An Index and Bibliography of Eel Larvae
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THE J.L.B. SMITH INSTITUTE OF ICHTHYOLOGY Rhodes University, Grahamstown SPECIAL PUBLICATION No. 7 Issued 12th December, 1969 AN INDEX AND BIBLIOGRAPHY OF EEL LARVAE by P. H. J. CASTLE Anglo American Corporation of South Africa Advanced Research Fellow in Ichthyology (Published by the J.L.B. Smith Institute of Ichthyology, Rhodes University, Grahamstown, South Africa) Long & Co. (Pty) Limited Printers Port Elizabeth Republic of South Africa 1 AN INDEX AND BIBLIOGRAPHY OF EEL LARVAE by P. H. J. CASTLE’ ABSTRACT A survey o f ichthyological literature from 1758 until the end of 1968 reveals that information on eel larvae {leptocephali) is widely scattered and contained in at least 600 different papers. A synthesis of tlris information, essential for the correlation of larval eels with their adult species, is attempted here as a necessary adjunct to the further development o f systematic studies on adults. About 450 forms of eel larvae have been described, of which nearly 100 have been at least tentatively identified; some 200 have been named as species of die genus Leptocephahis Grono- vius, 1763 (now a rejected name in Zoology), or of other larval genera; the remainder have not been named. Systematic and bibliographic information on these cel larvae is presented togedier with a geographic checklist and a list of myomere numbers in leptocephali. Reports of eel eggs and prelarvae are not included but these may be found in some of the papers listed. The leptocephalous larvae of other groups of fishes are excluded from this index, aldiough references are made to relevant sources of information. CONTENTS page I — Introduction .. .. .. .. .. .. .. .. .. 1 II — Notes on Generic Groupings in Leptocephali .. .. .. .. 5 III — Checklist of Eel Species Whose Larvae are Known .. .. .. 12 IV — Checklist o f Larvae Whose Genera are Known .. .. 28 V — Checklist o f the Species of the Genus Leptocephahts.......................... 31 VI — Checklist of Numbered or Lettered Species .. .. .. .. 58 VII — Checklist of Un-named Species .. .. .. .. .. .. 61 VIII — Systematic Checklist of Eel Larvae .. .. .. .. .. 63 IX — Geographical Checklist of Eel Larvae .. .. .. .. .. 70 X — A Bibliography of Eel Larvae .. .. .. .. .. .. 75 XI — List of Periodicals Consulted .. .. .. .. .. .. 113 1 Anglo American Corporation of South Africa Advanced Research Fellow in Ichthyology, Rhodes University, Grahamstown. Present address: Department of Zoology, Victoria University of Wellington, Wellington, New Zealand. 2 I — INTRODUCTION Amongst the major divisions o f the Teleostei, the Order AnguiUiformes (or eels) is possibly the most specialised. This specialisation is manifested externally in an attenuate body form which seems to be correlated with the habitat o f the adults. Eels also have an unusual life-history. The early stages o f most eel species have yet to be recognised but it is almost certain that all eels pass through an extended pelagic larval phase. The latter, in all species for which the larvae are known, differs conspicuously in body form and anatomy from that found in the adult. The larval phase is known as a leptocephalus. The elongate body form, however, is not restricted to eels, and occurs in many widely separated groups o f fishes. Furthermore, certain other groups o f teleosts, to which eels are no doubt phylogenetically related, have leptocephalous larvae. The term “leptocephalus” thus now has a much broader meaning than when it was first introduced to characterise the larvae o f eels. It is currently used to refer to all the elongate, transparent, larvae o f the Anguilliformes (including the Anguilloidei and the Saccopharyngoidei), the Elopiformes (Elopoidei and Albuloidei) and the Nota- canthiformes (Halosauridae and almost certainly also the Notacanthidae and Lipo- genyidae). Where the term “leptocephalus” is used in tills index it specifically refers to a larva of the true eels, for the sake of convenience. There are many descriptive accounts of elopiform leptocephali from various parts o f the world but these are not included in tills index. Tchernavin (1947: 299) and Bohlke (1966: 606-607) give useful summaries o f the literature pertaining to the leptocephali o f the Saccopharyngoidei, including the detailed accounts o f Lea (1913), Bertin (1938) and Orton (1963). Reference may be made to these papers for information on larval gulpers o f the type presented below for the eels. It is therefore considered unnecessary to duplicate it here. Mead (1965), Harrisson (1966) and Smith (1969, in press) discuss the larvae o f the Notacanthiformes. Some of these have been included in this index because they have become almost inextricably confused with larval eels, such is their close similarity to this group. The marked specialisation in both morphology and life-history in the eels is not combined in any other major division o f bony fishes. Within the Order itself the most extreme modifications, particularly in body form, are shown by the gulpers (Sub-order Saccopharyngoidei) which are closely related to the true eels (Sub-order Anguilloidei). The numerous unique features o f eels have led ichthyologists to con sider them to be the result o f an early diversification from the main line o f teleost evolution. The details of the evolutionary pathway have been a matter o f speculation but Greenwood et al. (1966: 350, fig. 1) present a strong case for the belief that it occurred by way of elopiform ancestors. Together with the percoids and siluroids, the sub-order Anguilloidei stands as one o f the larger sub-groups of teleosts. About 20 families of eels are currently recog nised, excluding the poorly-known fossil eels. Three o f these — the Congridae, 3 Muraenidae and Ophichthidae — are large and diverse, with more than ten genera in each. The remainder, in general, contain a smaller number of more specialised genera and relatively few species. There are probably about 140 valid genera within the sub-order. The considerable diversity of eel families is based essentially on major structural differences, some o f which are reflected in external features. Attention to osteological characters in adults has been a feature o f the development of the present-day classi fication o f eels. Accepted generic criteria vary from family to family but more com monly genera are separated by conspicuous differences in external characters. For example, there may be differences in the nature and distribution o f the teeth, the structure and position o f the paired nostrils, and the relative development of the fins and branchial apertures. Some of these are nonetheless manifestations o f differences in osteology. In general, it might be observed that individual eel genera are becoming more firmly established today, as a result of detailed studies on the essential distinctions in hard structure. In some families, however, the limits, exact relationships and validity o f many genera have yet to be clearly elucidated. Many of the early accounts largely ignored the particular characters which are now proving o f value in the recognition o f eel species. The earliest-named species often seem to have been distinguished on relatively superficial features, while species limits currently tend to be drawn on more obscure characters. These include the pattern o f distribution o f teeth, the configurement o f sensory pores, the structure of the caudal fm, the number of vertebrae and fm-rays and certain body proportions. The latter must be evaluated in each species with the effects of allometric growth in mind. Larval characters, however, have not been used in the establishment o f the present eel classification, which is still essentially based on Regan (1912). This is no doubt related to the difficulty that has been met with in identifying all but a few eel larvae at the specific, generic and even the familial level. As a result, there have been few serious attempts to integrate systematic studies on both larvae and adults. A situation has thus developed where important collections of larvae have been set aside as too difficult for study. This is unfortunate since eel larvae make up a significant part of the oceanic plankton and could be expected to provide valuable information in planktology. Some extensive collections o f leptocephali still remain largely unworked. However, apart from oceanographic considerations, larval studies may sometimes offer new leads in eel classification, as some current work suggests. With the increased attention being given in recent years to the biological resources of the oceans, and particularly to plankton, collections of eel larvae have also vastly increased. This increase has not been matched by collections o f adults which in general are more difficult to capture (particularly the deepwater forms). In any case, the adults seem to have been considered of largely academic interest. The larval features of 13 eel families are now known. Generic distinctions, based essentially on differences in pigmentation, are also recognised as such, even though many definitive generic identities have yet to be worked out. This is particularly so in the larger eel families where generic relationships are not well known. Collections of metamorphic specimens which usually show characters of both larvae and adults and thus allow the recognition of genera amongst larvae, are urgently required on a 4 large scale to overcome this difficulty. Eel larvae are currently being distinguished at the specific level by differences in meristic features which equate with similar differences in the adult species. There is, however, a general lack of knowledge of such characters in adults, a situation which would be corrected by a broad survey of eels for just this precise information, as a necessary adjunct to systematic studies on larvae. Leptocephali show a wide diversity at the generic level. In contrast, adults tend towards a morphological conformity, at least externally. It is tempting therefore to suggest that convergence or parallelism, perhaps has occurred in adults while the larvae might be regarded as more clearly reflecting phylogenetic relationships.