ABSTRACT
Fish display a wide range of developmental ontogenies. These distinctions have taxonomic, evolutionary, and ecological causes. The concept of saltatory ontogeny suggests that development is not gradual but proceeds in leaps separated by a series of stable developmental stages. In this context, endogenous/exogenous feeding also distinguishes the developmental phases of embryo (egg), eleutheroembryo (feeding off the yolk sac) and larvae (exogenous feeding) in fi sh (Belanger et al., 2010). Two fundamentally different strategies characterize the early development of marine teleosts (Pfeiler, 1986). The fi rst and the most common strategy (type 1) consists of a post-hatch period in which the yolk-sac is resorbed; exogenous feeding begins immediately thereafter and it continues throughout the larval period as the larvae grow into juvenile fi sh. In the second strategy (type 2), after a similar post-hatch period during which the yolk-sac is resorbed, the larval fi sh shows a dramatic increase in size. Two potential sources of nutrition have been proposed for type 2 larvae: dissolved organic carbon and particulate organic carbon in the form of zooplankton fecal pellets and larvacean houses. Type 2 larvae may remain in the plankton for several months and are typical of fi ve orders of bony fi shes: the albuliformes (the bonefi sh), the anguilliformes (the eels), the elopiformes (the tarpon and ladyfi sh), the notacanthiformes (the spiny eels) and the saccopharyngiformes (the gulper eels). The type 2 larva has an unusual morphology. It is decidedly laterally compressed, almost leaf-like in appearance, with a perfectly clear body and a slender head that gives it its name: the leptocephalus. Development proceeds in two main phases. In phase I, the larvae grow in size until they reach a maximum that is typical of the species. During phase I, unlike most (type 1) fi sh larvae, energy reserves are accumulated within
the leptocephalus as lipid and an acellular mass within a mucinous pouch. The mucinous pouch consists of proteoglycans, compounds made up of a conjugated peptide and glycosaminoglycan carbohydrates, most familiar as mucus and cartilage. Phase II of leptocephalus development consists of a size shrinkage and a profound change in shape to the juvenile morph, fueled by combustion of most of the accumulated energy reserves in the form of glycosaminoglycans and lipids (Pfeiler, 1996). The two elements in the energy budget equation that infl uence the amount of energy available for growth in larval fi sh are metabolic rate and excretion. Metabolic rate receives the greatest allocation of energy: 80-85% of the total ingested energy. Excretion commands a much lower percentage of ingested energy, with values ranging between 4-40% (Bishop and Torres, 1999). In early fi sh life the changes of metabolic rate, oxygen consumption and energetic metabolites lead to alterations in biomarker activities and concentrations.
