Proximate and Elemental Composition of Lanternfish (Pisces: Myctophidae) Larvae with Estimates of Energetic and Prey Requirements

The larvae of five species of myctophids [Benthosema suborbitale, Ceratoscopelus townsendi, Hygophum taaningi, Myctophum selenops, and Notolychnus valdiviae] from the eastern Gulf of Mexico were examined to estimate energetic and prey requirements during early growth. Age was determined from examination of sagittal otoliths. Larval period ranged from 36 days for the rapidly growing C. townsendi to 67 days for the diminutive N. valdiviae. Growth rate ranged from 0.14mmSL day-1 for B. suborbitale to 0.38 mmSL day-1 for C. townsendi. Energetic demands were estimated from a slightly modified version of the equation for the bioenergetic model where; Qc = Qg + Qs + Qm where Qc = energy consumed, Qg = food energy converted into potential energy through growth, Qs = includes energy lost in feces, excretions, and secretions, and Qm = includes energy consumed in routine metabolism [Qr] and energy consumed in active metabolism [Qa]. Proximate and elemental composition of larvae was measured and converted to calories using standard factors. Daily growth in calories was estimated from age and growth information combined with chemical composition [protein, lipid, and carbohydrate]. Time of active metabolism was determined from information on gut fullness evaluated during a diel cycle. Metabolic costs were then estimated from the measured respiratory activity of other Atlantic tropical-subtropical species. Caloric requirements for growth varied among species and size of larvae, but the greatest variation resulted from assumption regarding assimilation efficiency. Daily requirements ranged from less than one to 33 day-1. Using known caloric value of preferred food, prey requirements were calculated for larvae by matching larval size to prey of the appropriate size and taxa. From these data, daily prey requirements ranged from <10 to >300 items.