DEVELOPMENT AND RELATIONSHIPS 



Elopiformes: Development 

 W. J. Richards 



THE Elopiformes comprises four genera of recent fishes and 

 each of these genera is composed of at least two species. 

 The species are found in tropical waters of the Atlantic, Indian 

 and Pacific oceans. Elops, a cosmopolitan genus, is composed 

 of several species and Megalops is composed of two species. M. 

 atlantica Valenciennes is found in both the eastern and western 

 Atlantic and M. cyprinoides (Broussonet) is found in the Indian 

 and western Pacific Oceans. Alhula has two recognized species. 

 A. vulpes is cosmopolitan and A. nemoptera is found on the 

 Atlantic and Pacific coasts of the Americas. Recent electropho- 

 retic work indicates that there may be additional species (Shak- 

 lee and Tamaru, 1981). Pterothnssus has one species along the 

 coast of West Africa, P. helloci Cadenat, and one off Japan, P. 

 gissu Hilgendorf 



Larval stages of elopiform fishes have attracted great interest 

 among ichthyologists because of their unusual leptocephalus 

 development, a stage found in no other group but the Anguil- 

 liformes and Notacanthiformes. Consequently most recent clas- 

 sifications have combined all fish with leptocephalus larvae 

 into the Elopomorpha (Patterson and Rosen, 1977). Forked tails 

 of the elopiform leptocephali provide an easy means of sepa- 

 rating them from other leptocephali which have reduced or no 

 tails at all. The non-fork tailed leptocephali are treated sepa- 

 rately in the three subsequent papers in this volume. 



Recent classifications have altered our classical view of elo- 

 piform fishes by suggesting a much closer relationship with eels. 

 Greenwood et al. (1966) included all fishes with leptocephalus 

 larvae in the superorder (Elopomorpha). This superorder con- 

 tained: Elopiformes with two suborders, the Elopoidei (Elopidae 

 and Megalopidae) and the Albuloidei ( Albulidae including Pter- 

 othrissidae); Anguilliformes with two suborders, the Anguil- 

 loidei and Saccopharyngoidei; and Notacanthiformes with two 

 families (Notacanthidae and Halosauridae). A number of papers 

 have discussed this proposed classification and a majority has 

 sustained the opinion that the Elopomorpha is a monophyletic 

 assemblage. Forey (1973a) discussed the intragroup relation- 

 ships and made some interesting observations on leptocephali 

 in a second paper (1973b). Two significant classifications ap- 

 peared in 1977, one by Greenwood and one by Patterson and 

 Rosen. Both classifications concluded that Elopomorpha is a 

 natural, monophyletic group and that Albula and Pterothrissus 

 are related to the Halosauridae and Notacanthidae. Greenwood 

 (1977) presented a concept of Elopomorpha as a Cohort Tae- 

 niopaedia with two superorders: Elopomorpha comprised of 

 Elops and Megalops in the Order Elopiformes (Suborder Elo- 

 poidei) and Anguillomorpha comprised of two orders, the Al- 

 buliformes with two suborders (Albuloidei and Halosauroidei) 

 and the Anguilliformes. Patterson and Rosen (1977) defined a 

 cohort Elopomorpha of three orders: Elopiformes, Megalopi- 

 formes and Anguilliformes, the latter with two suborders— the 

 Anguilloidei and Albuloidei. Patterson and Rosen (1977) con- 



cluded that the interrelationships of the Elopidae, Megalopidae 

 and Anguilliformes are best represented by an unresolved tri- 

 chotomy. However, it would seem that those with forked tails 

 would be monophyletic and the reduced or tailless leptocephali 

 would be derived from those with tails. The trichotomy scheme 

 results in paraphyletic forked tailed forms. 



With the exception of the species of Pterothrissus. the species 

 of the remaining genera are coastal with some stages entering 

 hyposaline environments. Pterothrissus helloci occurs benthi- 

 cally from 70 to 500 m, most abundantly from 120 to 250 m, 

 off the coast of West Africa from 9°N latitude to 20°S latitude 

 (Poll, 1953). All elopiforms are presumed to have pelagic eggs 

 although the eggs of all are undescribed. According to Smith 

 and Potthoff (1975) the eggs and early larvae of Harengula 

 jaguana were erroneously attributed to Megalops atlanticus by 

 Breder (1944), Mansueti and Hardy (1967), and Mercado and 

 Ciardelh (1972). 



The larval stages have been well described for all genera and 

 are unique (Fig. 28). The larval stage is represented by the lep- 

 tocephalus which has been defined by Hulet (1978) and Smith 

 (1979). The leptocephalus is compressed, transparent and leaf- 

 like with a mucinous pouch which distinguishes it from all other 

 fish larvae. It grows to large size compared to other fish larvae, 

 it has fang-like teeth at the early stages which are subsequently 

 lost (possibly reabsorbed), its viscera is confined to a narrow 

 strand along the ventral midline, its musculature forms a thin 

 layer outside of the mucmous pouch and the remainder of the 

 pouch consists of a mass of acellular material composed of 

 mucoproteins and polysaccharides enclosed by a continuous 

 layer of epithelial cells. Its gut is in two sections, an esophagus 

 and an intestine which are separated by a gastric region com- 

 posed of the stomach, liver and gallbladder. The kidney, of 

 various lengths, lies over the gut beginning near the gastric region 

 and contmuing posteriorly. Ventral blood vessels conspicuously 

 appear between the aorta and the kidney and gut. In elopiform 

 leptocephali dorsal, anal, pectoral and pelvic fins are present 

 and the caudal fin is large and forked. 



Genera of elopiform leptocephali are easily identified except 

 at small sizes prior to caudal development when myomeres are 

 difficult to count. The number of myomeres for elopiforms ranges 

 from 51 to 92 whereas most anguilliform leptocephali have 

 more than 95. Leptocephali of the Cyemidae have 80 myomeres. 

 Smith ( 1 979) provides a key, characterizations and illustrations 

 of the genera. Many other workers have described complete 

 series or individual stages. Complete series of Elops have been 

 described by Gehringer (1959a), Megalops by Wade (1962), 

 Alhula by Alexander (1961), and Pterothrissus by Matsubara 

 (1942). Among other papers which describe and illustrate var- 

 ious stages are: oi Megalops by Delsman (1926b), Mercado and 

 Ciardelli ( 1 972), Gehringer ( 1 959b), Eldred ( 1 967b, 1 972) and 

 Richards (1969); of Pterothrissus by Smith (1966b) and Rich- 



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