Intra- and interspecific variability in prey size and niche breadth of myctophiform fish larvae

Prey size is one of the major factors involved in prey selection of fish la rvae, and is closely related to their foraging abilities and mouth size. Th is study examines the feeding habits of 6 coexisting fish larvae species of the order Myctophiformes in the western Mediterranean. The Myctophiformes are the dominant teleosts of the open oceans worldwide, and their larvae ar e characterized by high morphological variability. In the present study, si ze of ingested prey increased through larval development for all species ex amined, but at different rates. Niche breadth (i.e. range of sizes of inges ted prey, estimated as the standard deviation of log-transformed prey sizes ) was constant throughout growth. The larvae actively selected certain prey sizes; selection changed during development, and varied between species. T his variability appears to be related to interspecific morphological differ ences, and may constitute a strategy to optimize the utilization of trophic resources in the oligotrophic open-ocean habitat.