taiwanese eel species are reviewed by examining the original descriptions and figures, vouchers, as well as the recently collected specimens, where available. This represents the first detailed checklist of eels from Taiwanese waters.

In our earlier paper (Ho et al., 2013), we described three new species of the eel genus Neenchelys Bamber, 1915. Unfortunately, the critical step of registering the work in ZooBank was omitted, thus rendering the names unavailable. In this paper, we present abbreviated descriptions of the new species in order to make these names available in accordance with the amended Article 8.5 of the International Code of Zoological Nomenclature.

Three new species of ophichthid eels, subfamily Ophichthinae, are described and illustrated from specimens collected at fish markets in Taiwan. Included are: Echelus polyspondylus sp. nov., which is unique in its vertebral number (172-183), dorsal-fin origin (behind gill openings), and coloration (anal-fin membrane black posteriorly); Ophichthus bicolor sp. nov., which is unique in its mean vertebral formula (18-64-158), dorsal-fin origin (well behind gill openings), dentition (teeth large and conical), and contrasting coloration (tan dorsally, white ventrally); and O. shaoi sp. nov., which is unique in its mean vertebral formula (11-69-159), and prominent lip barbels. A key to Taiwanese species of Echelus and Ophichthus is provided.

The following nine elongate unpatterned muraenid species of the subfamily Muraeninae, including one new species, are recognized from Taiwan and adjacent waters: Gymnothorax albimarginatus (Temminck & Schlegel), G. dorsalis Seale, G. melanosomatus Loh, Shao & Chen, G. phasmatodes (Smith), G. prolatus Sasaki & Amaoka, G. sagmacephalus Böhlke, Pseudechidna brummeri (Bleeker), Strophidon sathete (Hamilton) and G. pseudomelanosomatus new species, described from two specimens. This new moray eel is distinguished from its similar species, G. melanosomatus, by the following features: grey brown body (vs. black), snout length 20.5% (vs. 17.8%) of head length, smaller eye diameter 8.2% (vs. 10.0%) of head length; preanal length 49.5% (vs. 58.5%) total length, and preanal vertebrae 89-89 (vs. 105-109). Phylogenetic relationships of the nine species were examined using nucleotide sequence data from partial sequences of mitochondrial ND5 gene (600 bp), and seven species form COI (600 bp). The genetic analyses suggest that G. pseudomelanosomatus is distinct from G. melanosomatus and the other six species of Gymnothorax. Morphological features and mitogenetic affinities strongly suggest that "G." dorsalis should be placed in Strophidon rather than in Gymnothorax. The results also suggest that employment of ND5 and COI gene sequences are rather useful for identification of species and for obtaining reasonable insights into the phylogeny of the muraenid species.