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 larvae, and is closely related to their foraging abilities and mouth size. This 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 are characterized by high morphological variability. In the present study, size of ingested prey increased through larval development for all species examined, but at different rates. Niche breadth (i.e. range of sizes of ingested 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. This variability appears to be related to interspecific morphological differences, and may constitute a strategy to optimize the utilization of trophic resources in the oligotrophic open-ocean habitat.