Eels and Eel-Larvae of Australasian Waters

Abstract

Studies made on a large collection of 1,100 specimens assembled from various parts of the world "but mainly from the southwest Pacific and the southeast Indian Oceans recognise 95 species, referred to 43 genera and 10 families. In this first major attempt to integrate studies on adult eels and the more difficult systematic group, the eel-larvae, the characteristic morphology of many genera of leptocephali is established for the first time, enabling future recognition of species, as soon as the definitive meristic characters are determined from adults. The study material was collected almost entirely in a "broad area from the equator to 45°S. and between 140°E. and 180°E. (that is, including New Caledonia, eastern Australia and New Zealand) although some specimens were taken off Western Australia, in other parts of the Indo-Pacific and in the western Atlantic. A small number of adult specimens available are referable to the deepwater family Synaphobranchidae and are sufficiently representative of this group to provide finally a full coverage of the family enabling the family Ilyophidae to be abandoned, the inclusion of Ilyophis with the synaphobranchids and the reinstatement of Histiobranchus as a full genus in the Synaphobranchidae. Other adult and juvenile eels examined, referable to six species, belong in the Congridae and are characteristic of the New Zealand region. Descriptions of adult eels should always include vertebral and fin-ray counts at least, to enable the identification of their larvae. Collections of leptocephali are increasing more rapidly than those of the adults but have remained unworked through the difficulties encountered in recognising even generic categories in the larvae. The characteristic morphology of the larvae of 13 eel families is new known and generic distinctions, based largely upon conspicuous differences in pigmentation as in other groups of fishes, are also now recognised as such. This pigmentation is usually retained long enough during metamorphosis to relate the leptocephalus to the Juvenile. Collection of such metamorphic specimens is a prime need in the development of systematic studies on leptocephali. At the specific level in eel-larvae distinctions are shown mainly in marked differences in myomere counts. The general lack of knowledge of this character in the adults would be corrected by a broad survey of eels for this precise information. The stability of characters in larval morphology suggests the future importance of larvae in the development of eel classification and in the determination of the relationships of the various groups of eels. The eels of New Zealand and southern Australia are relatively recent developments in an essentially temperate region from a northern, widely distributed Indo-Pacific stock. While some of these more southern eels are now known to spawn in the New Caledonia area at least two of them spawn off the eastern Australian coast, with a similar spawning area off Western Australia, giving rise to what are possibly distinct populations of these species. Eel-larvae of deepwater cosmopolitan eels, temperate eels and typical Indo-Pacific species at all stages of development are abundant in New Caledonia waters, indicating the close proximity of spawning areas and the importance of this region. Larvae are dispersed westwards and southwards to Australia by the prevailing east-southwest oceanic current system. Larval life is probably about six months in the Moringuidae and from one to two years in the temperate Anguillidae and Congridae, but intermediate in the Muraenidae and Ophichthidae.