98 



ONTOGENY AND SYSTEMATICS OF HSHES- AHLSTROM SYMPOSIUM 



ELOPIDAE 



MEGAL - 

 OP I DAE 



ALBUL- 

 IDAE 



HALO- 

 SAURIDAE 



NOTA- 

 CANTHIDAE 



EELS- 

 21 FAM5. 



ELOPIFORMES 



ANGUILLOIDEI 



Fig. 49. Hypothesis of relationships between major groups of elopomorphs. 



(Fig. 52E) have a looped gut and superficially resemble certain 

 ophichthids (Fig. 51 A); on the other hand, some ophichthid 

 larvae (for example, Basicanichthys. Fig. 52D) have a weakly 

 looped gut and superficially resemble congrid larvae (Smith and 

 Leiby, 1980). 



A contraindication of relationship may be shown by the larvae 

 of the Nemichthyidae and Cyematidae. It was mentioned above 

 that these two families differ in many characters and that their 

 traditional association must be questioned. The larvae of these 

 families are as different from each other as any two leptocephali 

 can be. Nemichthyid larvae are long and slender with a simple 

 gut that reaches almost to the tip of the tail. Cyematid larvae, 

 on the other hand, are high and deep and their gut contains 

 several characteristic loops (Fig. 48). Some observers have no- 

 ticed a resemblance between cyematid larvae and saccopha- 

 ryngoid larvae and have suggested that these families are related 

 (Benin, 1937; Raju, 1974). 



Despite the caveats that must be invoked when dealing with 

 the systematic implications of leptocephali, these larvae play an 

 important role in systematic studies of eels. They provide ad- 

 ditional characters to be used in systematic analysis, and they 

 are often more readily accessible than adults. The cryptic or 

 burrowing habits of most adult eels make them difficult to collect 

 in large numbers. The larvae, on the other hand, live in open 

 water near the surface and can easily be collected with plankton 

 nets or midwater trawls. In many cases, larvae provide data on 

 distribution and species structure that are unavailable from adults 

 (Smith and Castle, 1972, 1982). 



Elopomorphs 



The Notacanthiformes and Anguilliformes belong to a group 

 of fishes called elopomorphs, along with the Megalopidae, Elo- 

 pidae, and Albulidae (including Pterothrissidae). Current con- 

 cepts of the interrelationships of the major groups of elopo- 

 morphs are illustrated in Fig. 49 (Greenwood, 1977; Patterson 

 and Rosen, 1977; Lauder and Liem, 1983). The trichotomy 

 exists because there seem to be no derived characters that clearly 

 link any of the three main branches with any of the others. 



Elops and Megalops (including Tarpon) seem more similar 

 to each other than either is to Alhida. but this may be because 

 they are both midwater feeders with terminal mouths, whereas 

 A/hula is a bottom feeder. Alhula has several specializations 

 (enlarged cephalic canals, prolonged snout) that are lacking in 

 Elops and Megalops. Most if not all of the resemblances between 

 Elops and Megalops may be explained either as primitive char- 

 acters or as adaptations to a similar way of life. Megalops has 

 several derived characters not found in Elops, most notably the 

 vascular air bladder and the otophysic connection. Elops does 

 not seem to have any feature that is derived relative to other 

 elopomorphs. 



Several synapomorphies can be cited to link the Notacanthi- 

 formes and the Albulidae (Nelson, 1973; Greenwood, 1977). 

 The eels are usually placed on the albulid branch as well, but 

 this is still an open question. The Anguilliformes and Notacan- 

 thiformes share a similar elongate body form, but this feature 

 has evolved so many times in fishes that it means little by itself 



