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

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.

Feeding on protists and particulates by the leptocephali of the worm eels <i>Myrophis</i> spp. (Teleostei: Anguilliformes: Ophichthidae), and the potential energy contribution of large aloricate protozoa

“…The leptocephali examined here were from the same collections as other larval fishes examined from the northern Gulf of Mexico; consequently, the availability of ciliates as food for leptocephali was the same. Leptocephali are fully capable of sensing (Hulet, 1978;Okamura et al, 2002;Døving and Kasumyan, 2008) and capturing (Pfeiler, 1989) protozoa, including aloricate ciliates. Ciliates are likely to be easy prey for leptocephali in the ocean and they are eaten readily by other larval fishes (Figueiredo et al, 2007).…”

“…No leptocephalus, however, has been found attached to another organism in a parasitic mode of behavior. While the alimentary canal of leptocephali has been reported to be non-functional and occluded (Hulet, 1978), recent evidence indicates that the alimentary canal is fully functional (Tamura et al, 1993;Otake, 1996;Pedersen et al, 2003), and can serve in the uptake of DOM through the endocytotic absorption of seawater containing DOM by the gut epithelium (Otake et al, 1993). Laboratory experiments using DOM-enriched water (Liao and Chang, 2001), however, indicate that concentrations of DOM in the ocean are unlikely to be high enough to meet the nutritional requirements of leptocephali.…”

Dissolved and particulate organic matter as possible food sources for eel leptocephali

“…In particular their nutrition is enigmatic, since no food has ever been found in their guts (Moser 1981). Their poorly differentiated gut (Hulet 1978), equilibrium of body ionic composition with seawater (Hulet et al 1972). and high level of essential amino acids (Pfeiler 1986) provoked the hypothesis that nutrition is furnished by epidermal uptake of dissolved organic matter (DOM) (Hulet 1978, Pfeiler 1986).…”

Drastic changes in otolith strontium/calcium ratios in leptocephali and glass eels of Japanese eel Anguilia /aponica

“…Since such sulfated polysaccharides indicate an affinity to alkali earth elements, particularly high to Sr (Nishizawa 1978), GAG formation and breakdown may influence the body Sr content. In eel leptocephali, which have a simple circulatory system (Hulet 1978), elements contained in GAG -extracellular matrix seem to be directly taken up, under an enzymatic reaction, by the saccular epithelium, and transported into the endolymph. Although data on GAG content in eel leptocephali are not available, rapid GAG breakdown in metamorphosing leptocephali, causing a loss of Sr from the body, may result in a drastic decrease of otolith Sr content and, consequently, Sr/Ca ratios.…”