CASTLE: NOTACANTHIFORMES, ANGUILLIFORMES 



69 



Metamorphosis follows the euryodontic stage. It is relatively 

 abrupt and involves the replacement of many of the character- 

 istic leptocephalus features by those of the juvenile. The body 

 rounds up in section, tissue transparency is lost, the postorbital 

 portion of the head lengthens, the larval teeth are lost and the 

 definitive teeth are gradually substituted. The anus and median 

 fin origins move forwards, though not in all species. Pectoral 

 and caudal fins are lost late in metamorphosis in those species 

 which lack the fin in the juvenile and adult. There may be a 

 substantial reduction in body length, extremely so in the No- 

 tacanthiformes. The principal characters which are retained are 

 the definitive number of myomeres/vertebrae which is estab- 

 lished very early in larval life, the number of dorsal and anal 

 fin-rays which is attained rather late in development, and for 

 some species the larval pigment. The maintenance of larval 

 pigment through metamorphosis is of prime importance in iden- 

 tification at the generic level. However, metamorphic larvae are 

 relatively rare in collections, possibly because they are in any 

 case a transient stage; metamorphics are also benthic and hence 

 less accessible to collection. Information on these important 

 stages is therefore sparse. 



Identification 



Leptocephali are thus readily recognisable amongst other fish 

 larvae, apparently abundant in the warmer ocean, and accessible 

 near the surface. Large collections of leptocephali have accu- 

 mulated, for some families and genera there being many more 

 specimens available than of the adults (e.g., the moringuid, Neo- 

 conger. Smith and Castle, 1972; the Nettastomatidae, Smith 

 and Castle, 1982). The availability of such collections and the 

 need for identification of leptocephali have resulted in the recent 

 rapid advance of larval studies (Castle. 1969; Blache. 1977; 

 Smith. 1979; Fahay. 1 983). These studies have, understandably, 

 emphasized identification rather than inter-relationships based 

 on larval characters. 



Larvae of all but the monotypic families Simenchelyidae and 

 Myrocongridae and those of about half (82) of the genera are 

 known. Several distinctive larval forms, possibly of undescribed 

 genera rather than families, are also known (e.g., the congrid- 

 like Leptocephalus thorianus Schmidt, Smith, 1979). Family 

 identification, largely by morphological and pigment characters, 

 may be arrived at from Table 13, which incorporates infor- 

 mation set out in key form by Smith (1979) and Fahay (1983). 

 This "look-alike" approach to identifying leptocephali largely 

 suffices at the family level but is less satisfactory in identifying 

 genera, especially of the Ophichthidae and Congridae (Leiby, 

 1981). More detailed information may be necessary, especially 

 for species identification, but this will be slow to accumulate. 

 Some attempt to collate available data for identification pur- 

 poses is made in Tables 14-23, with their complementary figures 

 (Figs. 34 to 43). 



More than 500 different leptocephali have been described, 

 200 as nominal species of the invalid genus Leptocephalus Gron- 

 ovius, 1763. The procedure of formally naming eel larvae in 

 this way has been both opposed (Bohike and Smith, 1968) and 

 advocated (Castle, 1969). However, nomenclatural problems 

 associated with naming larval forms will not be readily over- 

 come by ignoring the priority of larval names or attempting to 

 apply a blanket restriction on their use. Some alternative ref- 

 erence scheme, or at least an agreed descriptive procedure, does 

 seem appropriate (Fahay and Obenchain, 1978) to accommo- 

 date the large number of distinctive ontogenetic stages of eels. 



Fig. 31. Anterior region of leptocephalus of an unidentified ?net- 

 tastomatid (DANA St. 4181 II, 34<'23'N, 25°53'W, 9 June 1931), show- 

 ing tab-like extensions of the intestine. 



Few complete growth series have been described and illus- 

 trated, and developmental osteology is known only for Anguilla 

 anguilla (Norman, 1926b), Serrivoiner spp. (Bauchot. 1959). 

 Ariosorna baleancum (Hulet. 1977). Ophichthus gomesi (Leiby, 

 1979a), and Atyrophispunctatus {Leiby, 1979b). At least in Oph- 

 ichthus gomesi ossification of the head skeleton does not occur 

 for most elements until metamorphosis, although the jaws, sus- 

 pensorium and branchial skeleton are present as cartilage during 

 the pre-metamorphic stage. Leiby's recent papers (1979b, 1981) 

 contain detailed information on the sequence of development 

 of the skeleton and emphasize the relevance of a more thorough 

 evaluation of developmental osteology in identification of lep- 

 tocephali. 



In overall body form leptocephali range from the greatly elon- 

 gate notacanths (Castle, 1973, for references; Smith, 1979; Fig. 

 29), Nemichthys (Nielsen and Smith, 1978; Smith, 1979; Table 

 19) and some Nettastomatidae (Smith and Castle, 1982) to the 

 short, deep larvae of Thalassenchelys (Castle and Raju, 1975; 

 Table 22 and Fig. 42). the Xenocongridac (Smith. 1969; Table 

 22 and Fig. 42) and Cyema atrum (Smith, 1979; Table 23 and 

 Fig. 43). 



The snout is typically rather sharp, especially so in some 

 Notacanthiformes (Fig. 29), Dysommatidae (Table 14 and Fig. 



