Discussion

Few external characters survive the metamorphosis from leptocephalus to juvenile eel. Typically, there is a loss of larval teeth, the snout becomes relatively shorter, the vent moves forwards, the body becomes rounder, segmentation becomes less obvious, and larval pigmentation is replaced by juvenile colouration. The morphological features of larvae diverge so markedly from those of the adult that the systematic determination of a leptocephalus can be made with our present knowledge only in terms of the number of myomeres. This number remains almost constant through the metamorphosis to the juvenile and to the adult and also appears to be constant within limits for each species although two or more species may have approximately the same number of myomeres. With the number at about 480, the leptocephalus described here can be referred at this time to only one known species in the whole of the Apodes, Nemichthys scolopaceus, in which the myomeres number from 450 to 500 or more in the adult. In all other eels the myomeres never exceed 300 in number, the majority having less than 200. N. scolopaceus has been recorded from New Zealand waters and is known from a single specimen of 835 mm from Cook Strait (Richardson and Garrick, 1953). Roule and Bertin (1929, pp. 61–67) and Beebe and Crane (1937, pp. 357–365) have fully described the leptocephalus of N. scolopaceus. This metamorphoses at a maximum of 253 mm. All the larval stages agree in having less than 450 W-shaped myomeres, of which 93–254 lie before the level of the vent, an elongate snout concave in lateral profile, subterminal vent only in the smaller specimens, and with large chromatophores scattered laterally on the body as well as minute ones along the dorsal aspect of the intestine, features which are not present in the leptocephalus described, so that the equivalence in the number of myomeres cannot be used to assign this larva to N. scolopaceus. Roule and Bertin also described a larger leptocephalus, designated Leptocephalus B, having 179–320 preanal myomeres and reaching 359 mm just before metamorphosis, characters which preclude the identification of this larva with L. giganteus. The adult of Leptocephalus B is at present unknown, though as Beebe and Crane suggest, it is probably a nemichthyid-like form.

The length of the specimen described above is sufficient to warrant speculation as to the size of the adult. L. giganteus shows no indication of approaching metamorphosis and its length may well be less than that at which metamorphosis should take place. It is most regrettable that the Dana “giant” leptocephalids have not been described in sufficient detail to allow comparison of the present specimen with one or other of these extremely large creatures. Bertin (1954, p. 313) records that there have been suggestions that the Dana “giant” leptocephali may represent the larvae of a giant eel. These suggestions were based on the known growth of Anguilla from a leptocephalus of 45 mm to an adult of about 800 mm, an increase in length in the order of about 18 times that of the leptocephalus. This would suggest that the 1,800 mm “giant” leptocephali would reach an adult length in the order of 30 metres, compatible with the length of some of the anguilliform “sea-monsters” which have been recorded. Bertin considers it improper in the present state of our knowledge to use the same ratio in what may be widely differing eels. Nevertheless, if we use the far lower ratio known in Nemichthys scolopaceus, in which the length of the grown adult is only three to six times that of the leptocephalus, the specimen here would develop to an eel of some 2.7 m to 5.4 m, approximately 9 to 18 feet. The Mediterranean and Indo-Pacific eel, Ophisurus serpens L. found also around the New Zealand coast is recognised as probably the largest known eel, and is reported to reach about 10 feet in length in tropical waters (Smith, 1953, p. 391). O. serpens has only 210 myomeres and a posteriorly-directed suspensorium, characters preventing