Zoology Publications from Victoria University of Wellington—Nos. 33 and 34
Günther's Catalogue of the Fishes of the British Museum (1870) provides the first clue to the correct generic name of the Australasian silver conger. Because of certain similarities in dentition, Günther (1870, pp. 42–43) included as a possible synonym of Richardson's Congrus habenatus a species which had been described as Myrophis heterognathos by Bleeker (1859, p. 9) from Nagasaki, Japan. Kaup (1859, p. 7) based his new genus Gnathophis on Bleeker's species, clearly distinguishing Gnathophis from Myrophis Lütken, 1851, in having three prong-like preorbital bones in the upper lip, the dorsal originating over the middle of the pectoral fin and the head length equal to the length of the abdomen. Mr R. H. Kanazawa, U.S. National Museum, has kindly provided me with both Bleeker's and Kaup's original descriptions and also informs me that Myrophis indeed does not have the prong-like bones in the upper lip. Bleeker (1864, p. 29) later admitted his error in referring the Nagasaki specimen to Myrophis (although Schultz et al. (1953, p. 70) perpetuate the mistake) and stated that his specimen was a second species of Uroconger Kaup, 1856. Asano (1962, p. 114) supports Bleeker's view in part and places Myrophis heterognathos as a synonym of Uroconger lepturus (Richardson, 1844). But a comparison of Bleeker's illustration of Myrophis heterognathos with Asano's description of Uroconger lepturus shows conclusively that the Nagasaki specimen cannot be a Uroconger.
In his useful and detailed account of the Japanese congrid eels Asano recognises on morphological grounds two divisions of the Congridae (a) the Anagostem, page 17 including Anago, Ariosoma, Alloconger, Chiloconger and Thyreoconger (which is a synonym of Ariosoma; see Rosenblatt, 1958, p. 54) all of which, among other characters, lack the supraoccipital bone and (b) the Conger-stem, including the other congrid genera examined, which all have the supraoccipital bone except Congriscus. Reference may be made to Asano's account for the numerous other characters which serve to distinguish the genera of the two groups. Bleeker's illustration of Myrophis heterognathos shows an eel with the dorsal fin originating over the middle of the pectoral, a character possessed only by Congriscus megastomus, Conger cinereus, Rhynchocymba nystromi, R. xenica and Congrina retrotincta of the Japanese eels; the tail is relatively short, referring it to Congriscus, Conger or Rhynchocymba; the vomerine teeth are molariform and in several rows, while the maxillary teeth are conical, both characters which are indicative of all genera except Uroconger and Congrina; the prong-like preorbital bones are directed forwards and outwards. Uroconger has a dorsal origin over the branchial aperture, a very long tail and preorbital prongs which are directed backwards and outwards; Congriscus and Conger have no preorbital prongs. Myrophis heterognathos can therefore be referred only to Rhynchocymba under Asano's system and clearly not to Uroconger. Since Myrophis heterognathos is the type of Kaup's genus Gnathophis then Rhynchocymba (Jordan & Hubbs, 1925) is a synonym of this genus.
Returning to Günther's suggestion that Gnathophis heterognathus is very closely similar to the Australasian silver conger, an examination of various osteological characters of the two shows them to be congeneric. The silver conger is immediately referable to Asano's Conger-stem in having a supraoccipital present, a reduced ethmoid process, four suborbital bones and lateral line ossicles of the Conger type. Further, there are three prong-like bony projections into the upper lip from the ventral aspect of the preorbital bone and the upturned labial flange of the upper lip is present but reduced. By Asano's account these characters restrict Richardson's Congrus habenatus to "Rhynchocymba "—i.e., Gnathophis. Phillipps (1927, p. 17) has already listed the silver conger as Gnathophis habenata but Gnathophis is masculine gender and the specific name therefore becomes G. habenatus (Richardson, 1848). The type species of Gnathophis Is G. heterognathus (Bleeker, 1859) from Nagasaki, Japan (type specimen 140mm total length held in the British Museum as BMNH 1867-11-28-305) and not G. habenatus as I had already noted in error (1960, p. 464).
Wade (1946, p. 194) describes a species of Gnathophis from the coast of California and thus the distribution of the genus is further extended from the Japanese and New Zealand regions, but the range is probably not confined to the Pacific. It is my opinion that Muraena mystax Delaroche, 1809, from the Mediterranean (in lit. variously Congermuraena, Bathycongrus, etc.) is also a Gnathophis but lacking specimens I am unable to confirm this. Lozano Rey (1947, pp. 531–534, pl. 7, fig. 3) describes and figures this species. His illustration shows an eel with essentially ventrally directed anterior nostrils, a labial flange not especially well developed, a relatively short tail and from the description, a silver iris. All of these features are characteristic of Gnathophis. Further, the larvae of Delaroche's species, as described and figured by Lea (1913, pp. 18–21, figs. 12–15, pl. 3, nos. 1–2) are strikingly similar to those of Gnathophis habenatus as described later in this account. They have the vent placed relatively far back (in fully developed larvae), there is a crescentic patch of black pigment below the posteroventral corner of the iris, there is a series of regularly-spaced pigment spots at the level of the intestine along the ventral body-wall and a series of spots on the bases of most anal rays. Although it would be unwise at this stage of knowledge to speculate too much on the relationships of adult eels as shown by page 18 their larvae, in view of the fact that leptocephali show fairly clearly defined generic categories it seems likely that Delaroche's "Muraena" mystax is a species of Gnathophis.
Gnathophis habenatus has been known under a variety of generic names, the first of which is Congermuraena Kaup, 1856. Kaup (1856, p. 105) first applied Congermuraena (or Congromuraena as emended by Günther, 1870, p. 40) to Congrus habenatus but in doing so referred two other species, Muraena balearica Delaroche, 1809, and Muraena mystax Delaroche, 1809, to this genus. Kaup gave a long description of Congermuraena habenata (copied from Richardson) in comparison with only brief mention of the other two species and Bleeker later selected Muraena balearica as the genotype. However, Swainson (1838, p. 220) had already diagnosed a new genus Ariosoma from a species well-known from Sicilian shores which was "richly coloured with silver reflections, very different from the lurid hues of the true eels". As Ogilby (1898, p. 287) points out Swainson was probably referring to Delaroche's Muraena balearica when he established Ariosoma. It is unlikely that he had Muraena mystax in mind since this species has only a silver iris compared with the broad silver colouration in Ariosoma balearica (Lozano Rey, 1947, pp. 530 & 533). Swainson's "nostrils not tubular" in his description of Ariosoma was probably meant to distinguish this genus from Muraena which has conspicuously tubular posterior nostrils.
As Griffin (1936, 16–17) indicates, Bleeker's action in selecting Muraena balearica as the genotype of Congermuraena makes the latter a synonym of Ariosoma. Griffin recognised the generic distinction of Richardson's Congrus habenatus from Ariosoma balearica and that the former was apparently left without a valid generic name. He therefore proposed Poutawa for this species and thus Poutawa is a synonym of Gnathophis.