Between the jaws of the leptocephalus larva: biomechanically approaching a rarely observed organism

Being part of the Elopomorph group of fishes, Anguillidae species have a leptocephalus larval stage. Unfortunately, due to (mostly) unknown deep-water marine birthplaces, a catadromous lifestyle, and a transparent body morphology, these Anguilla larvae are rarely encountered in nature. Therefore, information regarding the early development of these larvae, including the exogenous feeding strategy and feeding performance, is rather scarce. To get some insight into these early ontogenetic changes and their influence on the functionality of the developing feeding apparatus, an ontogenetic series is put together from two artificially bred Anguillids. Throughout this series, graphical three-dimensional reconstructions (based on histological sections) of the musculoskeletal system of European (Anguilla anguilla) and Japanese eel (Anguilla japonica) larvae provide detailed descriptions of the changing feeding apparatus. Subsequently, theoretical bite forces are calculated for every reconstructed phase, using 3D data of joints, levers, and muscles derived from these reconstructions. Although the expected increase in bite force is observed with progressing age of the larvae, the obtained forces remain rather small (several µN). As a result, leptocephalus larvae are hypothesized to be anatomically constrained to feed only on soft and/or small food particles, which is in line with the current observations of small and/or gelatinous prey items (Hydrozoa, Thaliacea, Ctenophora, Polycystenia) in the guts of these larvae