70 



ONTOGENY AND SYSTEMATICS OF FISHES-AHLSTROM SYMPOSIUM 



20 3 mm 



ENGYODONTIC 



37'» mm 



Gnathophis 



856 mm 



EURYODONTIC 

 Fig. 32. Development of teeth-series in the congrid Gnathophis. 



34), Nettastomatidae (Table 19 and Fig. 39) and Cyematidae 

 (Table 23 and Fig. 43), but characteristically short and rounded 

 in the Heterenchelyidae (Table 18 and Fig. 38) and Muraenidae 

 (Table 21 and Fig. 41), especially near metamorphosis. In some 

 Dysommatidae (Table 14 and Fig. 34) it is produced forwards 

 as a conspicuous, narrow, ethmoid rostrum bearing at its tip a 

 pair of "premaxillary" teeth and, in some also, fleshy tabs or 

 tentacles along its length. The rostrum itself is lost at meta- 

 morphosis so that the snouts of post-metamorphic dysomma- 

 tids, apart from their characteristic papillae and plicae, are sim- 

 ilar to those of other eels. 



In full-grown leptocephali the anus lies just in advance of the 

 midpoint (some Nettastomatidae, Table 19 and Fig. 39; some 

 Muraenidae, Table 21 and Fig. 41; some Xenocongridae, Table 

 22 and Fig. 42), well behind the midpoint (most genera), or is 

 subterminal (the congrid group Ariosoma-Bathymyrus. Table 

 1 7 and Fig. 37). For those in which it is subterminal, it advances 

 during metamorphosis, taking with it the anal fin origin and the 

 developing pterygiophores and actinotrichia. Its position in these 

 species is thus a very rough measure of the stage of metamor- 

 phosis. Broadly speaking, the amount of forward movement of 

 the anus is correlated with the length of larval life, generally 

 long in Notacanthiformes, Anguillidae (1-3 years) and Congri- 

 dae (10 months for species of Gnathophis, Castle, 1968; Castle 

 and Robertson, 1974) but much shorter in Moringuidae (3'/2 

 months for Moringua edwardsi. Castle, 1979) and probably also 

 for Muraenidae, Xenocongridae and many Ophichthidae. How- 

 ever, little is known of the duration of larval life in most eels. 



A special feature of some Ariosoma-Bathymyrus larvae is an 

 exterilium or external intestine (Mochioka et a!., 1982; Table 

 17Q and Fig. 37) and in the unidentified larva illustrated by 

 Weber (1913) and Smith (1979), there are tab-like extensions 

 of the intestine, of unknown significance (Fig. 31). 



The olfactory organ is a round to oval sac immediately in 



front of the eye. As growth proceeds its single aperture pro- 

 gressively becomes vertically subdivided by flaps growing from 

 the upper and lower margins. After separation of the two nos- 

 trils, the olfactory sac lengthens in many leptocephali, except 

 the Cyematidae, Nemichthyidae and Serrivomeridae, so that 

 the anterior nostril moves forwards to near the tip of the snout. 

 There it becomes subtubular and often turns downwards; late 

 in metamorphosis the posterior nostril may move dorsally or 

 ventrally to adopt its final position above or behind the eye or 

 ventrally on or through the upper lip. 



The eye is usually round, but in the notacanthiform larvae 

 referred to the larval genus Tiluropsis, and in Leptocephaliis 

 attemiatus, it is characteristically oval, with the long axis ver- 

 tical. In all Synaphobranchoidea, probably also including the 

 Simenchelyidae, the eye assumes a so-called "telescopic" or 

 "tubular" shape (Table 14 and Fig. 34) and the body of the eye 

 faces anterodorsally and is elongate, with a very deep retina. 



Teeth develop shortly after hatching. These engyodontic teeth 

 (Fig. 32) are few, needle-like, forwardly directed, each one pro- 

 gressively shorter along the rami of the jaws; typically there is 

 a pair of larger teeth anteriorly. The engyodontic teeth are shed 

 at the beginning of the euryodontic growth stage and are pro- 

 gressively replaced with the 3 series of shorter, broad-based teeth 

 in upper and lower jaws; the upper teeth are preceded by an 

 anteriormost pair, slightly smaller than the first maxillary pair, 

 which are very large in the supposed xenocongrid Thalassenche- 

 lys (Table 22 and Fig. 42). As growth proceeds teeth are added 

 progressively, to reach 40-50 at metamorphosis. They are blade- 

 like and slightly recurved in Paraconger, bicuspid in Coloconger 

 (Table 1 8 and Fig. 38), or needle-like and distinctly spaced in 

 the Heterenchelyidae (Table 18 and Fig. 38). Leiby (1979b) 

 notes that the splanchnocranium is so weakly developed in the 

 engyodontic stage of the ophichthid Myrophis pimctatus that 

 the first series of larval teeth cannot be used in feeding. 



I 



