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Zoology Publications from Victoria University of Wellington—Nos. 33 and 34



(1)The small silver eel originally described by Richardson (1848, p. 109) from Cook Strait, New Zealand, as Congrus habenatus should now be known as Gnathophis habenatus. Osteological and other characters show that it is congeneric with Myrophis heterognathos Bleeker, 1859, from Nagasaki, Japan, the type species of Gnathophis Kaup, 1859.
(2) Gnathophis is now recognised to have species in California and probably the Atlantic in addition to those known in Japan and New Zealand.
(3) G. habenatus has 120–129 vertebrae, scroll-like anterior nostrils and a small, triangular premaxillary-ethmoid patch of teeth and inhabits the shallow waters of harbours and river mouths from New Zealand to Lord Howe Island, south-east Australia, south-west Australia, and possibly St. Paul's Island, South Indian Ocean. It is distinct from G. incognitus, a new species described here, which has 139–147 vertebrae, minute epidermal papillae, anterior nostril with a simple free flap and a round premaxillary-ethmoid patch of teeth extending conspicuously in advance of the maxillary patches, known from deeper waters of New Zealand as far north as the Kermadec Islands.
(4) G. habenatus has been divided into two subspecies, G. habenatus habenatus and G. habenatus longicaudatus (Ramsay & Ogilby, 1888), which are separated solely on the length of the preanal region. G. h. habenatus is known from the New Zealand region and always has the vent a little in page 46 advance of the middle of the body so that the preanal length is never less than 40% of the total. G. h. longicaudatus is typical of the Australian region and has a short preanal length, never greater than 40% of the total. This difference in preanal lengths may be due to differences in the lengths of the pelagic larval life of the two subspecies.
(5) G. habenatus habenatus is found abundantly in Wellington harbour in a gravid condition during late autumn and winter, suggesting a nearby spawning area.
(6)A large collection of leptocephali of the two species, some 250 specimens, shows that both G. habenatus and G. incognitus spawn off the New South Wales coast and off Rottnest Island, Western Australia, over the continental slope, although there is probably only a limited spawning of G. incognitus in the western area.
(7)During growth of the larvae of G. habenatus from 5mm to 30mm the number of preanal myomeres increases rapidly from about 55 to 100 in the total of about 125. The vent remains at this level until the total length reaches about 80mm at which time metamorphosis begins and the vent moves forwards to its final position in the elver at the 37th myomere. Small leptocephali have a few large somatic chromatophores in a ventral series from the pectoral region to the vent as well as small pigment spots on the lower jaw and the caudal tip. As growth proceeds to the full-grown leptocephalus the ventral spots increase in number to about 90 (beginning at the 10th myomere), spots appear on the throat, on the dorsal and anal bases, and a characteristic black crescent appears below the iris at a length of about 30mm. Leptocephali of G. incognitus undergo similar changes during growth but are distinguished from those of G. habenatus in having 134–150 myomeres and the last vertical blood-vessel at myomere 46. G. habenatus has leptocephali with 116–132 myomeres and the last vessel at myomere 41.
(8)Leptocephali of both species have been collected from New Caledonia, but as yet the adults are unknown from this area. Adults of G. incognitus are known only from New Zealand and the adult of G. habenatus has yet to be recorded from Western Australia.