A rare species of Uropterygius (Anguilliformes, Muraenidae) found in the stomach content of a Yellow-lipped Sea Krait from Japan

A single muraenid specimen (genus Uropterygius), recovered from the stomach of a live Yellow-lipped Sea Krait, Laticauda colubrina (Schneider, 1799), captured at Iriomote Island, Japan, was found to possess intermediate traits between 2 rare species, Uropterygius xenodontus McCosker & Smith, 1997 and Uropterygius golanii McCosker & Smith, 1997. The specimen is provisionally identified as U. xenodontus and is described below with its morphs, as an important addition to material of rare specimens of the genus in the western North Pacific.


Introduction
Sea kraits (Squamata, Elapidae, Laticauda spp.) have a key role as predators of coastal fishes in tropical and subtropical regions.Some species specialize on feeding of anguilliform fishes, despite a large variety of co-existing fishes (McCosker 1975, Ineich et al. 2007, Tabata et al. 2017).When studying the stomach contents of Laticauda spp. at Iriomote and Ishigaki islands in Japan, 2 moray eel specimens of the genus Uropterygius Rüppell, 1838 (Muraenidae, Uropterygiinae) were recovered from Laticauda colubrina (Schneider, 1799).One was identified as Uropterygius nagoensis Hatooka, 1984, while the other remained unidentified (Tabata et al. 2017).We provisionally identify this specimen as Uropterygius xenodontus McCosker & Smith, 1997-a species known only from the type series collected from central and western South Pacific.The present report documents a significant range extension of this species, and is the first record from Japanese waters.

Methods
Details of specimen collection were given by Tabata et al. (2017).Counts and measurements generally follow McCosker and Smith (1997), measurements being made to the nearest 1 mm with a metal ruler for the total, trunk, preanal and tail lengths; and to 0.1 mm with digital calipers for other measurements.Vertebral counts were made from radiographs.Total and head lengths are abbreviated as TL and HL, respectively.Institutional abbreviations follow Sabaj (2016).Description.Selected morphological characters, generally reported for members of the family Muraenidae, are summarized in Table 1 and as follows.Body elongate, laterally compressed; head moderate; snout obtuse, short; anus situated about midway along body.Dorsal and anal-fins inconspicuous, origins situated near tail tip.
Distribution.Uropterygius xenodontus is widely distributed in shallow tropical and subtropical waters of the central to western North Pacific (Fig. 3).However, specimen records are limited to only the Coral Sea, American Samoa, the Marshall Islands, Johnston Island, and Iriomote Island (McCosker and Smith 1997, this study).identify the Iriomote specimen as U. xenodontus based on its collection locality (= Iriomote Island), located in the southern part of the western North Pacific.This specimen is also the first record from Japanese waters and well distant from the previously known distribution of the species (Fig. 3).
Exhaustive specimen collection efforts, primarily by net fishing and hook and line, were conducted from the early 20th century for the purpose of understanding fish biodiversity in Japanese waters.However, uncertainties remain in coral reef and rocky fish assemblages when compared to other coastal habitats, mainly due to the difficulty in capturing specimens.In addition to U. xenodontus reported here, other poorly known fish species have been found in recent investigations of sea krait stomach contents (Tabata et al. 2017).As evident here and also suggested by Ineich et al. (2007) and Séret et al. (2008), examination of the stomach contents of sea kraits provides an additional useful tool for investigating the diversity of tropical and subtropical fishes.

Discussion
Böhlke et al. (1989) recognized 4 genera, AnarchiasJordan & Starks, 1906, Channomuraena Richardson, 1848,  Scuticaria Jordan & Snyder, 1901, and Uropterygius, within the subfamily Uropterygiinae.Morphological features of this specimen correspond to the genus Uropterygius that comprises 21 valid species(Böhlke et al. 1989, Smith 2012).Vertebral numbers and wedge-shaped jaw teeth of our specimen were most similar to U. xenodontus, found in the western and central Pacific (Fig.3), and Uropterygius golanii McCosker & Smith, 1997, which is endemic to the Red Sea (Fig.3).Uropterygius xenodontus and U. golanii are recognized as representing a sister species relationship(McCosker and Smith 1997,  DiBattista et al. 2016), and are distinguished only by differences in vertebral numbers, that is, 143-146 before dorsal fin, 144-149 before anal fin, and 154-157 in total in the former vs134-138, 136-141, and 145-148  in the latter.Vertebral counts in our specimen are 139, 141 and 152 respectively, and represent an intermediate condition between U. xenodontus and U. golanii.We provisionally

Table 1 .
Counts and proportional measurements of Uropterygius xenodontus and U. golanii.