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. These larvae have a transparent, elongated and laterally compressed body, as well as needle-like prognathous teeth. Information regarding the physiology, morphology, ecology and behavior of these Anguilla larvae is available in literature for specimens caught during their catadromous migration. Unfortunately, due to deep-water marine birthplaces and the fragility of the organisms, information regarding the early development of these larvae, including the feeding strategy at the onset of exogenous feeding and their general 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 artificially bred European (Anguilla anguilla) and Japanese (Anguilla japonica) eel larvae. Graphical three-dimensional reconstructions (based on histological sections) of the musculoskeletal system provide detailed descriptions of the changing feeding apparatus throughout the early ontogenetic stages. These reconstructions allow not only to compare the early development of both species, but also to obtain an estimation on the theoretical bite forces (using 3D data of joints, levers, and muscles derived from the reconstructions) these early larvae can generate. Three larval stages, representing the transition from endogenous to exogenous feeding, are investigated in both A. anguilla and A. japonica. Although the exogenous feeding larvae of A. japonica are younger (days post hatching) and even smaller (total length) than the endogenous feeding larvae of A. anguilla, both species undergo similar morphological changes in their musculoskeletal topography of the feeding apparatus throughout the transition. As a result, bite forces obtained in an exogenously feeding larva of A. anguilla are comparable to forces calculated for an exogenously feeding larva of A. japonica. Although an increase in bite force is observed with progressing age of the larvae in both species, the obtained forces remain rather small (several µN). Combined with a rather limited gape angle (several 100µm), these leptocephalus larvae are hypothesized to be anatomically constrained to feed only on soft and/or small food particles. This hypothesis is in line with the retrieval of small and/or gelatinous prey items (Hydrozoa, Thaliacea, Ctenophora, Polycystenia) in the guts of leptocephali captured in nature.

Publication year:2015