Structure and functional development of the eel leptocephalus Ariosoma balearicum (De La Roche, 1809)

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Abstract
Eels, elopids, notacanths and other elopomorph fishes spawn in the ocean and the hatchlings spend their larval life as pelagic, planktonic organisms. The larvae are known as leptocephali and their transparent, leaf-like body characteristically bears little resemblance to the respective adult form. Planktonic life in the ocean may last for years before the leptocephalus undergoes a morphological transformation and takes on a recognizable fish-like appearance. A prolonged larval life suggests a delay in structural and functional development and this premise was the basis for the present study. Before this work on morphology, chemical analysis of the whole leptocephalous body indicated an internal salinity and osmolality far and above the known values for marine teleost fishes. For that reason, particular attention was given to the structural maturation of the gills, gut and kidney, all of which are intimately concerned with osmoregulation. The leptocephalus of the bandtooth conger Ariosoma balearicum was chosen for morphological studies because of its abundance in the tropical western Atlantic and because the Bathymyrinae, which includes , is a reasonably generalized subfamily of congrid eels. The congrids are primitive and representative of the basic eel plan and, as such, are accorded a rather ancestral position. In order to grasp the fundamental organization of a leptocephalus it was soon discovered that all parts of the body needed to be examined. One by one, a long list of structural peculiarities came to light, each of which seemed to have a direct relation to the functional adaptation of the leptocephalus for a prolonged oceanic existence. The section describing the chondrocranium is particularly detailed and permits comparison with an examination of the leptocephalous skull in Anguilla performed by Norman some fifty years ago. The advantages of current methods for processing tissue and the electron microscope are evident in the micrographs of cell surfaces and intracellular organelles. Gill function in the premetamorphic leptocephalus is limited by structure to a mandibular hemibranch. The four gill arches are present, but the vasculature is undeveloped and gill filaments are absent. The gastrointestinal tract is always devoid of food material and some sections of the intestine do not have a discernible lumen. The size, shape and appearance of the teeth seem unrelated to the source of nutrition. All kidney tubules are aglomerular. In summary, the leptocephalus demonstrates many features of structural and functional immaturity and, until transformed by metamorphosis, appears to be an obligatory inhabitant of the open ocean.