Reference: Biol. Bull 185: 277-283. (October, 1993) Metabolic Rates in Early Life History Stages of Elopomorph Fishes EDWARD PFEILER 1 * AND JOHN J. GOVONI-* Department of Biology, University of Puerto Rico, Mayagiie:, Puerto Rico 00681, and 2 NOAA, National Marine Fisheries Service, Southeast Fisheries Science Center, Beaufort Laboratory, Beaufort, North Carolina 28516 Abstract. The respiratory electron transport system (ETS) assay was used to estimate metabolic rates in four species of eel (Anguilliformes: Ophichthidae and Congri-dae) leptocephali (Myrophis pitnctatus, Ophichthus sp., Hildebrandia flava, and one unidentified congrid) and the bonefish (Albuliformes: Albulidae: Albula sp.)-Wet-weight-specific ETS values in whole-body homogenates, assayed at physiological temperatures, ranged from 4-20 Mg-at O h~' (g wet wt)"'. Arrhenius activation energies ( a ) ranged from 1 1.0-15.7 kcal mole" 1 . Both wet-weight-specific ETS activity and oxygen consumption rate in-creased approximately fivefold during metamorphosis of leptocephali of A/bu/a sp. Wet-weight-specific ETS activity showed little change as leptocephali of M. punctatus transformed into glass eels, but increased about fivefold as glass eels metamorphosed into elvers. No significant difference was found in ETS activity measured in fresh early metamorphic leptocephali of Albula sp. and lepto-cephali that had been stored frozen at -70C for up to 1 5 months. The data suggest that metabolic rates are low in leptocephali, which implies that the demand for nu-trients is also relatively low. We argue that the apparent diet of these larvae seems capable of providing a sufficient supply of nutrients under these conditions. Introduction A leptocephalous larva is characteristic of marine teleost fishes (superorder Elopomorpha) comprising the orders Received 3 December 1992; accepted 24 June 1993. * Present address: Departamento de Ciencias Mannas, Institute Tec-nologico y de Estudios Superiores de Monterrey (ITESM), Campus Guaymas, Apartado Postal 484, Guaymas, Sonora 85400, Mexico. Abbreviations: ETS, electron transport system: NADH, reduced nico-tmarmde adenine dinucleotide: NADPH, reduced nicotinamide adeninc dinudeotide phosphate; INT. /Modomtrotetrazolium violet, K,, Arrhenius activation energy; !' ,, routine oxygen consumption rate: R:ETS. ratio of wet-weight-specinc I , to ETS activity. Albuliformes (including Notacanthiformes), Anguilli-formes, Elopiformes, and Saccopharyngiformes (Robins, 1989). In addition to their distinct morphological char-acteristics, including the presence of a transparent, lat-erally compressed body composed mainly of a central core of gelatinous material (Smith, 1984), leptocephali differ from larvae of most other marine teleosts in having an extended larval period, referred to here as the premeta-morphic period. This period, during which the leptoceph-alus increases in size as the gelatinous matrix is formed, may last from a few months within the albuliforms and elopiforms (Smith, 1980; Pfeiler et a/.. 1988) to several years within the anguilliforms (Schmidt, 1925). The metamorphic period, during which the leptocephalus transforms into a juvenile fish, is usually completed within 2-3 weeks (Pfeiler, 1986). In addition to increased devel-opment of muscle, bone, and internal organs, metamor-phosis is characterized by a decrease in size as the gelat-inous matrix is broken down (Pfeiler, 1989). The source of nutrition in premetamorphic leptocephali has puzzled biologists for decades. Most of the premeta-morphic period is passed in the absence of a yolk sac (Pfeiler, 1986), and identifiable food material has not been observed in the gut of many species (for references see Pfeiler, 1989). The presence of a functional digestive sys-tem also has been questioned (Rasquin. 1955; Hulet, 1978). Premetamorphic leptocephali may be receiving nutrients by absorption of dissolved organic matter across surface epithelia and by ingestion of particulate organic matter and microscopic organisms (Pfeiler, 1986; Hulet and Robins, 1989; Otake et a/., 1993), but the relative contribution of these potential sources of nutrients has not been determined. Most of the nutritional requirements of metamorphic leptocephali are thought to be provided by breakdown and utilization of organic material stored in the extracel-277
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